Why exact factors matter
Most unit conversions on the web use rounded factors that are good enough for casual use and disastrous for engineering. 1 inch is exactly 2.54 cm — not 2.54000508 (the US Survey Foot, deprecated 2022) and not 2.5 (the rounded value many older sources still use). 1 lb is exactly 0.45359237 kg, not 0.4536. The Mars Climate Orbiter failed in 1999 because Lockheed Martin used pound-seconds while NASA expected newton-seconds — a unit-conversion error that destroyed a $327 million spacecraft. We use exact post-1959 factors throughout.
Volume and cooking measurements are the second-biggest trap. A US cup is 240 ml, a metric cup is 250 ml, a UK cup is 284 ml, and a US legal cup (used on nutrition labels by FDA rule) is also 240 ml — but a US customary cup outside of nutrition labelling is technically 236.588 ml. Recipes that don't state which cup they use are quietly wrong by 4–18%, which is why baking instructions move toward grams as soon as precision matters.
All eight conversion tools
Unit Converter
Multi-purpose converter with length, weight, volume, and temperature in one place. SI-based conversion factors.
Fahrenheit ↔ Celsius
F = C×9/5 + 32. Both scales meet at -40°. Why the US still uses Fahrenheit.
Inches ↔ Centimeters
1 inch = 2.54 cm exactly (1959 international yard agreement). The US Survey Foot was deprecated in 2022.
Pounds ↔ Kilograms
1 lb = 0.45359237 kg exactly. Avoirdupois vs troy pounds, and the 2019 kilogram redefinition (Planck constant).
MPG Calculator & Converter
Miles per gallon, US vs UK gallon trap, MPG ↔ L/100km conversion (235.214 ÷ MPG), MPGe for EVs.
Cooking Converter
US cup vs metric cup vs UK cup. Volume vs weight in baking, and why a cup of flour ≠ a cup of sugar.
Shoe Size Converter
US, UK, EU (Paris point), and Japanese (cm) sizing systems. Brannock device standard, kids vs adult offsets.
Time Between Dates
Calendar arithmetic between two dates, with leap-year and ISO 8601 handling. Calendar vs business vs trading days.
Recommended reading
Editorial standards for this category
Conversion math is deceptively simple — the formula fits on one line, but the constants behind it have to be exact. We verify every factor in this category against three sources: NIST SP 811 (Guide for the Use of the International System of Units), the BIPM SI Brochure (current 9th edition), and the relevant ISO standard (ISO 80000 for general units, ISO 8601 for date and time, ISO 9407 for shoe sizing). Where US customary units have multiple definitions (the survey foot vs the international foot, the US cup vs the legal cup), we state which one we use and why.
CalcNow Tools Team
A small team of contributors who research, build, and review the unit conversion, percentage, date, and everyday utility calculators on CalcNow.
Coverage: Unit conversion (length, weight, temperature, volume), percentage math, date arithmetic, password entropy, GPA scales, age calculations
Editorial standard: Every conversion factor is verified against NIST SP 811, the BIPM SI Brochure, and the relevant ISO standard (ISO 80000, ISO 8601). Calendar arithmetic follows the Gregorian rules used by the US Naval Observatory.