21 in Roman numerals
21 is written XXI. In chunks: X (10) + X (10) + I (1) = 21.
The answer
XXI
21 in Roman numerals is XXI.
How XXI is built
To write it, split 21 into 20 + 1. 20 is XX, 1 is I, so 21 is XX then I: XXI.
To read it, XXI has 3 chunks, and none of them is a take-away pair, so every chunk is added.
| Chunk | Worth | Why |
|---|---|---|
| X | 10 | Add |
| X | 10 | Add |
| I | 1 | Add |
The mistake to watch for
Writing 21 as XIX
Wrong: XIX
XIX has the same letters in a different order, and order matters. A smaller letter before a bigger one means take away, so XIX is 19, not 21. 21 is XXI.
Check it with the converter
Type XIX to see why XIX is not 21. The rule numbers it gives are the three rules on the Roman numerals page.
Converter set to XXI (21).
Keep going
Questions parents ask
What is 21 in Roman numerals?
21 is XXI.
What comes before and after XXI?
20 is XX and 22 is XXII.
Does XXI use subtraction?
No. Every letter in XXI is the same size as, or smaller than, the letter before it, so you just add them all.
What is the biggest letter in XXI?
X, which is 10. XXI has 3 letters.
Careful reading is an 11+ skill
Reading all 3 letters of XXI in order is the careful checking 11+ papers reward. The free diagnostic covers all four subjects, with no card needed.