“Millimeters” and “millimetres” are classic examples of homophones — words that sound the same but have different spellings and are used in different variants of English.

This can lead to confusion, especially in writing, where regional spelling conventions matter. In this post, we’ll explain the difference between “millimeters” and “millimetres”, provide examples, and help you avoid mistakes with a simple quiz and tips.

What Does “Millimeters” Mean?

Part of Speech: Noun (plural of “millimeter”)

“Millimeters” is the American English spelling for the metric unit of length equal to one-thousandth of a meter.

Examples:

• “The part measures 15 millimeters.”
• “Rainfall is expected to reach 50 millimeters.”

What Does “Millimetres” Mean?

Part of Speech: Noun (plural of “millimetre”)

“Millimetres” is the British English spelling for the same metric unit of length.

Examples:

• “The part measures 15 millimetres.”
• “Rainfall is expected to reach 50 millimetres.”

Key Differences: Millimeters vs Millimetres

Variant	Spelling	Usage
American English	Millimeters	Used in the United States
British English	Millimetres	Used in the UK, Australia, and other Commonwealth countries

Common Mistakes

• ✘ “The length is 20 millimetres.” (in American English) → ❌ Incorrect
• ✔ “The length is 20 millimeters.” (in American English) → ✅ Correct
• ✘ “The rainfall was 30 millimeters.” (in British English) → ❌ Incorrect
• ✔ “The rainfall was 30 millimetres.” (in British English) → ✅ Correct

Easy Tricks to Remember

• Millimeters: American English uses “-er” endings.
• Millimetres: British English uses “-re” endings.

Quick Practice Exercise

Choose the correct spelling (millimeters or millimetres):

1. The component is 10 --Choose-- millimeters millimetres long. (American English)
2. The rainfall was 25 --Choose-- millimeters millimetres yesterday. (British English)
3. The screw is 5 --Choose-- millimeters millimetres in diameter. (American English)
4. The fabric shrank by 2 --Choose-- millimeters millimetres after washing. (British English)

Check Answers

Bonus Resources

• Listen to the pronunciation of “millimeters” and “millimetres” (Cambridge Dictionary)
• Explore more homophones → See the full category

Conclusion

Millimeters and millimetres are spelled differently depending on whether you're using American or British English, but they mean the exact same thing and sound the same.

Now you know how to choose the correct version for your audience or region — and you won’t confuse your readers again!

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