Metric ↔ Imperial
Key anchor equivalences to keep in your head, with handy mnemonics and tips.
Length & Distance
| Measure | Equals | Precision | Tip |
|---|---|---|---|
| 1 inch | 2.54 cm | Exact | An inch is about the width of your thumb. |
| 1 foot | 30.48 cm | Exact | A foot is almost exactly 30 cm. |
| 1 yard | 0.9144 m | Exact | A metre is a yard and a bit (about 10% more). |
| 1 mile | 1.609 km | Exact | Multiply miles by 1.6 for a quick km estimate. |
| 1 cm | 0.3937 in | Exact | Roughly 2.5 cm per inch. |
| 1 km | 0.6214 mi | Approx | 5 miles ≈ 8 km (the Fibonacci trick). |
Weight & Mass
| Measure | Equals | Precision | Tip |
|---|---|---|---|
| 1 oz (avdp) | 28.35 g | Exact | About the weight of a slice of bread. |
| 1 pound | 453.6 g | Exact | A kilogram is about 2.2 pounds. |
| 1 stone | 6.35 kg | Exact | Used in the UK for body weight: 14 lb = 1 stone. |
| 1 kg | 2.205 lb | Approx | A bag of sugar is typically 1 kg (2.2 lb). |
| 1 troy oz | 31.10 g | Exact | For gold/silver — heavier than a regular oz. |
Volume (Liquid)
| Measure | Equals | Precision | Tip |
|---|---|---|---|
| 1 US fl oz | 29.57 mL | Exact | About 2 tablespoons. |
| 1 US cup | 236.6 mL | Exact | Just under a quarter litre. |
| 1 US pint | 473.2 mL | Exact | A US pint is smaller than an Imperial pint. |
| 1 US gallon | 3.785 L | Exact | About 4 litres (Imperial gallon is 4.55 L). |
| 1 litre | 33.81 US fl oz | Approx | A litre is close to a quart (just 5% more). |
| 1 Imp pint | 568.3 mL | Exact | A proper British pint — 20% bigger than US. |
Area
| Measure | Equals | Precision | Tip |
|---|---|---|---|
| 1 sq ft | 929 cm² | Approx | About the area of a large floor tile. |
| 1 acre | 4,047 m² | Exact | About 75% of a football field. |
| 1 sq mi | 2.59 km² | Approx | 1 sq mi = 640 acres. |
| 1 hectare | 2.471 acres | Approx | About 2.5 acres, or 100m × 100m. |
Temperature Landmarks
| Fahrenheit | Celsius | Significance | |
|---|---|---|---|
| 32°F | = | 0°C | Freezing point of water |
| 72°F | = | 22.2°C | Comfortable room temperature |
| 98.6°F | = | 37°C | Normal body temperature |
| 212°F | = | 100°C | Boiling point of water |
| 350°F | = | 176.7°C | Common baking temperature |
| −40°F | = | −40°C | Where both scales meet! |
Quick Formulas
Converting in your head
Exact factors are best left to the converter, but a few mental shortcuts get you within a few percent — good enough to read a road sign, judge a recipe, or sanity-check a number before you trust it.
Temperature: double and add 30
To turn Celsius into Fahrenheit, double it and add 30. 20°C becomes 70°F (exact: 68°F). Going the other way, subtract 30 and halve: 80°F becomes 25°C (exact: 26.7°C). The shortcut is very close between about 0°C and 25°C, the range most weather falls in, and drifts further off at oven temperatures — use the formula above for cooking.
Distance: the Fibonacci trick
The ratio between consecutive Fibonacci numbers (3, 5, 8, 13, 21, 34…) is about 1.618, almost the same as the 1.609 km in a mile. So 5 miles ≈ 8 km, 8 miles ≈ 13 km and 21 miles ≈ 34 km. For other numbers, add 60%: 50 mph ≈ 80 km/h. To go from km to miles, multiply by 6 and drop the last digit: 100 km ≈ 60 miles.
Weight: halve and take off 10%
To turn pounds into kilograms, halve the number then subtract a tenth of the result. 180 lb → 90 → 81 kg (exact: 81.6 kg). For kilograms to pounds, double and add 10%: 70 kg → 140 → 154 lb (exact: 154.3 lb).
Volume: a litre is a quart, plus a mouthful
A litre is about 5.7% larger than a US quart, so a 2-litre bottle holds a little more than half a US gallon. A US gallon is just under 4 litres; a British imperial gallon is about 4½ litres. When a UK fuel economy figure in miles per gallon looks surprisingly good, that larger imperial gallon is usually the reason.
Area: a hectare is two and a half acres
A hectare is a square 100 m on each side; an acre is roughly 64 m square. One square metre is close to 10.8 square feet, so a 20 m² room is about 215 sq ft — multiply by 10 and add 8% for a quick estimate.
Why some values are exact
The tables above mark each value as Exact or Approx. “Exact” does not just mean very precise: since the International Yard and Pound Agreement of 1959, the yard has been defined as exactly 0.9144 metres and the pound as exactly 0.45359237 kilograms. Every customary unit built from those (the inch, foot, mile, ounce and stone) therefore has an exact metric equivalent by definition. The US gallon is likewise defined as exactly 231 cubic inches, which fixes the fluid ounce and cup exactly too.
“Approx” values appear when you go the other direction. Because 1 inch is exactly 2.54 cm, 1 cm is 0.393700787… inches — a never-ending decimal that has to be rounded. Our converters carry these factors to full precision and only round the displayed answer, so chained conversions don’t accumulate error. For the story of how these definitions came about, see our history of measurement.