How we test our calculators

Last updated 29 October 2026

A calculator is only useful if you can trust the answer. Every Calcoway calculator goes through the same checks before it goes live, and again whenever we change it.

1. Worked examples

Each formula is checked against worked examples with known answers, such as textbook loan payments, published compound interest examples and simple cases you can do by hand (for example, 20% VAT on 100 is 20). These tests run every time the site is rebuilt. If one fails, the update doesn't ship.

2. Randomised cross-checks

Worked examples only cover the cases someone thought of. So we also generate thousands of random inputs for each calculator and check the results in a different way, for example:

• A loan payment is fed back into a month-by-month schedule to confirm the balance reaches exactly zero.
• Adding tax and then removing it returns the original price.
• Margin and markup, and hourly pay and salary, convert back and forth to the same numbers.
• Compound interest from the formula matches a period-by-period simulation.

3. Edge cases and bad input

We test zero, negative, very large and missing values, zero interest rates, partial years and prices below cost. Where a question has no sensible answer (such as a margin on a zero selling price), the calculator says so in plain words instead of showing a wrong number.

4. Real browsers and phones

Every calculator is tested in a real browser at desktop and small phone sizes, including typing, switching modes, resetting, the example button, share links, CSV downloads and different currencies and number styles (such as 1,250.50, 1.250,50 and 12,50,000).

5. Rates that change

Some inputs, like VAT and GST rates or marketplace fees, are set by governments and companies and change over time. We never hide these inside the formula. They appear as boxes you can edit, with the date we last checked them and where they come from. We review tax rates every quarter.

6. Showing the working

Every result shows the steps used to get it, so you can check the logic yourself or with your accountant. Estimates, such as future investment growth, are labelled as estimates.

What we can't promise

Our calculators do the maths you ask for, with the numbers you enter. They don't know your full situation and they aren't financial, tax or legal advice. For important decisions, check with a qualified professional.

Found a mistake?

Email [email protected] with the page, the numbers you entered and the answer you expected. We check every report, and if we find an error we fix it and update the date on the page.