How We Check Our Calculators
Page updated November 4, 2026.
A calculator is only useful if you can trust the answer. This page explains how we test every calculator on this site before it goes live, and how you can check any result yourself.
Every calculator has its own tests
Each calculator comes with a file of test cases, between 10 and 24 per calculator. A test types numbers into the real page, presses Calculate, and reads the answer exactly as you'd see it. Every calculator is tested in six ways:
| Kind of test | What it checks | Example |
|---|---|---|
| Normal | Everyday inputs give the right answer | A 10 × 12 ft room with 12 inch tiles needs 130 tiles with 10% extra |
| Reference | Known, published values come out exactly | $10,000 at 5% compounded monthly for 10 years is $16,470.09 |
| Invalid | Bad input gets a clear message, never a made-up answer | A negative length or a zero denominator is refused |
| Boundary | Edge cases behave sensibly | Leap days, a 0% interest rate, the exact top of an income range |
| Rounding | Rounding happens once, at the end | A box count rounds up; a dollar amount rounds to the cent |
| Clear | The Clear button really empties the form | No old answer is left on screen |
Every test also checks that the working shown under the answer ends on the same number as the answer, and that nothing like "NaN", "Infinity" or "undefined" ever appears on the page.
Where the expected answers come from
A test is only as good as the answer it expects. We never copy the expected answer from the calculator itself. Each one is worked out separately: by hand, with a separate piece of code, or from a published reference value. When a calculator has a worked example on its page, the example uses the calculator's starting values, so you can check the two against each other.
Nothing is published until every test passes
The tests run in a real web browser every time anything on the site changes, and again on the morning a new calculator is released. If a single test fails, the release stops automatically and the site stays as it was. A calculator also isn't listed in the sitemap or on its category page until its release date.
An independent review
On November 4, 2026, a separate review rechecked the first 30 calculators against high-precision reference calculations (60-digit decimal arithmetic, exact fractions and calendar date maths), using inputs our own tests didn't cover. It ran 223 extra checks plus 2,000 random factoring checks. It found problems in 19 calculators, for example:
- Some spans that end just after a month end showed negative days (Jan 31 to Mar 1 came out as "1 month, -2 days" instead of 29 days).
- Some construction calculators accepted negative lengths and still gave an answer.
- The gravel calculator rounded the tonnage before working out the weight and price, so both were slightly off.
All of them were fixed the same day, and each problem became a permanent test, so it can't come back unnoticed.
The rules our maths follows
- Round once, at the end. Intermediate values keep full precision; only what's shown on screen is rounded.
- Money is shown to the cent. Counts you have to buy whole, such as bags, boxes and posts, round up.
- Dates are calendar dates. Day counts don't shift with time zones or daylight saving time. If you were born on February 29, your birthday in other years is counted as March 1.
- No guessing. If an input is missing or doesn't make sense, the calculator asks for it instead of filling in a value.
- Assumptions are written down. Each calculator says what it assumes, for example that loan payments are fixed and monthly, or that contributions go in at the start of each year.
Rates and rules that change
Some calculators depend on figures set by governments or published by researchers, such as yearly contribution limits or sales tax rates. Those figures come from the official or original source, show the date they apply to, and link to the source on the calculator's page. Sources we use include:
- IRS: retirement account limits and tax on interest (Publication 550)
- FDIC: deposit insurance
- U.S. Department of Labor and the Code of Federal Regulations: overtime and time-clock rounding
- Tax Foundation: state and local sales tax rates
When a figure changes, we update the calculator and its tests together, and change the date on the page.
What testing can't tell you
Tests prove that a calculator does what it says. They can't know your situation. A loan estimate doesn't include a lender's own fees, and a concrete estimate can't see your site. Read the assumptions under each calculator, and for decisions that matter, confirm the numbers with a qualified professional. More about the site is on the About page.