Reading Roman numerals is easy enough. A film's closing credits, a clock face, a chapter heading — you can usually work out the value. Writing one is a different task. Asked to put 1994 down on paper, most people stall.
Dotori Roman Numerals converts both ways, and then shows the working. The second part is what makes the notation stick.
Two boxes, either one live
The screen holds two boxes. Arabic digits above, Roman letters below. Tap the one you want to edit, and the other updates on every keystroke. Type 1994 and MCMXCIV appears. Type MCMXCIV and 1994 appears.
The system keyboard never opens. Each box has its own keypad — digits for one side, the seven Roman letters for the other. The controls stay put, and half the screen is never covered.
Seven letters do all the work: I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, M is 1,000. The keypad puts them in that order, not alphabetically. That is how you end up remembering them.
The breakdown shows where the letters came from
Open "how it breaks down" under the result and the pieces appear. 1994 is M (1,000) + CM (900) + XC (90) + IV (4).
That single line answers the two questions people actually have. Why is 900 written CM and not DCCCC? Why is 4 written IV and not IIII?
Both come from the same rule. A smaller letter placed before a larger one is subtracted from it. Only six such pairs are standard: IV (4), IX (9), XL (40), XC (90), CD (400), CM (900). Anything else is not a subtractive pair, however sensible it looks.
Non-standard spellings are read, not rejected
Type IIII and the app gives you 4, with the standard spelling beside it. The same goes for VV and IC. Clock faces really do print IIII. Refusing to answer would be the wrong response to a real question.
The help page covers the rest of the rules:
- the seven symbols and their values
- the six subtractive pairs, and only those six
- how many times each letter may repeat
- the overline rule for large numbers
Every example there is tappable. Press one and it loads into the converter.
Past 3999, a bar goes over the letters
M is the largest symbol, so plain letters stop at 3,999. Beyond that, classical notation draws a bar above a group to multiply it by 1,000. A second bar multiplies it again. So 4,000 is IV with one bar, and 4,000,000 is IV with two.
The app draws each bar itself rather than relying on a font to supply one. That is why it lands in the same place at the same length on every device. The range runs from 1 to 3,999,999,999.
It starts at 1 for a reason worth knowing. Roman numerals have no symbol for zero. There is no way to write a negative number or a fraction.
One accessibility note, since a drawn line is invisible to a screen reader. The value box says it in words instead: "overlined I V times one thousand, then X I I, value 4012".
The app works entirely offline, with no account and no network access beyond the ad banner.
Wrap-up
Converting is the easy half. Seeing 1994 fall apart into M, CM, XC, and IV is the other half. The notation stops being memorisation and becomes a rule.
Put your birth year in and open the breakdown. The letters stop looking arbitrary after that.
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