Length, weight, volume, temperature, area & speed — plus the real story behind ounces, pounds, and the systems we measure with. Works offline too.
Avoirdupois, troy, fluid, apothecary — untangle the four different ounce systems.
From Egyptian cubits to the Planck constant — how humanity learned to measure.
Key anchor equivalences and mnemonic tips for metric–imperial conversion.
Pick a category tab, type a number into the Value box, and choose the units you are converting from and to. The result updates as you type, the swap button reverses the direction, and the copy button puts the answer on your clipboard. Below is a short, practical guide to each category — what the units actually are, where people get caught out, and a worked example you can check against the tool.
Since the 1959 international yard and pound agreement, one inch has been defined as exactly 2.54 centimeters. Every other imperial length hangs off that number: a foot is 30.48 cm, a yard is 0.9144 m, and a mile is 1,609.344 m. When you only need a rough figure, a meter is "a yard plus a hand-width" and a kilometer is a little over six-tenths of a mile.
The avoirdupois pound is defined as exactly 0.45359237 kilograms, and the everyday ounce is one-sixteenth of that (about 28.35 g). Be careful with precious metals: gold and silver are priced in troy ounces, which are roughly 10% heavier at 31.1 g. Our Weight tab uses avoirdupois units, the ones on food labels and bathroom scales.
This is where most kitchen mistakes happen. A US gallon is 3.785 liters, but a British (imperial) gallon is 4.546 liters — about 20% bigger. Fluid ounces differ too: a US fl oz is 29.57 mL, an imperial fl oz is 28.41 mL. If a recipe comes from the UK, Australia, or an older Canadian cookbook, check which system it uses before you pour.
The United States keeps a separate set of dry measures for produce and grain. A US dry pint is about 551 mL, noticeably larger than the 473 mL liquid pint, and a US bushel holds about 35.2 liters. You will mostly meet these at farm stands and in agricultural statistics — a "pint of blueberries" is a dry pint.
Temperature is the one conversion that is not a simple multiplication, because the scales start at different zero points. Subtract 32 from Fahrenheit, then multiply by 5/9 to get Celsius. Kelvin uses Celsius-sized steps but starts at absolute zero, so add 273.15. Curiously, −40 is the same in both Fahrenheit and Celsius.
Area units scale by the square of the length factor, which trips people up: one square foot is 0.0929 m², not 0.3048. For land, one hectare (a 100 m × 100 m square) equals about 2.471 acres, and an acre is roughly 4,047 m² — a bit smaller than an American football field without the end zones.
Road signs use miles per hour in the US and UK and kilometers per hour almost everywhere else; multiply mph by 1.609 to get km/h. Ships and aircraft use knots — one nautical mile per hour, defined as exactly 1.852 km/h. Scientists prefer meters per second; divide km/h by 3.6 to get there.
The conversion factors are the official exact definitions (for example, 1 in = 2.54 cm and 1 lb = 0.45359237 kg), and results are shown to eight significant digits. For everyday use, rounding to two or three digits is plenty.
Yes. All the measurement math runs in your browser, and once you have visited the site it can be installed as an app and used offline. The only exception is the currency converter, which needs a connection to fetch the latest exchange rates.
The word "ounce" survived from several older systems: avoirdupois for everyday goods, troy for precious metals, apothecaries' for old pharmacy work, and the fluid ounce for volume. Our Ounce Explained page untangles all four.
Not quite. A US customary cup is about 236.6 mL, while the "metric cup" used in Australia, New Zealand, and Canada is 250 mL. The difference is small for a single cup but adds up in baking, where precision matters.
The US actually uses "US customary" units, which are close cousins of the British imperial system. Metric conversion was legally authorized in 1866 and promoted in the 1970s, but it was voluntary, and road signs, recipes, and construction stayed customary. Our History page covers the full story.
Want to go deeper? Read The Ounce Explained, browse the history of measurement, or keep the quick reference tables handy.