Zellio.io

Roman Numeral Converter

Convert numbers to and from Roman numerals instantly.

Runs locallyWorks offlineShare link carries settings, never your data
Send output toMetrics & Unit ConverterOn-Screen RulerAngle Finder & ProtractorBuild a workflow from thisOutput stays on this page until you send it.

Frequently asked questions

How does subtractive notation work?

A smaller numeral before a larger one is subtracted: IV is 4 and IX is 9, rather than IIII and VIIII. Only I, X and C are used this way, and only before the next two higher symbols.

What is the largest number that can be written?

3,999 (MMMCMXCIX) using standard numerals, because there is no symbol above M and no more than three may repeat. Larger values historically used a bar above a numeral to mean multiply by a thousand.

Why is there no zero?

The system was built for counting and commerce, where nothing simply was not written down. Zero as a number arrived in Europe much later via Arabic numerals, which is a large part of why they displaced Roman ones.

Pro tips

  • Read a Roman numeral left to right and subtract only where a smaller symbol precedes a larger one; everything else adds.
  • Check whether a stated file size is decimal or binary before concluding storage is missing — the difference is about 7% at terabyte scale.
  • Remember that MPG and L/100km invert. Comparing two cars across the two systems without converting will reliably pick the wrong one.
  • Use the binary units (KiB, MiB, GiB) explicitly when precision matters; they exist specifically to remove this ambiguity.
  • Watch for US and imperial gallons in fuel figures — they differ by about 20%, which is more than the difference between most cars.

About Everyday Converters

Each of the three has a trap. Roman numerals carry no zero and no place value, a kilobyte means 1,000 or 1,024 bytes depending on who is counting, and fuel economy inverts between the two systems — a higher MPG is better while a higher L/100km is worse.

Convert numbers to Roman numerals and Roman numerals back to numbers, from 1 to 3999, with validation of canonical forms like MMXXIV. It works offline in your browser.

Convert numbers to and from Roman numerals, change data sizes between bytes, kilobytes, megabytes, gigabytes and beyond, and translate fuel economy across MPG, L/100km and km/L. It bundles three common conversions you reach for surprisingly often.

Everything works offline in your browser. Decode a Roman numeral on a monument, size a file or download, or compare a car's fuel efficiency between regional standards in seconds.

Roman numerals have no zero and no place value, which is why they are read by addition and subtraction rather than by position. A smaller numeral before a larger one is subtracted — IV is four, IX is nine — and only certain pairs are permitted in the standard form, which is why 1999 is MCMXCIX rather than the tidier-looking MIM. That constraint is the usual reason a hand-written conversion disagrees with a correct one.

Data sizes are ambiguous by convention rather than by error. Drive manufacturers count a gigabyte as 1,000,000,000 bytes while many operating systems report binary gibibytes of 1,073,741,824, and the gap widens as the units grow — which is the entire explanation for a 1 TB drive showing up as roughly 931 GB with nothing missing and nobody at fault. Fuel economy carries a different trap: MPG and L/100km run in opposite directions, so a higher number is better in one and worse in the other.

Common use cases

  • Reading a date on a building, monument or film credit
  • Working out a Super Bowl or monarch's number
  • Checking a numeral for a tattoo or engraving before committing
How it comparesEach of these three has dedicated single-purpose sites, and needing two of them means opening two tabs. Bundling the conversions people actually reach for occasionally is the point; a specialist tool is better if you do one of them constantly.