One converter for everything
Most "unit converters" are really one-trick calculators — a length converter, a weight converter, a temperature converter, each on its own page. The Unit Converter rolls all of them into one place. Pick a category (length, weight, volume, temperature, speed, area), pick your two units, type a number. You get the answer below. No category-hopping, no bookmarking five separate pages.
This matters because real life doesn't respect category boundaries. You're following a recipe that calls for 200 grams of flour and your scale reads ounces. The IKEA box says the shelf is 80 cm wide but your tape measure is in inches. The weather app told you it's 32 degrees and you don't know if that's a sweater day or a parka day. One tool that handles all of it saves the tab-juggling.
The conversions themselves are exact, not approximate. Since 1959, the foot, pound, and gallon have been defined in terms of metric units — so 1 inch is exactly 2.54 cm, 1 pound is exactly 0.45359237 kg, and 1 US gallon is exactly 3.785411784 liters. The numbers the Unit Converter produces come from those exact definitions.
How to use the Unit Converter
The flow is the same regardless of which category you're working in:
- Pick the category from the top selector — length, weight, volume, temperature, speed, or area.
- Pick your "from" unit. The options change based on the category (meters and feet for length; kilograms and pounds for weight; Celsius and Fahrenheit for temperature).
- Pick your "to" unit.
- Type the value you want to convert. The answer updates as you type — no Calculate button.
The math happens in your browser. Nothing leaves your device, which means it works offline once the page is cached, and there's no analytics watching what you're converting. Useful when you're prepping a freight quote or doing recipe math you'd rather not have logged somewhere.
The idea of a conversion factor
Almost every unit conversion is a multiplication. You take the value in one unit, multiply by a fixed number (the conversion factor), and get the value in the other unit. Temperature is the one exception — Celsius and Fahrenheit don't share a zero point, so you need to add or subtract as well as multiply.
Length: feet × 0.3048 = meters
Weight: pounds × 0.45359237 = kilograms
Volume: US gallons × 3.785411784 = liters
Temperature: (Fahrenheit − 32) × 5/9 = Celsius
Knowing the factor gives you a sanity check on any converter's output. If a tool tells you 5 feet is 50 meters, you know something's wrong because 5 × 0.3048 is nowhere near 50. The Unit Converter shows the result to enough decimal places that you can verify the math by hand if you want to.
Worked example: 5 feet to meters, 5 kilograms to pounds
Two of the conversions people ask about most often. The math is short enough to do on paper, which is a good way to confirm the calculator is doing what you think it's doing.
5 feet to meters:
- 1 foot = 0.3048 meters (exact, by international agreement)
- 5 × 0.3048 = 1.524 meters
So someone who is 5 feet tall is 1.524 meters, or 152.4 centimeters. That's a useful number to know because 152 cm sits at the lower end of adult human height, and 1.524 m is the standard width of railway track in most of the world (called "standard gauge"). Coincidence, but a memorable one.
5 kilograms to pounds:
- 1 kilogram = 2.20462262 pounds (1 ÷ 0.45359237)
- 5 × 2.20462262 = 11.023 pounds
So a 5 kg dumbbell is about 11 pounds. If your gym is in the US and sells weights in 5-pound increments, the nearest pair below 5 kg is the 10-pound dumbbells; the nearest above is 12.5 pounds. Useful when you're trying to match a European workout plan to American equipment.
Common conversions across the categories
If you're going to use the Unit Converter regularly, a handful of conversion factors are worth committing to memory. Here are the ones that come up most often in everyday life:
| From | To | Multiply by | Example |
|---|---|---|---|
| Inches | Centimeters | 2.54 | 10 in = 25.4 cm |
| Feet | Meters | 0.3048 | 6 ft = 1.829 m |
| Miles | Kilometers | 1.609344 | 26.2 mi = 42.2 km (a marathon) |
| Pounds | Kilograms | 0.453592 | 150 lb = 68 kg |
| Ounces | Grams | 28.3495 | 4 oz = 113 g (a stick of butter) |
| US gallons | Liters | 3.785412 | 15 gal = 56.8 L (a typical car tank) |
| US fluid ounces | Milliliters | 29.5735 | 12 fl oz = 355 mL (a soda can) |
| Square feet | Square meters | 0.092903 | 1000 sq ft = 92.9 m² |
| Miles per hour | Kilometers per hour | 1.609344 | 60 mph = 96.6 km/h |
| Celsius | Fahrenheit | × 9/5 + 32 | 20 °C = 68 °F |
Three of those — inches to centimeters, pounds to kilograms, miles to kilometers — cover maybe 80% of everyday conversion needs. The rest are useful when they come up.
Why the imperial system is messy and metric isn't
The metric system was designed in post-Revolution France in the 1790s by a committee of scientists. They picked base units (meter, gram, liter) and built everything else with prefixes — kilo for 1000, centi for 1/100, milli for 1/1000. So 1 kilometer is exactly 1000 meters and 1 milliliter is exactly 1/1000 of a liter. The math is always a decimal shift.
The imperial system grew up over centuries, with units tied to whatever was convenient at the time. A foot was the length of a foot. A mile was 1000 paces of a Roman soldier. An acre was as much land as one ox could plow in a day. A gallon was different sizes in different places — the US "gallon" and the UK "imperial gallon" still differ by about 20%. Converting within imperial is rarely a clean multiplication (12 inches to a foot, 3 feet to a yard, 1760 yards to a mile, 5280 feet to a mile — none of those numbers rhyme).
That's why the conversions in the table above always go between imperial and metric, not within imperial. Once you're in metric, the conversions inside the system are decimal shifts you can do in your head. If you need to know how many millimeters are in 5.7 meters, the answer is 5700 — you moved the decimal three places. There's nothing to look up.
Edge cases worth knowing
A few traps that catch people:
- US vs UK gallons. One US gallon is 3.785 liters; one UK (imperial) gallon is 4.546 liters. About 20% difference. The Unit Converter uses US gallons by default because the US is where the imperial volume units are still in active use. If you're following a UK recipe or filling up in Canada (where UK gallons used to be standard), check which one the source meant.
- Ounces of weight vs ounces of volume. A US fluid ounce is a volume (about 29.6 mL). A regular ounce is a weight (about 28.3 grams). They're different units that share a name, and the numbers happen to be close, which makes the trap worse. The Unit Converter keeps them in separate categories (weight vs volume) to avoid the confusion.
- Temperature isn't proportional. 40 °C is not "twice as hot" as 20 °C in any meaningful sense, because Celsius doesn't start at absolute zero. Doubling a temperature value doesn't double the heat. Conversions between Celsius and Fahrenheit involve both a multiplication and an offset.
- Tons are three different things. A US short ton is 2000 pounds (907.2 kg). A UK long ton is 2240 pounds (1016 kg). A metric ton (tonne) is exactly 1000 kg (2204.6 pounds). When a shipping document says "tons," it might mean any of them. The Unit Converter's weight category uses metric tonne by default; if you need short or long tons, do the math against pounds.
Related calculators
The Unit Converter is the general-purpose tool, but a few of the conversions have dedicated pages with extra context for that specific case:
- Length Converter — same length math, with longer examples on feet/meters/miles and the history of why imperial measurements ended up the way they did.
- Weight Converter — same weight math, with a longer note on the kilogram-pound relationship (and why "weight" and "mass" are technically different things outside Earth's gravity).
- Temperature Converter — Celsius, Fahrenheit, and Kelvin, with the formulas spelled out and a guide to which scale shows up where.
- mL to oz Converter — the specific case people ask about most, with the fluid-ounce-vs-weight-ounce trap explained in detail.
- Speed Calculator — useful if you're trying to figure out distance, time, or speed from any two of the three.
Frequently asked questions
Why does my conversion show so many decimal places?
Because that's the exact answer. The Unit Converter doesn't round unless rounding is needed for display. If you convert 1 inch to centimeters, the answer is exactly 2.54 — no decimals needed. If you convert 1 mile to feet, the answer is exactly 5280. If you convert 1 kilogram to pounds, the answer is 2.20462262... going on forever, so the calculator displays a useful number of decimals. Round to whatever precision you need for your purpose.
Is "1 inch = 2.54 cm" exact or approximate?
Exact, by international agreement signed in 1959. Before that, the US and UK had slightly different definitions of the inch, and the agreement merged them by defining one yard as exactly 0.9144 meters. The inch (1/36 of a yard) is 0.0254 meters or 2.54 cm. There's no rounding involved. The metric system is the reference; imperial units are now defined in terms of it.
Why are US and UK gallons different sizes?
Historical drift. Before American independence, the British "wine gallon" of 231 cubic inches was used in the colonies. After independence, the US kept the wine gallon and called it the US gallon. Britain reorganized its measures in 1824, replacing the wine gallon with the imperial gallon (about 277 cubic inches). The two countries have been out of sync ever since. The Unit Converter uses US gallons by default; for UK recipes, check whether the source means imperial.
Can I convert between currencies?
No — currency conversion needs live exchange rates that change every second, and this tool is built around fixed conversion factors. For currencies you'd want a dedicated FX converter that pulls live rates. The Unit Converter is for physical units (length, weight, volume, temperature, speed, area) where the conversion factors don't change.
Does it handle area conversions?
Yes — square meters, square feet, square yards, acres, hectares, and square miles are all in the area category. Area conversions can be slightly counterintuitive because the factor is the length factor squared. 1 foot is 0.3048 meters, so 1 square foot is 0.3048² ≈ 0.0929 square meters. The Unit Converter handles the squaring; you just pick the units.
Why is converting temperature different from converting length?
Because Celsius and Fahrenheit don't share a zero point. The Celsius scale puts zero at the freezing point of water; Fahrenheit puts zero at the freezing point of a specific brine solution that Daniel Fahrenheit used as his reference around 1724. So converting between them needs both a multiplication (to handle the different size of one degree) and an offset (to handle the different zero points). Kelvin shares its scale size with Celsius and just shifts the zero to absolute zero, so Celsius-to-Kelvin is a simple addition (K = C + 273.15).
Can I bookmark a specific conversion?
Not directly — the Unit Converter doesn't put the inputs in the URL. The dedicated converter pages (Length, Weight, Temperature, mL to oz) might be a better fit if you keep coming back to the same kind of conversion — they're shorter, focused, and the URL itself tells you what they do. The Unit Converter is for when you're doing a mix of conversion types in one session.