In this article
A recipe from abroad asks for a quart. A property listing gives the plot in acres and your form wants square feet. A parcel is priced by the pound. Every one of these is a single multiplication, so why do so many people end up with the wrong number?
The Unit Converter lets you pick a category, choose the unit you have and the unit you want, type a number and read the answer straight away. This guide gives the numbers worth remembering, a few worked examples, and the mistakes that cause trouble.
What the tool converts
| Category | Units |
|---|---|
| Length | Millimetre, centimetre, metre, kilometre, inch, foot, yard, mile |
| Weight | Milligram, gram, kilogram, ton, ounce, pound |
| Temperature | Celsius, Fahrenheit, Kelvin |
| Area | Square metre, square kilometre, square foot, acre, hectare |
| Volume | Millilitre, litre, cubic metre, cup, pint, quart, gallon |
Need more units, such as quintal and tola for weight, or teaspoons and fluid ounces for volume? The dedicated Length, Weight, Area and Volume converters on Toolcloak cover those.
Results show up to eight decimal places, with trailing zeros removed, so a clean answer like 2.54 stays tidy and a long one is still precise. For more units in one category, try the Length Converter, Weight Converter, Area Converter and Volume Converter.
The conversion factors worth knowing
| You have | Multiply by | To get | Check |
|---|---|---|---|
| Inches | 2.54 | Centimetres | Exact by definition |
| Feet | 0.3048 | Metres | Exact by definition |
| Miles | 1.609344 | Kilometres | Roughly 1.6 |
| Pounds | 0.4536 | Kilograms | Roughly 0.45 |
| Ounces | 28.35 | Grams | About 28 g |
| Square feet | 0.0929 | Square metres | 10.76 sq ft = 1 sq m |
| Acres | 0.4047 | Hectares | 1 hectare = 2.47 acres |
| US gallons | 3.785 | Litres | About 3.8 |
Rounded to four significant figures where marked. Inches, feet and miles are defined exactly in metric.
Three quick ratios cover most everyday checks. A mile is about 1.6 km. A kilogram is about 2.2 pounds. An inch is 2.54 cm, so four inches is just over ten centimetres. If the answer from any tool is wildly off from these, you picked the wrong unit.
Worked examples
- 1Turn it all into inches5 ft × 12 = 60, plus 10 = 70 in.
- 2Multiply by 2.5470 × 2.54 = 177.8.
- 3Answer177.8 cm.
Feet and inches are two units, so combine them first before converting.
- 100 km in miles: 100 ÷ 1.609344 = 62.14 miles.
- 70 kg in pounds: 70 ÷ 0.4536 = 154.3 lb.
- 1 acre in square feet: 4,046.86 ÷ 0.0929 = about 43,560 sq ft.
- 2 US gallons in litres: 2 × 3.785 = 7.57 litres.
Trap 1: two different gallons
The US gallon is about 3.785 litres. The UK (imperial) gallon is about 4.546 litres, which is roughly 20% bigger. Pints, quarts and cups have the same split, because they are fractions of the gallon. The Unit Converter uses the US measures. If a British recipe, a petrol price or a fuel-economy figure uses imperial units, a US conversion will be about a fifth too low.
Same name, different size. Always check which country’s unit a recipe or a spec sheet means.
Trap 2: three different tons
“Ton” is slippery. In the converter it means the metric ton (tonne), which is exactly 1,000 kg. In the US a ton usually means the short ton of 2,000 pounds, about 907 kg. In older British usage there is also the long ton of 2,240 pounds, about 1,016 kg. A shipping figure can be out by 10% depending on which one was meant.
When money or safety depends on the weight, write out the unit in full, for example “tonne” or “short ton”.
Trap 3: areas scale by the square
One metre is about 3.28 feet, but one square metre is about 10.76 square feet, not 3.28. Areas multiply a length by a length, so the factor is squared (3.28 × 3.28 ≈ 10.76). Volumes are cubed. People who convert an area with a length factor get an answer more than three times too small.
Temperature is different
Temperature is the one category where multiplying alone is not enough, because the zero points differ. To go from Celsius to Fahrenheit, multiply by 9/5 and add 32. To get Kelvin, add 273.15. Also note that 20 °C is not “twice as hot” as 10 °C, because the scale starts at an arbitrary point. The full story, including the one temperature where Celsius and Fahrenheit agree, is in Celsius, Fahrenheit and Kelvin.
| Celsius | Fahrenheit | Kelvin | What it is |
|---|---|---|---|
| 0 °C | 32 °F | 273.15 K | Water freezes |
| 20 °C | 68 °F | 293.15 K | A mild room |
| 37 °C | 98.6 °F | 310.15 K | Normal body temperature |
| 100 °C | 212 °F | 373.15 K | Water boils at sea level |
Fahrenheit = Celsius × 9/5 + 32. Kelvin = Celsius + 273.15.
Habits that prevent mistakes
- Convert one thing at a time. Combine feet and inches into one unit first, then convert.
- Check the direction. If you are going from a small unit to a big one, the number should get smaller.
- Round at the end. Keep all the digits during the sum and round only the final answer.
- Match the category. Length for lengths, Area for areas, Volume for volumes.
- Check the country. Gallons, pints, cups and tons differ between the US and the UK.
- For data sizes, use a different tool. Kilobytes and kibibytes are not the same, as explained in why a 1 TB drive shows 931 GB.
For kitchen measures where cups and spoons have to become grams, the volume of a cup of flour is not the same weight as a cup of sugar, so use the Cooking Measurement Converter, which accounts for typical ingredient densities.
See also: cm to feet and inches, with a height chart, how a gold jewellery bill is worked out.
Questions people ask
How do I convert miles to kilometres?
Multiply by 1.609344. A quick check is that a mile is about 1.6 km, so 10 miles is about 16 km. In the Unit Converter, choose Length, set From to Mile and To to Kilometer.
How do I convert kg to pounds?
Multiply kilograms by about 2.2046, or divide by 0.4536. For 70 kg the answer is about 154.3 lb. The Weight category does this instantly.
How many square feet are in an acre?
One acre is about 43,560 square feet, or roughly 4,047 square metres. In the converter choose Area, then Acre to Square Foot.
Is a US gallon the same as a UK gallon?
No. A US gallon is about 3.785 litres and a UK (imperial) gallon is about 4.546 litres. The Unit Converter uses US measures, so adjust if your source uses imperial units.
What does ton mean in the converter?
It means the metric ton (tonne), which is 1,000 kilograms. A US short ton is about 907 kg and a UK long ton is about 1,016 kg, so check which one your source means.
Why can’t I convert area with a length factor?
Because area is length times length, so the factor is squared. One metre is 3.28 feet, but one square metre is about 10.76 square feet.
How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5 and add 32. For example 37 °C × 1.8 + 32 = 98.6 °F. Choose Temperature in the converter and it does the sum for you.
Which units are in the Unit Converter?
Length, weight, temperature, area and volume, covering metric and common imperial and US units such as miles, feet, inches, pounds, ounces, acres, gallons and cups.