NumberRule is a set of free calculators for the arithmetic that comes up in ordinary life — how much concrete a slab takes, what a mortgage payment is made of, what a mile in your car actually costs.
Why the formula is on the page
Most calculator sites give you a number and no way to check it. That is fine until the number matters, and then you are trusting a stranger's arithmetic with no way to see where it came from.
So every page here publishes the formula it used, names each variable, and works through one realistic example. If you disagree with the answer you can see exactly which step you disagree with. If you would rather do it on paper, everything you need is on the page.
The worked example on each page is produced by the same code that runs the calculator, so the two cannot drift apart. When a formula is corrected, the example corrects itself.
What it will not do
There are no calculators here for anything whose answer changes without us touching the code — tax brackets, currency conversion, inflation, benefit estimates. A tool like that is wrong the moment a rate moves, and quietly wrong is worse than absent.
For the same reason, rates and prices are always fields you fill in rather than figures we publish. Your lender's rate and your merchant's price are the only ones worth calculating with.
We also leave alone the questions where being wrong is expensive: child support, medication dosage, injury settlements. Those need a person, not a form.
How it is built
About fifteen calculation engines sit behind the whole site — amortisation, compound growth, area, volume and density, unit conversion, date arithmetic and a few more. Each calculator is a thin configuration over one of them, which is why a correction in one place fixes every page that depends on it.
The arithmetic runs on the server rather than in your browser. Amounts you enter are used to work out the answer and are not stored, logged, or shared — see the privacy page for what that means in detail.
Corrections
If a figure here is wrong, we would rather know. Every number in the prose is checked against the engines before it ships, but that is a process run by people and people miss things. Tell us and it gets fixed.
Nothing on this site is professional advice. These are estimating aids and references. Anything that carries real money or real risk should be checked against whoever actually governs it — your supplier, your lender, your syllabus or your doctor.