Unit Converter

One converter for ten quantities, with the units picked from a list or written straight into the line.

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What this tool does

Unit Converter holds ten quantities in one page — length, weight and mass, area, volume, speed, temperature, pressure, energy, data size and time — and 128 units between them. Every conversion routes through that quantity's base unit, so the metre, the kilogram, the second and the bit are the pivot rather than a lookup table of pairs. Choose the quantity, choose From and To, and each non-empty line of the input is converted on its own.

Units can be typed rather than picked. A line reading 5 ft to in, or 100 km/h to mph, sets both ends by itself, and when the unit named belongs to a quantity other than the one selected, that line switches quantity to follow it. Spellings that several quantities share are decided in a fixed order. A written target unit settles the line first, which is why 1 c to km/h is the speed of light while 1 c to °F is Celsius. With no target written, the quantity already on screen keeps the spelling for itself, so 5 mb is millibars while Pressure is selected. Only when the selected quantity has no unit by that name does the strongest match among the other nine take the line, and that is what makes a bare MB a megabyte and a bare c Celsius.

Precision is either automatic — ten significant digits with trailing zeros trimmed, switching to eight-digit exponential form from 10¹⁵ upwards and below 10⁻⁶ — or a fixed count of 0, 1, 2, 3, 4, 6 or 8 decimals, applied to the value you typed as well as the answer, so 1,234.5 kg to lb at two decimals prints as 1234.50 kg = 2721.61 lb. Setting To to All units (table) prints the entire table for that quantity instead of a single pair. One formula line is footnoted at the end for the last pair converted, when there is one to give: the ratio between the two units for anything linear, and the written-out arithmetic for the everyday temperature pairs, though a rarer pairing such as °C to °R prints no formula at all.

The measurement work runs in this browser tab and no input is uploaded. What it will not do is worth knowing first. Quantities cannot be crossed — 1 kg to litres is refused with a message naming the mismatch rather than guessed at — and there is no currency here, because rates move by the minute and Currency Converter fetches them instead. The rule that lets the selected quantity claim a shared spelling has a sharp edge, too: with Speed selected, a line reading -300 °C is taken as -300 times the speed of light and converted without complaint, so write the target unit when the quantity on screen is not the one you mean. Time is arithmetic on fixed lengths, a month being the average 30.44 days and a year 365.25, so real calendar questions belong in Date Difference. Mach is pinned to the sea-level, 15 °C value of 340.29 m/s and ignores altitude, weight and mass share a single table with no gravity in it, and a cup of flour into grams needs an ingredient density this tool does not carry — that one is Cooking Converter.

How to use it

  1. Choose the quantity. Length, weight and mass, area, volume, speed, temperature, pressure, energy, data size or time. The From and To lists then offer only the units belonging to it.
  2. Enter your values, one per line. A bare number uses the two dropdowns. Feet-and-inches (6'2"), mixed fractions (1 1/2), vulgar fractions (½) and thousands separators written 1,234.5 or 1.234,5 all parse.
  3. Or write the units into the line instead. 5 ft to in and 100 km/h to mph override both dropdowns for that line, and a unit from another quantity moves the line to that quantity.
  4. Set the precision, or ask for the whole table. Automatic keeps ten significant digits; a fixed decimal count is better for a figure you are pasting into a document. Choosing All units (table) as the target lists every unit in the quantity at once.

Processing & privacy

Runs on your device. Your file is processed directly in this browser.

Common use cases

Following a US recipe in metric

Cups, tablespoons and fluid ounces convert straight to millilitres, since volume needs no knowledge of what is in the jug.

Checking a parcel against a courier limit

Girth in centimetres against a limit quoted in inches, and the weight in kilograms against one quoted in pounds or stones.

Reading a datasheet written for another market

Pressure given in bar against a gauge in psi, or a heater rated in BTU against a meter that reads in kilowatt-hours.

Sizing a room, a plot or a roll of material

Square metres to square feet, hectares to acres, and back again when the estate agent and the builder disagree.

Converting a pasted column

A list of numbers, one per line, comes back as a list of results, with a single formula line at the end for whichever pair happened to be converted last.

Questions

Are the values I type sent anywhere?
No. The unit tables and the arithmetic are part of the page itself, so every line is converted inside this browser tab and nothing you enter is transmitted or logged. Conversion carries on after the connection drops, although the site keeps no cached copy of itself, so the page has to be fetched while you still have one.
Can it convert kilograms to litres?
No. Those are different quantities and the bridge between them is the density of the substance, which the tool has no way to know. It refuses the line and names the mismatch rather than returning a number that looks plausible.
Why was my To selection ignored?
Because the line named a unit from a different quantity. When that happens the line moves to the quantity that owns the unit, and with no target unit written after it the result is printed as the full table for that quantity. Add to and your unit to pin the target.
Which cup, pint and gallon does it use?
US measures are the plain entries, so cup, pint and gallon are the American ones. The imperial versions are listed separately as fl oz (UK), pt (UK), qt (UK) and gal (UK), and the 250 ml metric cup has its own entry.
Is a kilobyte 1,000 or 1,024 bytes?
Both are available and they are separate units. kB, MB and GB are the decimal powers of ten; KiB, MiB and GiB are the binary powers of two. Everything in that quantity is counted in bits, so a byte is 8 bits and megabits sit alongside megabytes.
What happens below absolute zero?
With Temperature selected, or on any line whose target unit is a temperature, the conversion is refused and the message names the limit in the unit you were using, since -300 °C describes nothing physical and quietly returning a negative kelvin figure would be worse than stopping.
Should I use this or the single-quantity converters?
Use this one when the quantity changes from line to line or you cannot remember where a unit lives. Length Converter, Weight Converter and the rest share these tables, open with the right units already selected, and never reinterpret a unit as belonging to some other quantity.

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