How we check our calculators
Calculator Collection has 1,800+ calculators. A calculator is only useful if its answer is right, so every one goes through the same checks before and after it is published, and the most-used ones are reviewed again on a schedule. This page explains those checks and keeps a dated record of what our reviews found and fixed.
1. Built on a published method
Each calculator uses an established formula or reference: a standard financial formula, a published clinical equation, an engineering handbook relation or an official unit definition. The formula is shown on the page, and the sources it comes from are listed under “Sources”, so you can check the method yourself. Where sources disagree, the page says which convention it follows and why.
2. Recalculated independently
A reviewer recalculates the result from the method itself, not by copying the page’s formula, for the default inputs and for at least one other realistic set of inputs, and compares it with the live page, including units and rounding. The explanation, worked examples, FAQ answers and result bands are then read against that result and against the cited sources, because a correct number next to a wrong sentence still misleads.
3. Automated tests on every change
Almost 4,000 automated tests run the calculators exactly as the site does and compare them with independently worked reference values. A change to any calculator is only published when every test passes, and each correction found in a review gets its own test so the same mistake cannot return unnoticed.
4. Extra care for health and money
Medical, medication and financial calculators get additional review of units, input limits and defaults. Dosing calculators start with the weight field empty instead of a sample value, and health thresholds are checked against current guidance from bodies such as the AAP, ADA, CDC and NICE. These tools are for education and are not a substitute for professional advice.
5. Checked on real screens
Pages are loaded in a browser on a phone-sized and a desktop screen to confirm the result appears, charts agree with the headline number and nothing is cut off.
Review log
Calculators ranked 301–600 and every guide
The next 300 most-used calculators were recalculated independently (defaults and at least one other realistic input set) and checked on phone and desktop screens, and all 79 guides were checked for correct worked examples, working links and agreement with the calculators they promote.
Calculators: 234 needed no change, 57 had text or band corrections and 9 had a calculation or input bug; guides: 64 needed no change and 15 were corrected. All fixes are live.
- Fuel cost: the distance field is now one-way, so a round trip is no longer counted twice.
- Exoplanet habitability: the atmosphere factors now give Earth about 288 K, Venus about 738 K and Mars about 211 K.
- Fabric yardage: base yardages now follow a standard sewing yardage chart; they were 25–50% too low.
- Customs duty: the end of the EU €150 duty exemption on 1 July 2026 is reflected.
Hidden calculators brought back
The 246 hidden calculators with the clearest search demand were reviewed before being published again: results recomputed against references, defaults made realistic, worked examples recalculated, unsupported claims removed and sources replaced with specific references.
182 calculators are back (some renamed); 64 stay hidden because a visible page already answers the same question or the method could not be made credible, and they now redirect to that page.
- Restored: gross profit margin, markup vs margin, IRR, mortgage amortization schedule, capital gains tax, Social Security break-even, odds converter, golf handicap, Reynolds number.
- Kept hidden: a bike tyre pressure formula that gave 52 psi for mountain-bike tyres (real pressures are about 20–30 psi).
The 300 most-used calculators
The 300 calculators with the most visits and AI-assistant citations. Each was recalculated independently from the underlying formula or reference, with the default inputs and at least one other realistic set of inputs, and its explanation, worked examples, FAQ and result bands were checked against the result and against published sources. Every page was also loaded on a phone-sized and a desktop screen.
255 calculators needed no change. 41 had text that contradicted the result or a published figure, 3 had a calculation or rounding bug and 1 gave a result far from the applicable standard; all were corrected.
- Pool filtration: spa and public-pool pump flow now follows the CDC Model Aquatic Health Code turnover times (30 minutes for public spas, 6 hours for public pools); spas were previously undersized.
- Dovetail angle: softwood now returns the standard 1:6 slope (9.5°) and hardwood 1:8 (7.1°).
- Pediatric pain scale: the infant fever threshold in the FAQ is now 38.0 °C (100.4 °F), per AAP guidance.
- Orifice flow: small flow rates now keep three significant figures instead of rounding 0.00967 m³/s to 0.01.
- Driving breaks: the "stop every 2 hours" rule no longer counts one stop too many.
New and revised calculators
69 calculators added or rebuilt in October 2026 (time and date, medical units and dosing, history, waves, digital units, HVAC, sizing and game maths) were recalculated against references and checked live after publication.
55 needed no change; 1 bug and 13 text or display issues were corrected.
- Days-until countdown: the page now counts from the visitor’s date instead of the date the page was built.
- Cat age: the senior stage now starts at 10 years, matching the AAHA/AAFP feline life-stage guidelines.
- LDL cholesterol: the worked example in mmol/L now matches the calculator’s conversion.
Found a mistake?
Every calculator has a “Report a problem” link, or you can use our contact page. We check each report against the sources, correct confirmed errors and add them to the next review. See also our editorial standards.