Quick answer
British English distinguishes them: metre is the unit of length and the rhythm of verse, while meter is a measuring device — a gas meter, a parking meter. American English uses meter for everything. So the British split is device versus unit; the American one does not exist.
The core difference
A unit and a device, separated in British English only.
- metre (British) — the unit of length; also poetic rhythm: iambic metre.
- meter (British) — a measuring device: electricity meter.
- meter (American) — all of the above.
The compounds
British English keeps the split in compounds too: kilometre, centimetre, millimetre take the unit spelling; speedometer, thermometer, barometer take the device spelling. That is consistent once you see the logic — measuring instruments are meters.
The same pattern elsewhere
Litre/liter, theatre/theater, centre/center all follow the British -re versus American -er pattern, from the French-derived spellings that American reform simplified.
Frequently asked questions
Which spelling for the unit of length?
Metre in British English, meter in American English.
What about kilometre and speedometer?
British English keeps the split — units take -metre, instruments take -meter. So kilometre but speedometer.
What other words follow this pattern?
Litre/liter, theatre/theater, centre/center — the British -re versus American -er spellings.
