Unit conversion made simple: metric, imperial, temperature and the Unit Converter

Recipes written in cups, a flight measured in miles, a parcel weighed in pounds, a weather report in Fahrenheit: most of the world uses the metric system, a few countries still use imperial units, and everyone meets both on the internet. Converting between them in your head is error-prone, and one wrong factor can ruin a cake or a shipment. The free Unit Converter converts length, weight, volume and temperature instantly. This guide explains how conversions work, the exact factors the tool relies on, and the traps that give wrong answers even with a calculator.

What the tool converts

Category	Units
Length	millimetre, centimetre, metre, kilometre, inch, foot, yard, mile
Weight	milligram, gram, kilogram, tonne, ounce, pound, stone
Volume	millilitre, litre, US teaspoon, US tablespoon, US cup, US pint, US gallon
Temperature	Celsius, Fahrenheit, Kelvin

You pick a category, type a value and choose the units to convert from and to. The result updates as you type, and a list underneath shows the same value in other units of that category, so converting 10 kilometres also shows it in miles, yards, feet and more at once. Numbers are shown with up to eight significant digits.

How a conversion works

Every unit in a category is defined as a multiple of one base unit. To convert, you multiply by the factor to get to the base unit, then divide by the factor of the target unit. For length the base unit is the metre:

• 1 kilometre = 1,000 metres
• 1 mile = 1,609.344 metres
• So 10 km = 10,000 m, and 10,000 ÷ 1,609.344 = 6.2137 miles.

The tool does exactly this, so it works between any pair of units, not just the ones you thought of. The factors it uses are exact definitions, not rounded approximations:

Relationship	Exact value
1 inch	2.54 centimetres
1 foot	0.3048 metres
1 yard	0.9144 metres
1 mile	1,609.344 metres
1 pound	0.45359237 kilograms
1 ounce	0.028349523125 kilograms
1 stone	6.35029318 kilograms (14 pounds)
1 US gallon	3.785411784 litres
1 US cup	236.5882365 millilitres

Since 1959 the inch, foot, yard, mile and pound have all been defined in terms of metric units, so these factors are not estimates.

Quick mental checks

When you can't use a tool, these shortcuts are close enough for sanity checks:

• Kilometres to miles: multiply by 0.6. So 10 km is about 6 miles (exactly 6.21).
• Miles to kilometres: multiply by 1.6. A marathon of 26.2 miles is about 42.2 km.
• Kilograms to pounds: double it and add 10%. 80 kg is about 176 lb (exactly 176.4).
• Inches to centimetres: multiply by 2.5. So 6 inches is about 15 cm.
• Litres to US gallons: divide by about 4 (exactly 3.785).

A good habit: do the quick estimate first, then check it against the tool. If they disagree widely, you probably typed the wrong unit.

Temperature is different

Length, weight and volume scales all start at zero, so a single multiplier works. Temperature scales do not share a zero. Zero degrees Celsius is 32 degrees Fahrenheit and 273.15 kelvin, so you need a formula with an offset as well as a multiplier:

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

Some examples you can check in the converter:

Celsius	Fahrenheit	Kelvin
−40	−40	233.15
0	32	273.15
22.2	72	295.4
37 (body)	98.6	310.15
100 (boiling water at sea level)	212	373.15
176.7	350	449.8

The last row is why an oven set to 350 °F in an American recipe is about 177 °C, usually rounded to 175 °C or 180 °C. Note also that −40 is the one temperature where Celsius and Fahrenheit are equal.

A frequent mistake is converting a temperature difference with the full formula. A rise of 10 °C is a rise of 18 °F (just the multiplier), not 50 °F, because the offset of 32 only applies to a position on the scale, not to a change.

The gallon trap

The tool uses US sizes for volume, labelled as such. That matters because the US and the UK (imperial) systems share names but not sizes:

Unit	US	UK (imperial)
Gallon	3.785 litres	4.546 litres
Pint	473 ml	568 ml
Fluid ounce	29.6 ml	28.4 ml

A UK gallon is about 20% larger than a US gallon, so fuel economy figures such as "miles per gallon" differ by that much between the two countries. The same 40 mpg car is about 33 mpg in US terms. Cooking has the same issue: an American cup is 236.6 ml, but a "metric cup" in Australia and Canada is 250 ml, and recipes rarely say which they mean. For baking, weighing ingredients in grams is more reliable than measuring volume in any system.

Weight has its own trap: tonne (metric, 1,000 kg) is not the same as the US short ton (907 kg) or the UK long ton (1,016 kg). The tool offers the metric tonne.

Using the tool

1. Open the Unit Converter.
2. Choose a category: Length, Weight, Volume or Temperature.
3. Enter the value and choose the units to convert from and to.
4. Read the result. Use the list below it to see other units at a glance.

Once the page has loaded it needs no connection, and nothing you enter is sent anywhere.

Real situations

• Travel. Road signs, speeds and distances in miles; temperatures in Fahrenheit; petrol in gallons.
• Cooking. Convert cups and tablespoons to millilitres, and oven temperatures between scales.
• Shopping online. Clothing and furniture sizes in inches; shipping weights in pounds.
• Fitness and health. Body weight in stone and pounds or in kilograms, heights in feet and inches. For a health ratio calculated from them, use the BMI Calculator.
• Study and engineering. Science works in metric, textbooks and datasheets sometimes do not.
• Web and design. Printing sizes in inches versus screen sizes in pixels; the CSS units lesson explains how px, em, rem, cm and in relate on a page.

Rounding and precision

Show enough digits for the job. For cooking, whole millilitres are plenty. For a science assignment, keep the significant figures your data supports; a length measured as 5.2 m cannot honestly become 17.0603674 feet. Convert at the end of a calculation, not in the middle, to avoid rounding errors piling up. And remember that tiny differences between this tool and another converter come from rounding, not mistakes.

Common mistakes

• Using the wrong gallon, pint or ton.
• Applying a multiplier to a temperature instead of the full formula.
• Converting between area or volume units with a length factor: 1 metre is about 3.28 feet, but 1 square metre is about 10.76 square feet, because you must square the factor.
• Mixing up weight (pounds, kilograms) and volume (litres) for ingredients; a litre of flour does not weigh a kilogram.
• Rounding too early.

Related tools

Work out percentages of a measurement with the Percentage Calculator, or convert a number to words for a written quantity with the Number to Words Converter. If you are scaling a recipe or a bill, the Discount & Profit Margin Calculator handles the money side. For dates and durations rather than distances, see the Age & Date Difference Calculator.

Quick answers

Are the conversions exact? They use exact or internationally agreed factors and show up to eight significant digits.

Does it use US or UK gallons? US. UK gallons are about 20% larger.

Why can't I just multiply to convert temperature? The scales have different zero points, so conversion needs an offset.

Does it work offline? Once the page has loaded, yes.

Which is more accurate, miles to km by 1.6 or by 1.609? 1.609 is closer; the tool uses 1.609344, the exact value.