How we verify
How we check the calculators on this site:
- Formulas. Percentages, percent change, discounts, tax, tips, margins and temperature use standard arithmetic definitions, written out in the “How it is worked out” section of each page.
- Tests. Every calculator is tested against worked examples before it goes live, including edge cases such as a zero denominator or a temperature below absolute zero.
- Unit factors. Unit converters use the standard exact definitions: the international inch (25.4 mm), foot (0.3048 m), yard (0.9144 m), mile (1,609.344 m) and nautical mile (1,852 m); the pound (0.45359237 kg) and ounce (one sixteenth of a pound); the kilowatt-hour (3.6 million joules); the thermochemical calorie (4.184 joules); mechanical horsepower (550 foot-pounds force per second, 745.6998715822702 watts) and metric horsepower (735.49875 watts); the International Table BTU (1,055.05585262 joules). The size of each unit is shown on its page.
- Rates you enter. Sales tax rates, tip customs and grading scales differ by place and school, so the calculators use the number you type and do not look anything up for you.
- Two ways to calculate. The answer shown before the page runs and the answer the calculator gives while you type are produced by separate code (PHP on the server, JavaScript in your browser) and are compared in our tests.
If you find a wrong answer, tell us on the contact page.