About RiverMath
RiverMath is for the moment you need a number and do not want an account. Type height, a bill, a VAT rate or a finish time. The result appears on this page. If sync is on, a copy goes to the server and only the site owner can open it with a PIN.
What you can calculate
The tools cover three everyday jobs: a quick check on the body, a money figure you can explain, and conversions you would otherwise open a second tab for. Each calculator has its own page. The full catalogue is on the home page.
Open a group instead of scrolling every card here:
How it works
Every formula runs in your browser after the page loads. We do not ask you to sign in or upload a spreadsheet. Refresh the page and the fields reset. A server copy exists only if you turn sync on. Body, pregnancy and breathalyzer tools never leave this device.
Who it is for
Students checking homework, runners pacing a 10 km, freelancers adding VAT, or anyone comparing a sale price. The Polish salary tool is a planning sketch for an umowa o pracę in 2026 — useful before a conversation, not a substitute for a payroll office.
What this is not
These are educational estimates. A clinic will not treat a BMI or body-fat result as a diagnosis. A bank will not accept a loan or savings figure as advice. A tax office will not take a VAT or PIT estimate as a filing. If a decision has legal or medical weight, use the official source.
Longseries
Historical series and downloadable datasets live on Longseries, a sister site. RiverMath is the calculator. Longseries is the archive.
Questions people ask
- Is RiverMath free?
- Yes. The calculators are free to use in the browser. There is no paywall and no account.
- Do you store the numbers I type?
- The arithmetic runs in the browser. A copy of the result goes to the server only if you turn sync on, and only the site owner can read it with a PIN. Other visitors cannot. You can turn sync off. BMI, pregnancy, ovulation and breathalyzer results are never sent. The local list can be cleared separately.
- Are the salary and loan results official?
- No. They are simplified models for planning. Tax offices, banks and employers issue the documents that count.
- Which languages are available?
- English, Polish and German. Each page has its own URL, such as /en/bmi, /pl/bmi and /de/bmi.
- Can I use a result in a medical or tax file?
- No. Treat every result as an educational estimate. Confirm important figures with a clinician, accountant or the relevant office.