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Metric vs Imperial Measurements
Convert the quantity you actually have, state the unit standard and distinguish Imperial units from US customary measures.
Updated 16 October 2026 • Tools by Layna Editorial Team • General information
Metric, Imperial and US customary are not interchangeable labels
Metric measures commonly use metres, litres and grams with decimal prefixes. Imperial and US customary systems share some familiar unit names, but not every unit has the same size. In particular, an Imperial gallon and a US liquid gallon differ.
For everyday conversions, first identify the quantity: length, mass, area, volume, speed or temperature. A unit name without its context can be ambiguous: a fluid ounce is volume, while an avoirdupois ounce is mass.
Reference conversions and their bases
| Quantity | Relationship | What to label |
|---|---|---|
| Length | 1 international inch = 2.54 cm exactly | Inches, rather than a guessed ruler interval |
| Length | 1 international mile = 1.609344 km exactly | Miles as distance, or miles/hour for speed |
| Mass | 1 avoirdupois pound = 0.45359237 kg exactly | Mass, not fluid volume |
| Volume | 1 Imperial gallon = 4.54609 L exactly | Imperial gallon |
| Volume | 1 US liquid gallon = 3.785411784 L exactly | US liquid gallon |
| Temperature | °F = °C × 9/5 + 32 | A temperature value, not a temperature difference |
Worked examples: size, area and temperature
A 12-inch length is 12 × 2.54 = 30.48 cm. A one-foot square is 30.48 cm × 30.48 cm = 929.0304 cm²; an area conversion uses the square of the length factor.
For temperature, 20°C × 9/5 + 32 = 68°F. But a change of 20°C corresponds to a change of 36°F, without adding 32. The offset applies to a temperature reading, not to a difference.
A 5-US-gallon container holds 5 × 3.785411784 = 18.927 L when rounded to three decimals. Five Imperial gallons holds 22.730 L. Choosing the wrong gallon changes the answer by almost 3.8 L.
Keep precision appropriate to the measurement
Exact conversion factors do not make an approximate measurement exact. Preserve enough digits while calculating, then round the final result to suit the original measurement and intended use.
For shared plans and records, state the governing system and label every value. Changing 10 to 25.4 without changing “in” to “cm” leaves an incorrect record even when the arithmetic was right.
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Frequently asked questions
Is a US gallon the same as an Imperial gallon?
No. A US liquid gallon is 3.785411784 L; an Imperial gallon is 4.54609 L.
Can I convert grams to millilitres with a fixed factor?
Only if the substance’s density is known or an appropriate ingredient-specific estimate is supplied. Grams measure mass; millilitres measure volume.
Why is the square-foot conversion not the foot conversion?
Area has two length dimensions, so the length conversion factor must be squared.
Sources and method
The worked figures are illustrative arithmetic. The references below explain the underlying definitions and limits.