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RiverMathRiverMath

Do the maths before you decide.

Mortgages, pay, energy bills and health. Enter your own numbers, the result is worked out in your browser, and you can see where it came from.

For example:8500 grossBMI 82 kg 178 cm23% of 1200

What is the gross amount?

net
£1,000.00
VAT 20%
£200.00
gross
£1,200.00
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Common questions

How we calculate

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127 calculators
Each one explains its result, in English, Polish and German.
Rates from the source
Polish taxes, energy tariffs and inflation follow podatki.gov.pl, ure.gov.pl, stat.gov.pl. Last checked 16 October 2026.
What's new
Version 0.181.0, 9 November 2026. In the BMI, VAT, mortgage and FIRE number calculators a required field left empty stays empty and waits for a value instead of showing a result from a default, and the mortgage sensitivity panel disappears until the fields are filled in. All changes

By country

Language is not a country. Polish G11 tariffs, payroll and tax rules sit under Poland’s flag rather than cluttering the general English catalogue.

What do you want to calculate?

Choose a goal, then the calculator that answers your question.

Heating costs
Electricity costs
Pay and work arrangements in Poland
Buying a home

BMI

Body mass index from weight and height, using the standard WHO categories for adults.

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Body fat

Body-fat percentage using the US Navy circumference method — a tape-measure estimate, not a DEXA scan.

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Calories

Basal metabolic rate (BMR) from the Mifflin–St Jeor equation and total daily energy expenditure (TDEE) by activity level.

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Ideal weight (Devine)

Calculate reference body weight from sex and height using the clinical Devine formula.

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Daily water intake

Estimate daily fluid intake in litres from your body weight and minutes of physical activity.

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Macronutrients (Protein, Carbs, Fat)

Split your daily calorie target into exact grams of protein, carbohydrates and dietary fat.

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Waist-to-hip ratio (WHR)

Calculate your waist-to-hip ratio (WHR) from simple tape-measure readings.

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Weight-change timeline

Estimate how many weeks and days it takes to reach your target weight from a daily calorie deficit or surplus (at 7,700 kcal per kg).

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Steps to distance & calories

Convert your step count into kilometres walked and estimated calories burned based on height and weight.

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Running pace

Calculate your pace per kilometre (min/km) and average speed (km/h) from distance and finish time.

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Race predictor

Predict finish times for 5 km, 10 km, the half marathon and the marathon from a distance you have already raced.

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One-rep max (1RM)

Estimate your maximum one-repetition lift (1RM) from a submaximal working set using the Epley equation.

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Heart-rate zones (Karvonen)

Calculate your 5 Karvonen training heart-rate zones from age and resting heart rate.

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VO2 max (Cooper test)

Estimate your maximal oxygen uptake (VO2 max) from the distance covered in a 12-minute Cooper run test.

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Sleep cycles

Find optimal wake-up times or bedtimes based on 90-minute sleep cycles plus the time you need to fall asleep.

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Sleep debt

Calculate how many hours of sleep you are short of your personal target across a run of nights.

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Caffeine remaining

Estimate how many milligrams of caffeine remain active in your body after a chosen number of hours.

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Intermittent fasting window

Calculate your fasting hours and eating window from the clock times of your last and first meals.

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FFMI (Fat-Free Mass Index)

Calculate your lean body mass (LBM) and fat-free mass index (FFMI) from weight, height and body-fat percentage.

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Daily protein target

Calculate your daily protein target in grams from body weight and your chosen grams-per-kilogram multiplier.

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Max heart rate (HRmax)

Estimate your maximum heart rate (HRmax) from age using the classic Fox formula (220 − age) and the Tanaka equation.

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Body surface area (BSA)

Calculate body surface area (BSA) in square metres from height and weight using the Mosteller formula.

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Ovulation & fertile window

Estimate your ovulation date, fertile window and potential due date from the first day of your last period and cycle length.

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Pregnancy & due date

Calculate your estimated due date (EDD), current gestational age in weeks and days, and trimester from your last period.

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VAT

Add or extract VAT. Works with 23% in Poland, 19% in Germany, 20% in the UK, or any custom rate.

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Tip

Calculate a tip from the bill amount and chosen percentage, then split the total equally between people.

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Split a bill

Split a bill (and optional tip) across any number of people so the individual shares add back to the exact total.

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Discount

Enter the original price and discount percentage to see the final sale price and exact savings.

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Margin and markup

Calculate unit profit, gross profit margin (%) and markup (%) from unit cost and selling price.

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Break-even point

Calculate how many units you need to sell and what revenue is required to cover your fixed costs.

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E-commerce & Marketplace Profit Margin Calculator

Estimate unit net profit, actual net margin, operational markup, and break-even price including marketplace fees, payment gateways, shipping, and taxes.

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Loan overpayment

Fixed-rate loan amortisation. See how much time and total interest you save by paying extra each month.

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Debt payoff timeline

Calculate how many months and how much total interest it will take to pay off a balance at a fixed APR and monthly payment.

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Debt payoff strategy

List your debts (card, loan, instalments) and a monthly budget — see which payoff order costs less interest and which clears a debt sooner.

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Household budget

Find out what is left each month, balance your spending with the 50/30/20 rule and trace your money flow from paychecks to savings.

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Mortgage

Calculate monthly mortgage payments, loan-to-value (LTV) and total interest for equal (annuity) or declining instalments.

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Rent vs buy

Compare total cash spent on a mortgage and home ownership against renting and investing your down payment over a chosen horizon.

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Rental yield

Calculate annual gross and net rental yield from property purchase price, monthly rent, vacancy rate and running costs.

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Property sale tax

See whether the sale is exempt, the gain, the taxable part after a housing expense, the tax and the net amount.

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Polish inheritance and gift tax

Check the tax group, whether the close-family exemption applies, the tax-free amount, bracket tax and the filing deadline.

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Buying power (Poland CPI)

Convert an amount of money between years using the average annual Polish CPI (GUS) or your own custom annual rate.

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Future price (Inflation)

Project what today’s price or expense will grow to after N years at a constant annual inflation rate.

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Compound interest

Future value of a starting lump sum plus optional monthly contributions with compound growth.

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Investment fees impact

Compare two management fee levels and see how much capital and profit fees take over the long run.

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CAGR (Compound Annual Growth)

Calculate the steady compound annual growth rate (CAGR) that takes a starting value to an ending value over N years.

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ROI (Return on Investment)

Calculate total percentage return on investment (ROI) and the annualised return over your holding period.

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Simple payback period

Calculate how many years it takes for an annual saving or cash inflow to pay back an upfront investment.

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Savings goal timeline

Calculate how many months and years of regular monthly saving it takes to reach your target pot (with optional interest).

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Savings rate

Calculate what percentage of your monthly take-home income you set aside and your total annual savings.

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Emergency fund

Calculate your target emergency cash buffer from essential monthly expenses and see how many months it will take to build.

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FIRE number

Calculate your target financial-independence portfolio from annual spending and safe withdrawal rate, plus years to reach it.

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Dividend yield & income

Calculate dividend yield (%) and annual gross dividend cashflow from share price, annual dividend per share and share count.

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Subscriptions tracker

Add up your recurring subscriptions (Netflix, Spotify, YouTube, cloud storage, AI tools and custom plans) to see your monthly and yearly total.

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AI Subscriptions

Compare and sum costs for ChatGPT, Claude, Gemini, xAI Grok and developer AI tools.

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Cost of raising a child

Estimate cumulative spending on raising a child from average monthly expenses and number of years.

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Statutory interest (Poland)

Calculate Polish statutory capital interest, late-payment interest, or commercial B2B late-payment interest from the NBP reference rate.

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Rule of 72

Estimate how many years it takes for a lump sum to double at a constant annual rate of return.

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Real rate of return

Calculate your true inflation-adjusted annual return from a nominal return and inflation rate.

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NPV (Net Present Value)

Calculate the net present value (NPV) of a constant annual cash inflow minus an upfront capital outlay.

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Annuity payout / loan payment

Calculate the fixed monthly payout (or loan payment) that brings a balance to zero over N years at a constant interest rate.

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Effective annual rate (EAR)

Convert a nominal annual interest rate into the effective annual rate (EAR) based on compounding frequency.

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Solar PV yield

Estimate annual AC solar generation from system size, regional irradiation, and one or two roof orientations.

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Solar PV sizing

Calculate what solar array size (kWp) and panel count you need to cover a chosen share of your annual electricity consumption.

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Solar PV payback

Calculate the simple payback period from net installation cost, self-consumption share, and grid buy/sell rates.

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PV kit configurator

Build a PV kit from panels, inverters, batteries, mounting and labour and estimate its cost.

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Heat pump vs gas boiler

Estimate annual space-heating demand from floor area and insulation class, then compare heat-pump electricity cost (by SCOP) against a gas boiler.

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Air-conditioning cooling load

Estimate required air-conditioner cooling capacity in watts and BTU/h from room floor area and thermal load per m².

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Heating fuel comparison

Compare the annual cost of generating the same amount of useful heat across gas, pellets, coal, wood, heating oil, electric resistance and a heat pump.

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Domestic hot water

Calculate thermal energy, electricity use and running cost to heat daily hot water by a chosen temperature rise.

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Heat loss through a surface (U-value)

Calculate thermal energy lost (kWh) and heating cost through a wall, window or roof from its U-value, area and temperature difference.

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Gas bill

Convert gas meter readings from cubic metres (m³) into kilowatt-hours (kWh) and calculate your monthly bill including standing charges.

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Appliance electricity cost

Calculate electricity consumption (kWh) and running cost for any appliance from its wattage and daily operating hours.

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Water and sewage bill

Calculate your water and wastewater bill from meter consumption in cubic metres (m³) and local utility tariffs.

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Septic tank pump-out

Estimate daily wastewater inflow, how many days it takes to fill a holding tank, pump-outs per year and annual emptying cost.

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Rainwater tank

Work out how many litres of rain your roof collects, which tank size is enough and how much you save on water and sewage.

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EV vs petrol running cost

Compare the cost per 100 km and annual energy bill for an electric car (home and public charging) against a petrol car.

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EV charging time

Calculate how many hours it takes to charge an electric vehicle between two states of charge (SoC).

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EV range (summer & winter)

Estimate real-world electric vehicle range from battery capacity and kWh/100 km consumption in normal and winter conditions.

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Standby power cost

Calculate the annual energy use (kWh) and cost of electronics left on standby 24/7.

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LED bulb swap savings

Calculate the kilowatt-hours and money saved by replacing old bulbs with LEDs.

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Heating degree days (HDD)

Estimate seasonal space-heating energy from heating degree days (HDD) and your building’s heat-loss coefficient (UA).

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Percentage

Calculate what x% of y is, what percentage a is of b, and the percentage change from A to B.

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Percentage change

Calculate the percentage increase or decrease from a starting value to a new value.

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Trigonometry (sin, cos, tan)

Calculate sine, cosine, tangent and radian measure for an angle in degrees.

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Pythagorean theorem

Enter any two sides of a right triangle and the calculator solves for the third side and triangle area.

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Quadratic equation

Find the discriminant (Δ) and real roots (x₁, x₂) of a quadratic equation ax² + bx + c = 0.

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Logarithm

Calculate log_b(x) for any valid base, alongside the natural logarithm (ln) and common base-10 logarithm (log₁₀).

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Combinatorics (n!, nPr, nCr)

Calculate factorial (n!), permutations without repetition (nPr) and combinations (nCr) for whole numbers.

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GCD and LCM

Find the greatest common divisor (GCD) and least common multiple (LCM) of two integers.

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Circle area & circumference

Calculate diameter, circumference and area of a circle from its radius.

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Triangle area (Heron’s formula)

Calculate the area and semi-perimeter of any valid triangle from the lengths of its three sides.

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Ohm’s law & electrical power

Enter any two of the four quantities (voltage, current, resistance, power) to calculate the remaining two values.

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Newton’s second law (F = m·a)

Leave one of force, mass or acceleration empty to solve for the missing SI value.

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Kinetic & potential energy

Calculate translational kinetic energy and gravitational potential energy in joules (J).

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Free fall

Starting from rest in a vacuum: enter either fall time or drop distance and leave the other field blank.

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Ideal gas law (PV = nRT)

Leave one of pressure (P), volume (V), moles (n) or temperature (T) blank to solve the ideal gas equation.

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Wave speed (v = f · λ)

Leave one of wave speed, frequency or wavelength blank to calculate the missing value.

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Density, mass & volume

Leave one of mass, volume or density blank to solve ρ = m / V.

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Snell’s law of refraction

Calculate the angle of refraction across an optical boundary (n₁, n₂) or check for total internal reflection.

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Thin-lens equation

Leave one of focal length (f), object distance (u) or image distance (v) blank to solve for the third value and magnification.

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Unit converter

Weight, length, distance and temperature. Convert common metric and imperial units instantly.

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Currency

Euro, zloty, US dollar and other ECB currencies using the reference rate from the latest TARGET business day.

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Exact age

Calculate your exact chronological age today in full years, months and days from your date of birth.

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Days between dates

Count calendar days, weeks and weekdays (Mon–Fri) between two dates, or add/subtract days from a start date.

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Hours & work time

Calculate hours and minutes between two clock times minus a break, or add minutes to a starting time.

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Polish workdays

Count working days between two dates (excluding weekends and Polish public holidays) or add working days to a start date.

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Trip fuel cost

Calculate fuel required in litres and total trip cost from distance, fuel consumption (L/100 km) and pump price.

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Commute cost

Compare the monthly cost of driving to work (fuel and parking) against a monthly public-transit pass.

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Car running cost (TCO)

Calculate the true annual car running cost and total car cost per year, month and kilometre (TCO), including depreciation, fuel, insurance and maintenance.

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Car lease payment

Estimate a lease payment and total cost including buyout and fees. Enter your own terms or start from a documented offer and vehicle price on a selected date.

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Breathalyzer (Widmark BAC)

Log drinks with clock times to estimate blood-alcohol concentration (‰) at a chosen readout hour using the Widmark model.

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Alcohol Units and Grams

Calculate pure ethyl alcohol in grams, standard drinks (10 g), UK units, and alcohol calories.

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Driving time

Calculate travel time in hours and minutes from trip distance and average speed.

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Monthly fuel cost

Calculate how many litres of fuel you burn and your monthly pump bill from monthly mileage.

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Recipe scaler

Scale an ingredient amount up or down from the recipe’s original yield to the number of servings you want.

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Unit price comparison

Compare two pack sizes by price per unit (gram, millilitre or piece) and see which one is cheaper in percent.

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Wall paint needed

Calculate how many litres of paint you need from wall area, number of coats and tin coverage (m²/L).

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Flooring & tile packs

Calculate how many packs of laminate, wood flooring or tiles to buy from room area, cutting waste and pack coverage.

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Dew point

Calculate the Magnus dew-point temperature from air temperature and relative humidity.

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Wind chill

Calculate how cold the air feels on exposed skin from air temperature and wind speed.

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Wallpaper rolls needed

Calculate how many wallpaper rolls to buy from wall perimeter, room height, roll dimensions and pattern-matching waste.

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Concrete slab volume

Calculate cubic metres (m³) of concrete needed for a rectangular slab or screed, including a waste margin.

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