Write a length into the row of the unit you have: the other twenty rows fill in at once, each through the metre and rounded to twelve significant digits, so 1 mile returns 1.609344 km, 5280 feet and 63360 inches.
Runs locally in your browserWrite a length into the row of the unit you have and the other twenty rows fill in at once, each one in its own unit. Every row is converted through the metre, so the twenty numbers agree with each other instead of drifting apart row by row.
The whole calculation runs in this browser; nothing is sent anywhere. Convert (or Enter) in a row prints a confirmation line under the table, and Clear all empties the twenty-one fields.
Each row is a fixed number of metres: 1000 in a kilometre, 0.01 in a centimetre, 0.0001 in a decimillimetre, 500 in the Chinese li (里), 10/3 in a zhang (丈), 1/3 in a chi (尺), 1/30 in a cun (寸), 1/300 in a fen (分), 1/3000 in a li (厘), 1852 in a nautical mile, 1.8288 in a fathom, 1609.344 in a mile, 201.168 in a furlong, 0.9144 in a yard, 0.3048 in a foot, 0.0254 in an inch and 1e-9 in a nanometre.
The metric rows step down by ten from the kilometre to the decimetre and continue through millimetre, decimillimetre and micrometre, which is the scale a workshop uses: 1 decimillimetre is 0.1 mm and 100 µm.
The factors are the definitions, not approximations: 1 mile is 1760 yards = 1609.344 m, 1 nautical mile is 1852 m, 1 furlong is 1/8 mile, 1 fathom is 6 feet, 1 inch is 0.0254 m. Measured on the live page, 1 mile now returns 1.609344 kilometres and 1 nautical mile 1.852 kilometres; the old script carried four-digit factors and printed 1.6092693916961698 and 1.8518518518518519 instead.
Results are rounded to twelve significant digits: 1 m is 3.28083989501 feet, 39.3700787402 inches and 1000 millimetres, 1 inch is exactly 2.54 cm, 1 µm is 0.001 mm, and 1 nm is 1e-12 km. Values between 1e-6 and 1e12 are written as plain decimals, outside that window in exponent notation, and binary noise such as 2.5400000025908 is gone.
The six Chinese market units are in the table with their own names and their size in metres: li (里) 500 m, zhang (丈) 10/3 m, chi (尺) 1/3 m, cun (寸) 1/30 m, fen (分) 1/300 m and li (厘) 1/3000 m, so 1 m is 3 chi, 30 cun and 3000 fen. On the old page they were hidden on all six locales and the rows around them were labelled by machine translations.
An empty field, a lone point, "1.2.3" and a negative number are answered with a sentence in the page language instead of silence, the other rows are cleared so a rejected entry never sits next to stale results, and the row you typed in keeps your text until you replace it.