A calculator is only useful if you can trust the number it returns. Because our tools use standard, deterministic formulas (see Calculator Methodology), they can be checked in several concrete ways.
Against known formulas
Compare to the closed-form answer.
Most of what our tools compute has a known, published formula — an amortization payment, a compound-growth balance, a present value. We check a calculator's output against that formula worked independently. If a $20,000 loan at 6% over 60 months should have a monthly payment near $386.66, the tool has to return that.
Against worked examples
Match independent and published examples.
Where practical, we compare results to worked examples computed separately — sometimes with an independent script written specifically to reproduce a calculator's results, and sometimes against examples published in reputable references. When a page relies on a data pipeline, that pipeline is checked with its own automated tests so the numbers feeding the page are correct before the page ever displays them.
At the edges
Boundary conditions, zeros, and large values.
Bugs like to hide at the extremes, so we test them on purpose: zero values (a 0% rate, a $0 balance, a zero-month term), very large values, and the boundaries where a formula can misbehave — such as the special case of a zero-interest loan, where the usual amortization formula would divide by zero. The goal is a tool that stays sensible across the whole range of inputs, not just the typical ones.
Honesty
Bugs can still happen.
No amount of testing catches everything. Browsers differ, inputs combine in ways we did not anticipate, and reference figures change. We improve the calculators continuously, and we would rather hear about a problem than have you rely on a wrong number. If a result looks off, please tell us on our Contact page — the inputs you used and what you expected help us reproduce and fix it quickly.
Related reading: Calculator Methodology · Data Sources · Editorial Policy.