Unit converter

Unit conversion looks like the most solved problem on the internet, and it is the one where free tools most often disagree with each other. The constants are not the reason — almost all of them are defined values that are identical everywhere. The disagreements come from rounding, from units that share a name while differing by ten or twenty per cent, and from whole families of units that English-language tools simply do not carry.

Why two systems exist at all

The metric system was built during the French Revolution as a deliberate replacement for a patchwork in which the same unit name meant different amounts in different towns. Its two design decisions — everything scales by powers of ten, and the base units are tied to something reproducible rather than to an artefact or a body part — are the whole reason metric arithmetic is easy.

The imperial and US customary systems grew instead by accumulation, which is why their internal ratios are 12, 3, 16, 14 and 1760. Those numbers look arbitrary, and they are, but they divide in ways decimals do not: a foot splits into halves, thirds and quarters as whole inches, which is genuinely convenient when you are dividing a plank rather than multiplying on paper.

The United States never abandoned customary units for everyday use. The Metric Conversion Act of 1975 declared metric the preferred system but made conversion voluntary, and voluntary conversion did not happen outside science, medicine and the military. The practical consequence for the rest of the world is that American specifications, screen sizes, fastener standards and pipe threads still arrive in inches.

Every conversion here is exact, and that is a recent fact

Since 1959 the imperial units have been defined by reference to the metric ones: the yard is exactly 0.9144 m and the pound exactly 0.45359237 kg. That agreement, signed by six countries, replaced separate national definitions that disagreed with each other by a couple of parts per million. So 2.54 cm per inch is not a measurement that better instruments could refine — it is a definition, and the conversion is exact in both directions.

The metric side went the same way. In 1983 the metre stopped being a bar and became a distance defined by the speed of light. In 2019 the kilogram stopped being a cylinder near Paris — which had been drifting relative to its own copies by tens of micrograms — and became a value derived from the Planck constant. All seven SI base units are now defined by fixed constants of nature, which means no laboratory anywhere holds the master copy of anything.

What that means for a converter is simple: any disagreement between two conversion tools is a display decision, not a physical uncertainty.

Taiwanese units, and where they are still the only unit used

This is the part English-language converters omit entirely, and in Taiwan it is not historical trivia — several of these units are the only ones in active commercial use for their domain.

Property is priced in ping (坪), a square of six Taiwanese chi, which is exactly 400/121 m² or 3.3058 m². Farmland is measured in jia (甲) and fen (分), where a jia is 2934 ping, roughly 0.97 hectares. Interior fit-out, carpentry, doors and window frames are quoted in chi (台尺), which is 10/33 of a metre — 30.303 cm, not 30 cm, and that three-millimetre-per-chi difference is the most common measuring error on a Taiwanese building site.

Traditional markets sell by the catty (台斤) at exactly 600 g, which is not the mainland Chinese shijin at 500 g. Chinese medicine prescriptions are written in qian (錢) at 3.75 g, and gold shops price in qian rather than troy ounces. Plumbers and hardware shops use 分 for pipe sizes, but there it means one eighth of an inch, so a 4分 pipe is half an inch — a completely different quantity from the traditional 分, which is 3.03 mm.

Because several of those names are genuinely shared between units of different size, the tools here refuse to guess. Type an ambiguous unit and you get an explanation of the options instead of a number, on the principle that a plausible-looking wrong answer is worse than an error message.

Rounding, which is where converters actually differ

Fixing the display to a set number of decimal places is the most common mistake. Two decimals turns 0.0393701 inches into 0.04, which loses the information entirely, and it also breaks round-tripping: convert a value out and back and you no longer land on what you typed. Rounding to a set number of significant figures behaves sensibly at both ends of the scale, so that is what these tools do by default.

The second common mistake is printing raw floating-point output. Because 2.54 has no exact binary representation, converting 1 cm to inches and back can produce 0.9999999999999999. That is not a conversion error, it is a display error, and rounding to significant figures removes it.

The third is inserting thousands separators into a result you are about to paste into a spreadsheet or a script. These tools do not.

What is here now, and what is not

Length and weight are available: a combined page for each that shows one value in every unit at once, plus single-direction pages for the three conversions people search for most — centimetres to inches, millimetres to inches, and kilograms to pounds. Those three come with complete 1 to 100 reference tables, which is the thing a search-result widget cannot give you.

Area, temperature, volume, speed and data storage are not built yet, and we would rather say that than fill this page with links to pages that do not exist. The unit tables for area and temperature are already written and tested — including ping, jia and fen, and the offset arithmetic that Celsius and Fahrenheit need — so those pages are a small step rather than a large one.