Room I

Numeral converter

Two tablets, one truth. Write on either and the other one answers, with every step of the working laid out underneath.

Arabic number A whole number from 1 to 3999. Commas are fine.

Roman numeral The letters I, V, X, L, C, D and M, in any case.

Type on either tablet, or choose an example below.

The carved result

Nothing is carved yet. Write a number or a numeral above and it will appear here.

The working, stone by stone

The explanation appears here as soon as you have a valid number or numeral.

Beyond 3999: the vinculum

A bar drawn above a letter, called a vinculum, multiplies it by one thousand. V means 5,000 and X means 10,000. The plain letters stop at MMMCMXCIX (3999) because there is no fifth M in the standard system.

Enter a number from 4000 to 3,999,999 in the Arabic tablet and this converter will show how it would be written with bars. Bars are shown for reading only; they cannot be typed into the Roman tablet.

The pattern table

Every number is built place by place. Read the thousands, hundreds, tens and ones of any number off this table, left to right, and you have its numeral.

Digits 1 to 9 in each place
Digit	Thousands	Hundreds	Tens	Ones
1	M	C	X	I
2	MM	CC	XX	II
3	MMM	CCC	XXX	III
4	needs a bar	CD	XL	IV
5	needs a bar	D	L	V
6	needs a bar	DC	LX	VI
7	needs a bar	DCC	LXX	VII
8	needs a bar	DCCC	LXXX	VIII
9	needs a bar	CM	XC	IX

How to use it, and what it assumes

How to use it

Type a whole number from 1 to 3999 on the left tablet, or Roman letters on the right. You may use upper or lower case.
Read the carved result in the black slab. The other tablet fills itself in.
Follow the numbered working to see why the answer is what it is, then try one of the examples.
Use the buttons to copy a result or to carry the number over to the inscription maker.

What it assumes

It uses the modern standard form, with the six subtractive pairs IV, IX, XL, XC, CD and CM.
Letters V, L and D are never repeated, and I, X, C and M are never repeated more than three times in a row.
Numerals are read left to right, largest place first. Spaces and full stops between letters are ignored.
The plain range is 1 to 3999. Larger numbers are shown with a vinculum for reading only.

Edge cases, kindly handled

Zero, negatives and fractions have no standard Roman form here. You will get a short explanation, not an error code.
Non-standard numerals such as IIII, VX or IC are not rejected coldly. The tool explains the rule they break and offers the standard form.
Stray characters are named, so you can see which one to delete.
Very large input is declined politely once it passes what bars can reasonably express.

Want the reasons behind the rules? The Learn page has them, with a short practice drill.