Anyone can put a form on a web page and call it a calculator. This page explains exactly how ours are built, where the formulas come from, and how we know the answers are right — so you can judge the tool rather than trust it blindly.
1. Formulas come from published sources
Every calculator starts from a formula documented in standard references — finance and statistics textbooks, official standards bodies for units of measurement, and the defining institutions for a given quantity. Where a page relies on a specific external definition, that source is listed in a “Sources & Further Reading” section at the bottom of the page. Where more than one convention exists (quartile methods, rounding of exact halves, US vs. imperial gallons), we state which convention the calculator uses rather than silently picking one.
2. Every calculator is covered by automated tests
This is the part most calculator sites don't do. Each tool has test cases with independently known answers — worked examples from textbooks, exact unit definitions, and hand-checked edge cases. Those tests drive a real browser, type into the actual calculator, and compare the on-screen result against the expected value.
The suite runs before every release of the site. If a change breaks any calculator's arithmetic, the build fails and the change does not ship. All 33 calculators currently pass.
1/2 + 1/3 must display as 5/6 · √48 must simplify to 4√3 · a 30% discount followed by an extra 20% must produce a 44% effective discount · 5 miles must equal 8.04672 km exactly · a one-sample t-test on 5, 6, 7, 8, 9 against μ₀ = 5 must return t = 2.8284 and p ≈ 0.047.
3. Edge cases are handled explicitly, not ignored
Division by zero, negative inputs where they are meaningless, temperatures below absolute zero, requesting more unique random numbers than a range contains, matrix dimensions that make an operation undefined — each of these produces a clear explanation of the rule being broken, not a blank screen or a nonsense number. These behaviours are tested too.
4. Results show their working
Wherever it is useful, the calculator prints the intermediate steps that produced the answer. This lets you verify the result yourself, and it is the difference between a tool that gives you a number and one that teaches you the calculation.
5. Corrections
If you believe a result is wrong, tell us and we will investigate: email contact@calculatorsguide.com with the calculator name, the values you entered, and the answer you expected. Confirmed errors are fixed, a regression test is added so the same mistake cannot return, and the page's “last updated” date changes.
Authorship
Calculators and their explanations are written and reviewed by the CalculatorsGuide editorial team. Pages carry a “last updated” date so you can see how current the material is. Content was last reviewed on .
What we don't do
- We don't require sign-ups, and calculations never leave your browser.
- We don't present results as professional financial, legal or medical advice — see our terms.
- We don't copy explanations from other calculator sites; every page is written here.