Unit Converter: Length, Weight, Volume, Temperature and More

Convert units with the factor shown, and bridge mass to volume honestly

Two tools. The converter handles length, mass, volume, area, speed, time, temperature, pressure, energy and data, always inside one dimension (a length can become other lengths, never litres), prints the exact factor it used, and shows the reverse conversion so the result can be flipped and checked. US and imperial liquid measures are listed separately, because a US gallon and an imperial gallon are different sizes. The density bridge does the one cross-dimension job that is legitimate: mass, volume and density, solving for whichever one you leave out, because converting between mass and volume without a density is not a conversion at all.

What this converter does

Every conversion here runs through an exact definition: a unit is worth a stated number of base units (metres, kilograms, litres, seconds and so on), the value is multiplied into the base, then divided into the target. Nothing is approximated from memory: an inch is 0.0254 metres because it is defined to be, a pound is 0.45359237 kilograms by the same international agreement, and the US liquid gallon is 3.785411784 litres while the imperial gallon is 4.54609 litres, which is why they are separate entries rather than one “gallon” with an asterisk. Temperature is the exception that proves the rule: it converts by formula, not factor, because its scales start at different zeros.

How to use it

Stay inside the category. The lists only offer units of the same dimension. If the unit you want is not there, the conversion you are imagining crosses dimensions, and the density bridge (for mass and volume) or a different page is the honest route.
Check the reverse line. Every result prints what one unit of the target is worth in the source, so a swapped sanity check takes one glance: if the reverse factor looks absurd, the wrong gallon or ounce was probably picked.
Watch the two ounces. An ounce of mass (28.35 g) and a fluid ounce of volume (29.57 mL, US) share a name and nothing else. They live in different categories here on purpose.
For the bridge, the density is the substance’s, at your temperature. Water is 1.00 g/mL near 4°C and 0.997 at room temperature; oils sit around 0.91; honey near 1.42. A bridge result is only as good as the density typed into it.

The method

Factor conversions: result = value × (factor of source) / (factor of target), with factors stated against the SI base unit of the category.
Temperature: °F = °C × 9/5 + 32; K = °C + 273.15, applied in the order the direction requires.
Density bridge: density = mass / volume, rearranged to solve for whichever quantity is left blank: mass = density × volume; volume = mass / density.
Precision: factors are exact definitions where the unit is defined exactly, and results are shown to eight significant figures, which is more than any measuring tool feeding them can justify.

Worked examples

The road-trip conversion. 5 kilometres is 3.106856 miles. Run it backwards and 5 miles is 8.046720 kilometres; the pair of results is the reversibility check the results panel performs on every conversion.

The weather conversion. 100°F is 37.777778°C: hot by any scale, and a case where the formula, not a factor, does the work, since the two scales disagree about zero as well as step size.

The bridge. 500 grams of a liquid with density 1.2 g/mL occupies 500 / 1.2 = 416.67 mL. Without that density figure the question “how many millilitres is 500 grams” has no answer, which is the entire reason the bridge asks for it.

Reading your results

Read the factor line as the receipt: it states exactly what was multiplied, so a result can be reproduced in a spreadsheet or challenged on sight. Read long decimal tails as arithmetic, not accuracy: a kitchen measurement converted to eight figures was still measured to two. And read US and imperial entries with the suspicion they deserve: recipes, fuel economy and plumbing all cross the Atlantic badly when a “gallon” or “pint” is assumed rather than checked.

Limitations

One dimension at a time. Compound units (fuel economy, density itself, torque) are not single categories here; the density bridge covers the one everyday crossing, and dedicated calculators cover several compound cases.
Cooking measures are the standardised ones. The US cup, tablespoon and teaspoon here are the legal/metric-defined values; heirloom and regional utensils differ, which is a limitation of the physical spoon, honestly inherited.
Data uses both conventions, labelled. Kilobytes here are 1,000 bytes (the SI and storage-industry convention) and kibibytes 1,024 (the binary convention), listed as separate units so the famous 7 percent gap is a choice, not a surprise.
Concentration is not converted here. Turning a medicine’s mass into a volume needs that product’s labelled concentration (for example milligrams per millilitre) used as the bridge’s density figure, and the dose itself is a decision for the label and a professional, never for a converter.

Frequently asked questions

Why are there two gallons?

Because history left two: the US liquid gallon (3.785411784 litres, from the old wine gallon) and the imperial gallon (4.54609 litres, defined in Britain in 1824 and still used in a few places). They differ by about 20 percent, enough to wreck a fuel calculation or a recipe, so this page lists them as the separate units they are and suggests checking any source that just says “gallon” without saying whose.

How do I convert grams to millilitres?

Only through the substance’s density, which is what the bridge is for: divide the grams by the density in g/mL. For water the answer is the same number, which is how the shortcut got its reputation; for flour, oil, honey or anything powdered, aerated or viscous, the shortcut is wrong by margins that matter, and the density is the difference.

Why does temperature not use a factor like the others?

Because the scales are offset, not just scaled: 0°C is 32°F, not 0. A pure factor would send freezing water to a fictional temperature. So temperature converts by its formula in whichever direction is chosen, and it is the one category where doubling the input does not double the output (only differences double: a 10°C rise is an 18°F rise).

How precise are these conversions?

The factors are exact or standard to more digits than the results display, and the display runs to eight significant figures. The weak link is never the factor; it is the measurement being converted, which usually deserves two or three figures and gets carried through the rest out of politeness. Round the final answer to what the original measurement earned.

Related calculators

Method: exact definitional factors to SI base units per category; temperature by its scale formulas; mass-volume crossings only through a user-supplied density. Last reviewed: 15 October 2026.