How many conversions are there?
Every unit, every scale, even the obscure ones — one tool. Length, Roman numerals, shoe sizes, colour, with parsecs, smoots and beard-seconds included.
Length
Metric · imperial · obscure
Try 1 light-year — see how many bananas that is.
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What is a conversion?
A conversion is the same quantity expressed in a different unit of measurement. Five miles becomes 8.05 kilometres. One cup of flour becomes 125 grams. The number 2026 becomes MMXXVI. The thing being measured does not change. Only the language used to describe it changes.
Humans have been doing this for as long as we have been trading. Egyptian cubits became Greek feet became Roman miles became English statute miles became kilometres. Each step required somebody to know the ratio. This tool exists so you do not have to.
Every category, every unit
The plan is the most comprehensive conversion tool on the web. Available today, with more added every week.
Available now
- Length — millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, nautical miles, plus the obscure: cubits, hands, fathoms, leagues, astronomical units, light-years, parsecs, smoots, bananas and beard-seconds.
- Roman numerals — Arabic to Roman, 1 to 3999, both directions, with strict validation that rejects malformed input.
- Shoe size — UK, US, EU and Japanese sizing, men's and women's, with all four shown side-by-side.
- Colour — HEX, RGB, HSL, CMYK, plus the nearest named colour from a 30-name palette.
Coming soon
Weight and mass · Temperature · Volume · Speed · Time and duration · Data storage (with kibibyte vs kilobyte) · Energy · Force · Pressure · Power · Frequency · Angle · Fuel economy · Decibel scale · Earthquake magnitudes · Beaufort wind scale · Morse code (with audio) · NATO phonetic alphabet · Number bases (binary, hex, octal, base 64) · Braille · Pig latin and leet speak · Ring sizes · Hat sizes · Clothing sizes · Paper sizes · Cooking ingredient conversions · Date systems (Gregorian, Julian, Hijri, Hebrew, Persian, Chinese zodiac) · Dog and cat years · Astronomical distances · Musical frequency to note · BPM to delay milliseconds.
Want a specific conversion added? Most live in days, not weeks.
The weirdest units ever standardised
These are real units. Some are jokes that escaped containment. All have been used in published, citable contexts.
1. The Smoot
1.7018 mOliver Smoot was laid end-over-end across the Harvard Bridge by his MIT fraternity in October 1958. The bridge is still marked at 364.4 smoots plus one ear, repainted by the chapter every year.
2. The Beard-second
5 nmAverage beard growth in one second. Coined at SLAC as a unit deliberately too small to be useful — like a microcentury, but with more dignity.
3. The Jiffy
~10 msIn computing it is roughly one hundredth of a second. In electronics it is one cycle of mains power (1/60 sec US, 1/50 sec EU). In physics it is the time for light to travel one fermi — about 3 × 10⁻²⁴ seconds.
4. The Banana for Scale
17.78 cmThe internet meme that became so widely adopted that the US National Institute of Standards and Technology has informally acknowledged it. The standard banana is roughly seven inches, though brand variance applies.
5. The Microcentury
52.6 minutesJohn von Neumann allegedly used this unit to politely indicate to a lecturer that they were running long. A century of time, divided by one million.
6. The Hammer Unit
1.905 cmA unit of length used in the Source game engine, equivalent to 1/16 of a Half-Life foot. Half-Life feet are also fictional. There are about 53 hammer units in a real-world metre.
7. The Nibble
4 bitsHalf a byte. The spelling is deliberate. Two nibbles make a byte; four bytes used to make an entire computer.
8. The Hogshead
238.5 LA unit used historically for shipping wine, beer, cider and tobacco. There were also barrels, butts, firkins, kilderkins and tuns. Pre-metric brewing was a wonderful place.
9. The Mickey
~0.1 mmThe smallest detectable movement of a computer mouse. Named after Mickey Mouse. Mouse cursor sensitivity is sometimes still measured in mickeys per inch.
10. Furlongs per Fortnight
1.66 × 10⁻⁴ m/sA real velocity unit that exists mostly as a joke about archaic measurement. One furlong is 220 yards; one fortnight is 14 days. Light travels at about 1.8 × 10¹² furlongs per fortnight.
Mini-quiz: match the obscure unit
Five questions. No timer. Answer at your own pace.
A "smoot" is a unit of...
Frequently asked
What is a unit conversion?+
A unit conversion expresses the same quantity in a different unit of measurement. One mile becomes 1.609 kilometres; one cup of flour becomes 125 grams; the number 2026 becomes MMXXVI. The underlying quantity does not change — only the unit used to describe it.
How many unit conversions are there?+
Across all categories — length, mass, temperature, volume, speed, time, data, energy, force, pressure, power, angle, fuel economy, date systems, sizing for shoes and clothes and rings and hats, text encodings like Roman numerals and Morse and NATO phonetic and number bases, colour spaces and historical units — there are well over a thousand pairwise conversions in regular use. This tool is building the most comprehensive collection of them in one place.
How accurate are these conversions?+
Every constant in the converter is individually fact-checked. The inch, foot, yard and mile are exact post-1959. The astronomical unit, light-year and parsec use the IAU 2012 and 2015 exact definitions. The math engine self-tests on every page load — open your browser console and you will see a confirmation message that nothing has been altered.
What is the most obscure unit ever standardised?+
The smoot, at exactly 5 feet 7 inches or 1.7018 metres. In October 1958, MIT fraternity pledge Oliver Smoot was repeatedly laid end-over-end across the Harvard Bridge by his fraternity brothers to measure its length. The bridge is officially marked at 364.4 smoots plus one ear, repainted by the chapter every year. Smoot himself later became Chairman of the American National Standards Institute, which is excellent karma.
Are Roman numerals still used today?+
Yes. Standard usage continues for film and television copyright dates, monarch and pope names (King Charles III), the Super Bowl, watch faces, book chapter numbering and historical event names. The system maxes out at MMMCMXCIX (3999) without the vinculum overline notation, which is why this converter has the same upper limit.
Why are imperial and metric still both in use?+
The United States, Liberia and Myanmar are the only three countries that have not officially adopted the metric system as their primary system of weights and measures. The United Kingdom uses a mix — metric for science and engineering, but imperial persists for road distances (miles), beer (pints), and human height and weight in everyday conversation.
How do shoe sizes convert between countries?+
Each region uses a different reference point. UK sizing measures the last (the foot-shaped form a shoe is built on) starting from a 4-inch base at size zero. US sizing offsets UK by roughly 1 for men and 2 for women. European sizing uses the Paris point, where one unit equals two-thirds of a centimetre. Japanese sizing uses Mondopoint — the foot length in centimetres. Brand variance is significant, particularly for athletic footwear.
What is a beard-second?+
A unit of length equal to about five nanometres, defined as the average growth of a human beard in one second. It was coined as a deliberately too-small unit at SLAC National Accelerator Laboratory and is occasionally cited alongside the smoot as evidence that scientists have a healthy sense of humour about measurement.