Unit Converter Guide: Length, Weight, Volume, Temperature

Most unit conversion mistakes aren't arithmetic errors — they're wrong-unit errors. A US gallon isn't an imperial gallon, a fluid ounce isn't an ounce of weight, and "ton" means three different things depending on who wrote it. This guide covers the conversions people actually need, the exact factors behind them, and how to catch a bad answer before it ends up in a recipe, a drawing, or a shipping label.

Use the free tool: Unit Converter - private, in-browser conversion for length, weight, volume, and temperature.

What the converter handles

Four categories, each with the units most people actually encounter:

• Length: millimetres, centimetres, metres, kilometres, inches, feet, yards, miles
• Weight: milligrams, grams, kilograms, metric tonnes, ounces, pounds
• Volume: millilitres, litres, kilolitres, cups, pints, quarts, gallons
• Temperature: Celsius, Fahrenheit, Kelvin

There is no upload step and no server round trip. The maths runs locally in the page, so dimensions of a confidential part or a customer's shipping weight never leave your device. Everything else on the site works the same way — see browse all tools if you want the full list.

How to convert units, step by step

1. Open the Unit Converter.
2. Pick a category — length, weight, volume, or temperature. The unit dropdowns reset to sensible defaults for that category.
3. Type the value you want to convert. Results update as you type; invalid text is flagged, though an empty field is treated as 0.
4. Choose the from and to units.
5. Read the result. It is displayed to six significant figures — for example, 1 metre shows as 3.28084 feet.
6. Use Swap to invert the direction, and Copy to put the raw value on your clipboard.

The conversion factors worth remembering

Length

| From | To | Multiply by | | --- | --- | --- | | inches | centimetres | 2.54 (exact) | | feet | metres | 0.3048 (exact) | | yards | metres | 0.9144 (exact) | | miles | kilometres | 1.609344 (exact) | | metres | feet | 3.28084 |

The imperial length units are defined by the metre, which is why those factors are exact rather than measured. One inch has been exactly 25.4 mm since the 1959 international yard and pound agreement.

Weight (mass)

| From | To | Multiply by | | --- | --- | --- | | pounds | kilograms | 0.45359237 (exact) | | ounces | grams | 28.349523125 | | kilograms | pounds | 2.20462 | | metric tonne | kilograms | 1000 |

Note the word metric tonne. The converter's "t" is 1,000 kg. A US short ton is 907.18 kg and a UK long ton is 1,016.05 kg, so a freight quote that says "ton" needs clarification before you convert anything.

Volume — read the label

| From | To | Multiply by | | --- | --- | --- | | US gallon | litres | 3.785411784 | | US quart | litres | 0.946352946 | | US pint | litres | 0.473176473 | | cup (tool default) | litres | 0.24 |

Volume is where converters disagree most, because unit names collide:

• Gallons, quarts, and pints here are US customary. An imperial gallon is 4.54609 litres — about 20% larger. If a UK source says "gallon", converting it as a US gallon understates the amount.
• The cup is 240 ml, the US legal cup used on nutrition labels. A US customary cup is about 236.6 ml and a metric cup is 250 ml. For a recipe scaled eight cups at a time, that difference is real.
• Fluid ounces are not on the list, deliberately. A fluid ounce is volume (29.57 ml in the US, 28.41 ml in the UK) while an ounce is weight. Mixing them up is the single most common kitchen conversion error.

Temperature

Temperature is the one category that isn't a simple multiplication, because the scales have different zero points:

• Celsius to Fahrenheit: F = C × 9/5 + 32
• Fahrenheit to Celsius: C = (F − 32) × 5/9
• Celsius to Kelvin: K = C + 273.15
• Kelvin to Celsius: C = K − 273.15

Quick mental check: double the Celsius value and add 30. At 10 °C that gives 50 °F, which is exact. At 100 °C it gives 230 instead of 212, so treat it as an estimate and use the tool when precision matters. Also note that Kelvin has no degree symbol and no "degrees Kelvin" — it's just kelvin, from absolute zero.

Why two converters can disagree

Three legitimate reasons a result differs from another site:

1. Different unit definitions. US versus imperial gallons, 240 versus 250 ml cups, short versus metric tons. No rounding error involved.
2. Rounding at different stages. Converting metres to inches in one hop, versus metres to feet and then feet to inches, can shift the last digit. Convert directly whenever you can.
3. Truncation versus rounding. Cutting 2.205 lb down to 2.2 introduces a 0.2% error. That's harmless for a suitcase, not harmless for a medication dose or a torque spec.

The free converter computes in a single step through a base unit, so you avoid compounding error from chained conversions.

Sanity checks that catch bad answers

Before you trust a number, ask whether the direction makes sense:

• Metres to feet: the number should get bigger, roughly 3×. A 2 m board is about 6.5 ft.
• Kilograms to pounds: bigger, roughly 2.2×. A 70 kg person is about 154 lb.
• Litres to US gallons: smaller, roughly a quarter. A 4 L engine is about 1.06 US gallons.
• Celsius to Fahrenheit: bigger for anything above −40 °C (the two scales cross at −40). If 20 °C came out as 68 °F you're right; if not, check that Celsius is the From unit and Fahrenheit is the To unit.

If a check fails, the usual culprits are a swapped from/to pair — that's what the Swap button is for — or a value entered in the wrong category, such as a weight typed into volume.

Where conversions show up in real work

Print sizing is a frequent one: image specs are usually in pixels, print shops want millimetres, and both get quoted in inches. If you're sizing artwork, pair this guide with print size guidance for QR codes and the social image size reference, which covers pixel dimensions rather than physical units. Being able to move between the two without guessing is what keeps a design from coming back from the printer at the wrong scale.

Related tools

• Unit Converter — the converter used throughout this guide
• Image Compressor — shrink image files locally
• JSON Formatter — format and validate JSON without uploading it