Unit Converter

Unit conversion with trustworthy constants

This tool focuses on SI-derived and common US customary factors used in engineering and daily life. Temperature uses defined offsets (°F vs °C) rather than linear ratios, so the UI treats it specially.

When precision matters: aviation fuel pounds-to-gallons vs lab chemistry μL conversions may need instrument-specific densities—use this for everyday accuracy, not certification paperwork.

Pairs with BMI / TDEE calculators when you flip between metric and imperial health inputs.

What you can convert

• Length: metres, kilometres, centimetres, millimetres, miles, feet, inches and yards.
• Weight (mass): kilograms, grams, milligrams, metric tonnes, pounds and ounces.
• Temperature: Celsius, Fahrenheit and Kelvin.
• Area: square metres, square kilometres, square centimetres, square feet, acres and hectares.
• Speed: metres per second, kilometres per hour, miles per hour, knots and feet per second.

Each category converts through a base unit (metre, kilogram, square metre, metre per second), so any unit converts to any other in the same category in a single step.

Exact definitions, not rounded factors

Every factor is the exact legal definition: 1 inch = 2.54 cm, 1 foot = 0.3048 m, 1 mile = 1,609.344 m, 1 pound = 0.45359237 kg, 1 ounce = 28.349523125 g, 1 acre = 4,046.8564224 m², 1 km/h = 1/3.6 m/s and 1 knot = 1,852 m per hour. Rounded factors drift: using 0.27778 for km/h turns 100 km/h into 62.1376 mph instead of the correct 62.1371. Results are rounded only when displayed.

Worked examples

• Height: 5 ft 10 in is 70 inches × 2.54 = 177.8 cm.
• Running: a marathon (42.195 km) is 26.2188 miles.
• Driving: 100 km/h is 62.14 mph; 70 mph is 112.65 km/h.
• Temperature: 30 °C is 86 °F; body temperature of 98.6 °F is exactly 37 °C; −40 is the same in both scales.
• Land: 1 acre is 0.4047 hectares, and 1 hectare is 2.471 acres.
• Sailing and aviation: 1 knot is exactly 1.852 km/h, because a nautical mile is defined as 1,852 m.

Temperature is different

Length or weight conversions just multiply by a factor, because zero means the same thing in every unit. Temperature scales have different zero points, so they need an offset as well: °F = °C × 9⁄5 + 32 and K = °C + 273.15. That has a practical consequence for temperature differences: a rise of 10 °C is a rise of 18 °F, not 50 °F. Convert the difference with the factor alone (× 9⁄5), not with the full formula.

Common pitfalls

• US vs imperial: a US fluid ounce, pint and gallon are smaller than their British imperial namesakes (a US gallon is 3.785 L, an imperial gallon 4.546 L). This tool’s ounce and pound are the avoirdupois units used for body weight and groceries.
• Troy ounces: precious metals are priced in troy ounces of 31.1035 g, about 10% heavier than the ounce here.
• Stones: UK body weights are often given in stones and pounds. 1 stone = 14 lb = 6.35 kg, so 11 st 4 lb is 158 lb, or 71.7 kg — convert the total in pounds here.
• Weight vs volume: a cup of flour and a cup of water weigh different amounts, so grams cannot be converted to cups without knowing the ingredient’s density.
• Too many decimals: converting 6 ft to 1.8288 m implies a precision the original measurement never had. Round the result to match how precisely you measured.

Quick estimates in your head

• Inches to centimetres: multiply by 2.5 (exact: 2.54). A 10-inch tablet is about 25 cm.
• Kilograms to pounds: double and add 10% (exact: 2.2046). 70 kg → 140 + 14 = 154 lb.
• Miles to kilometres: multiply by 1.6, or use Fibonacci pairs — 5 miles ≈ 8 km, 8 miles ≈ 13 km.
• Celsius to Fahrenheit: double and add 30. Good to within about 2 °F between 0 °C and 20 °C, drifting further outside that range — 20 °C → 70 °F (exact 68 °F).
• Metric prefixes: kilo = 1,000, centi = 1/100, milli = 1/1,000, micro = 1/1,000,000 — so 1 kg = 1,000,000 mg.