Unit Converter

1. One base unit per measurement

Every conversion here goes through a base unit rather than a table of direct pairs. A value is multiplied by its own unit's factor to reach the base, then divided by the target unit's factor to leave it. Miles to nautical miles is mi → m → nmi, and no separate mile-to-nautical-mile factor has to exist or be kept correct.

That is why the console can show the same value in every unit of a category at once, and why adding a unit adds a row rather than a column of new pairs. 107 units across 12 measurements:

The Formula tile shows the factor being applied, so the arithmetic is checkable rather than taken on trust: mi = km × 0.621371192237. Results are given to eight significant figures and the factors themselves are exact where the definition is exact — an inch is exactly 25.4 mm, a mile is exactly 1,609.344 m, and a nautical mile is exactly 1,852 m by definition, not by measurement.

2. Temperature is the exception

Every other category scales through zero: nought metres is nought feet, so a single multiplication does the job. Temperature scales do not share a zero point, so they need an offset as well as a factor — an affine conversion rather than a proportional one.

The practical consequence is that you cannot convert a temperature difference the same way you convert a temperature. A room at 20 °C is 68 °F, but a rise of 20 °C is a rise of 36 °F, not 68 — for an interval you use the 9/5 factor without the offset. It is the most common temperature-conversion error in engineering copy.

Two landmarks worth knowing: −40 °C = −40 °F, the one point where the scales cross, and 0 K = −273.15 °C, absolute zero. Rankine is Fahrenheit's absolute scale, so it shares Kelvin's zero and Fahrenheit's degree size.

3. US and imperial units are not the same units

“Gallon” and “fluid ounce” mean different quantities either side of the Atlantic, which makes halved recipes and fuel-economy figures quietly wrong:

Note that the gallon and the fluid ounce disagree in opposite directions: the imperial gallon is bigger, but the imperial fluid ounce is smaller. That is because an imperial gallon is 160 imperial fluid ounces while a US gallon is 128 US fluid ounces. Both systems are listed separately in the volume category, labelled US or imperial, so the choice is explicit.

Mass has the same trap. A short ton (US) is 907.18474 kg, a long ton (UK) is 1,016.0469 kg, and a tonne is exactly 1,000 kg — three different units routinely written “ton”. Stone is listed too, at exactly 14 pounds.

4. Data sizes: 1,000 or 1,024

A gigabyte is not a gibibyte, and this is where a drive's advertised capacity goes missing. kB, MB, GB and TB are decimal — powers of 1,000, as the SI prefixes require and as drive manufacturers use. KiB, MiB, GiB and TiB are binary — powers of 1,024, standardised by IEC 80000-13, and what Windows means when it displays “GB”.

The gap compounds with every prefix: 2.4% at kilo, 4.9% at mega, 7.4% at giga and 10.0% at tera. A 1 TB drive holds 0.909 TiB, which is why a “1 TB” disk shows as about 931 GB in a file manager. Nothing has been lost and nobody is cheating — two different units are being reported under similar names. Both families are in the list here, spelled out, so you can convert between them explicitly.

5. Where a “unit” hides an assumption

Some conversions are exact by definition, some are exact by measurement, and a few are averages wearing the costume of a unit. It is worth knowing which is which:

• Months and years in the time category are averages — 30.436875 days and 365.2425 days, the Gregorian mean. No calendar month is that length. For real dates use the date calculator, which counts actual days.
• Horsepower comes in flavours. Mechanical horsepower is 745.7 W; metric horsepower (PS, used in European vehicle figures) is 735.5 W, about 1.4% smaller. Both are listed and labelled.
• Calories are two units. The food Calorie is a kilocalorie; the physics calorie is a thousandth of it. The energy category uses kcal and kJ, the pair on nutrition labels.
• Pressure gauges may read absolute or gauge pressure. The factors here convert the number you have; whether it already excludes atmospheric pressure is a property of the instrument, not of the arithmetic.
• Weight and mass are treated as the same quantity, as everyday use does. A kilogram is mass and a pound-force is weight, and the distinction only matters off this planet — but it is why the category is labelled “weight & mass”.

There are 81 named conversion pairs with a page of their own, and any combination of units inside a category can be converted here whether or not it has a page. Everything runs against a fixed factor table in your browser — no lookup, no API call, nothing uploaded, and it keeps working offline once the page has loaded.

6. All 81 conversions with a page of their own

Each link below opens a page for that one conversion: the factor, the formula written out, a reference table at the values people actually use, the reverse direction and the converter pre-filled. The converter above handles any pair inside a category whether or not it is listed here — these are the ones common enough to be worth explaining in full.

Every one of those pages carries the same factor table as this one, so the two can never disagree. If a conversion you need is missing, pick its category above and both units will be in the list.