401 in Roman numerals

401 in Roman numerals is CDI.

Pick any whole number from 1 to 3,999.

401 = \text{CDI}

How to work it out

1
Take the biggest Roman value that fits, write its letter, subtract it, and repeat.
2
Here: 400 (CD) + 1 (I) = 401.
3
Joined together: CDI. (Four of the same letter in a row is never used: 4 is IV, 9 is IX, 40 is XL, 90 is XC, 400 is CD, 900 is CM.)

Value	Letters
400	CD
1	I

What is 401 in Roman numerals?

CDI.

How do you read CDI?

CDI = 400 + 1 = 401.