Common Unit Conversions Cheat Sheet
A surprisingly small set of conversion factors covers almost every day-to-day need — cooking, travel, DIY, fitness, shipping. Here they are, with both the exact value and a “good enough for your head” approximation, plus the places where a sloppy conversion actually causes problems.
Length
| From | To | Exact factor | Quick mental version |
|---|---|---|---|
| 1 inch | cm | 2.54 | “two and a half” |
| 1 foot | m | 0.3048 | × 0.3 |
| 1 yard | m | 0.9144 | ≈ 0.9, or “a bit less than a metre” |
| 1 mile | km | 1.609344 | × 1.6, or add 60% |
| 1 metre | ft | 3.28084 | × 3.3 |
| 1 km | mile | 0.621371 | × 0.6, or “× 6 and drop a digit” |
Fast km → miles: multiply by 6 and drop the last digit. 100 km → 600 → 60 (true 62.1). 50 km → 300 → 30 (true 31.1). Close enough for a road sign.
Fast miles → km: halve it and add that back plus a bit. 60 mi → 30 → 90 → ~96 (true 96.6).
Weight and mass
| From | To | Exact | Quick |
|---|---|---|---|
| 1 kg | lb | 2.20462 | × 2.2 |
| 1 lb | kg | 0.453592 | ÷ 2.2, or × 0.45 |
| 1 oz | g | 28.3495 | “about 28”, or 30 for a rough guess |
| 1 stone | lb | 14 | exact (UK body-weight unit) |
| 1 US ton (short) | kg | 907.18 | ≈ 900 |
| 1 tonne (metric) | lb | 2204.62 | ≈ 2,200 |
Temperature
Temperature is not a simple multiplication — the scales have different zero points.
- °C → °F: × 9/5, then + 32. Quick: double it and add 30. (20 °C → 70; true 68. 30 °C → 90; true 86 — the quick version drifts high as you go up.)
- °F → °C: − 32, then × 5/9. Quick: subtract 30 and halve. (70 °F → 20; true 21.)
- Anchors worth memorising: −40° is identical on both scales. 0 °C = 32 °F (freezing). 21 °C ≈ 70 °F (room temperature). 37 °C = 98.6 °F (body). 100 °C = 212 °F (water boils at sea level).
Volume
| From | To | Exact |
|---|---|---|
| 1 US gallon | litres | 3.78541 |
| 1 litre | US fl oz | 33.814 |
| 1 US cup | ml | 236.6 (240 is fine for recipes) |
| 1 US tablespoon | ml | 14.79 (≈ 15) |
| 1 US teaspoon | ml | 4.93 (≈ 5) |
The imperial trap: UK (“imperial”) pints, quarts, and gallons are about 20% larger than the US versions (1 imperial gallon ≈ 4.546 L). A UK pint is 568 ml; a US pint is 473 ml. Recipes, fuel economy figures, and beer sizes all suffer from this — always check which system a source is using.
Cooking-specific notes
- A “stick” of US butter is 1/2 cup = 8 tbsp = 113 g.
- Flour and other dry goods do not convert cleanly between cups (volume) and grams (mass) — density varies. For baking, a kitchen scale beats any conversion.
- Oven temperatures: 350 °F ≈ 175–180 °C; 400 °F ≈ 200 °C; 425 °F ≈ 220 °C.
Digital storage: decimal vs binary
The confusion here is not the unit but the base:
- Drive and network makers use decimal: 1 GB = 1,000 MB = 1,000,000,000 bytes.
- Operating systems often display binary: 1 GiB = 1,024 MiB = 1,073,741,824 bytes.
That ~7.4% gap compounds: a “1 TB” drive shows as about 931 GiB in your file manager. Nothing is missing — the two are just counting differently. Network speeds add another trap: ISPs quote megabits (Mb/s), files download in megabytes (MB/s), and 8 bits = 1 byte, so a “100 Mb/s” line tops out around 12.5 MB/s.
Area, speed, pressure, and energy
Beyond the everyday four, a few more that come up:
Area
| From | To | Factor |
|---|---|---|
| 1 sq ft | sq m | 0.0929 |
| 1 sq m | sq ft | 10.764 |
| 1 acre | sq m | 4,046.86 |
| 1 acre | hectares | 0.4047 |
| 1 sq mile | sq km | 2.59 |
Speed
| From | To | Factor / trick |
|---|---|---|
| 1 mph | km/h | 1.60934 |
| 1 km/h | mph | 0.62137 |
| 1 knot | mph | 1.15078 |
| 1 knot | km/h | 1.852 (exact) |
| 1 m/s | km/h | × 3.6 (exact) |
| 1 m/s | mph | × 2.237 |
Pressure (tyres, weather)
| From | To | Factor |
|---|---|---|
| 1 psi | kPa | 6.895 |
| 1 psi | bar | 0.0689 |
| 1 bar | psi | 14.504 |
| 1 atm | psi | 14.696 |
| 1 inHg | mb / hPa | 33.864 |
Energy and power
| From | To | Factor |
|---|---|---|
| 1 kWh | MJ | 3.6 (exact) |
| 1 Calorie (food, kcal) | kJ | 4.184 |
| 1 BTU | kJ | 1.055 |
| 1 hp (mechanical) | watts | 745.7 |
| 1 kW | hp | 1.341 |
Fuel economy: the reciprocal trap
Miles per gallon and litres per 100 km are inverses, so you cannot convert by a simple multiplier — you divide:
L/100km = 235.215 ÷ mpg (US) and mpg (US) = 235.215 ÷ (L/100km)
30 mpg ≈ 7.84 L/100km. 40 mpg ≈ 5.88 L/100km. Also note US vs imperial gallons again: 30 US mpg ≈ 36 imperial mpg, because the imperial gallon is larger. UK car ads quote imperial mpg, which is why a British car “does more miles per gallon” than the same car sold in the US.
When you need it exact
Mental estimates are fine for travel, cooking, and casual conversation. For anything that matters — medication dosages, engineering tolerances, invoicing, lab work — use the unit converters, which apply the internationally defined factors at full precision, or a kitchen/bench scale for mass.
The bottom line
Memorise a handful: inch = 2.54 cm, mile ≈ 1.6 km, kg ≈ 2.2 lb, °C → °F is “double and add 30” (roughly), and 1 US gallon ≈ 3.79 L. Watch two traps — UK units are ~20% bigger than US units, and storage is quoted in decimal but shown in binary. For precision, use the converters rather than the mental shortcuts.