How to convert between units

Unit conversion sounds trivial — multiply by a factor — until you trip on Fahrenheit-to-Celsius (which needs an offset, not just a ratio), US vs Imperial gallons (20% different), or the international ambiguity of “ton.” This guide covers the conversion factors that matter, the ones that catch people out, and a mental-math toolkit for the conversions you’ll do most often.

The two types of conversions

Multiplicative: most conversions. Multiply by a constant factor. Meters → feet: × 3.281. Kilograms → pounds: × 2.205. Same formula both ways, with the reciprocal.

Linear (with offset): Fahrenheit ↔ Celsius needs both a ratio and an offset. You can’t “multiply by one number.”

Temperature — the one with the offset

C → F: F = (C × 9/5) + 32. Or approximately: double C, subtract 10%, add 32.

F → C: C = (F − 32) × 5/9. Or approximately: subtract 32, multiply by 0.56.

C → K (Kelvin): K = C + 273.15. No ratio, just an offset.

Anchors worth memorizing:

0°C = 32°F (freezing)

20°C = 68°F (room temperature)

37°C = 98.6°F (body temperature)

100°C = 212°F (boiling)

−40°C = −40°F (the only point where they match)

Length — metric vs imperial

1 inch = 2.54 cm (exact, by definition).

1 foot = 30.48 cm (exact).

1 yard = 0.9144 m (exact).

1 mile = 1.609344 km. Approximation: km ≈ miles × 1.6. Or: miles ≈ km × 0.62.

1 m = 3.281 ft = 39.37 in.

Mental math: for cm → inches, divide by 2.5 (or multiply by 0.4). 10 cm ≈ 4 in. For m → yards, they’re roughly equivalent (1 m ≈ 1.09 yd). Close enough for estimation.

Weight / mass

1 lb = 0.4536 kg. Approximation: lb × 0.45 or kg × 2.2.

1 kg = 2.2046 lb.

1 oz = 28.35 g.

Ton — three different values. US “short ton” = 2,000 lb = 907 kg. UK “long ton” = 2,240 lb = 1,016 kg. Metric ton (tonne) = 1,000 kg = 2,205 lb. Always clarify which.

Mental math: kg → lb, double and add 10%. 50 kg × 2 = 100, + 10 = 110 lb (actual: 110.23).

Volume — where US and UK diverge

1 US gallon = 3.785 L = 4 US quarts = 128 US fl oz.

1 Imperial (UK) gallon = 4.546 L — about 20% larger than US.

1 US fluid ounce = 29.57 mL.

1 Imperial fluid ounce = 28.41 mL. Slightly smaller than US fl oz, despite the gallon being larger.

1 cup (US) = 236.6 mL. Note: cup is US-specific; the UK uses “Imperial cup” (284 mL) rarely. UK recipes usually specify grams or mL.

1 cubic meter = 1,000 L.

Recipe-conversion gotcha: a US cup of butter ≈ 227 g; a UK “cup” of butter could mean 240 mL ≈ 230 g or 284 mL ≈ 270 g. Always weigh if precision matters.

Area

1 sq ft = 0.0929 m².

1 m² = 10.764 sq ft. Approximation: m² × 10.8 for sq ft.

1 acre = 4,047 m² = 0.4047 hectare = 43,560 sq ft.

1 hectare = 10,000 m² = 2.471 acres.

1 sq mile = 640 acres = 2.59 km².

Speed

mph → km/h: × 1.609. A US 65 mph speed limit = ~105 km/h.

km/h → mph: × 0.621. German autobahn at 130 km/h = ~81 mph.

Knots → mph: × 1.151. Knots → km/h: × 1.852.

m/s → km/h: × 3.6. Usain Bolt’s peak ~12 m/s = ~43 km/h.

Energy and power

1 calorie (small) = 4.184 J. But food “calories” are kilocalories (kcal) = 4,184 J. Confusing but entrenched.

1 kWh = 3,600,000 J = 3.6 MJ.

1 BTU = 1,055 J. 1,000 BTU/h = ~293 W.

1 horsepower (mechanical) = 745.7 W. 1 horsepower (metric) = 735.5 W. Yes, two different horsepowers.

Data sizes — base-10 vs base-2

Historically “MB” meant 2^20 = 1,048,576 bytes (binary). Now officially:

MB (megabyte) = 10^6 = 1,000,000 bytes (decimal, used by storage manufacturers).

MiB (mebibyte) = 2^20 = 1,048,576 bytes(binary, used by OS / RAM).

A “1 TB” hard drive shows as ~931 GiB in your OS. Not a scam — just the decimal-vs-binary mismatch. Same at every scale: KB/KiB, GB/GiB, TB/TiB.

Angle, pressure, and other niches

1 radian = 180/π degrees ≈ 57.296°. π radians = 180°.

1 atm = 101,325 Pa = 14.7 psi = 760 mmHg = 1.013 bar. Tire pressure 32 psi = 220 kPa = 2.2 bar.

Fuel economy: mpg (US) × 0.425 = km/L. Or 235.21 / mpg_US = L/100km. 30 mpg ≈ 7.8 L/100km.

UK mpg ≠ US mpg. UK uses Imperial gallons. 30 UK mpg = 25 US mpg. Always clarify.

Common conversion errors

1. Applying ratio where offset is needed.Especially temperatures. 20°C is not “20 × 1.8 = 36°F.” It’s 68°F. You need the +32 offset.

2. Losing track of US vs UK gallons. 20% difference — big in fuel economy and cooking.

3. Confusing “calories” with “kcal.”A 200-“calorie” cookie is 200 kcal = 200,000 small calories.

4. Using the wrong “ton.” US short ton, UK long ton, metric tonne — up to 12% differences.

5. Rounding too early. Chain conversions lose precision fast if each step rounds to 2 significant digits. Keep more digits in intermediate steps; round the final answer.

Run the numbers

Convert any unit with the unit converter. Pair with the temperature converter for the offset-aware temperature conversions, and the currency converter when you need cross-currency values alongside unit math.