Unit Converter

What is a unit converter and which units does this one cover?

A unit converter turns a measurement into another unit of the same quantity: inches into centimetres, pounds into kilograms, Fahrenheit into Celsius. This one covers six categories. More importantly, it converts your value into every unit of the category at once, so you see the answer and the scale side by side.

• Length: millimetre, centimetre, metre, kilometre, inch, foot, yard, mile and nautical mile.
• Weight: milligram, gram, kilogram, tonne, ounce, pound and stone.
• Temperature: Celsius, Fahrenheit and kelvin.
• Volume: millilitre, litre, cubic metre, US fluid ounce, US legal cup, US gallon and UK gallon.
• Area: square centimetre, square metre, dönüm, hectare, square kilometre, square foot and acre.
• Speed: metres per second, kilometres per hour, miles per hour, knots and feet per second.

I use it most on cross-border e-commerce projects, where product sizes have to appear in both systems. Everything runs in your browser; the values you type never reach a server.

Why do metric and imperial units still live side by side?

The metric system follows decimal logic: 1 kilometre is 1,000 metres and 1 litre is 1,000 millilitres. Imperial and US customary units grew out of history instead; a foot has 12 inches, a mile has 1,760 yards and a pound has 16 ounces. That is why mental conversions in imperial units go wrong more often.

However, the two systems do not stand apart. According to the conversion tables of the US National Institute of Standards and Technology (NIST), the inch equals exactly 2.54 centimetres and the pound exactly 0.45359237 kilograms. In other words, imperial units are fixed multiples of metric ones, and this tool works with those exact definitions.

Everyday life still mixes both, depending on where you are:

• In the United States, recipes, road signs and body weights mostly use customary units.
• In the United Kingdom, road distances stay in miles while shops weigh food in grams and kilograms.
• Screen sizes and wheel rims come in inches almost everywhere.
• Shipping and aviation measure distance in nautical miles and speed in knots.

So the need to convert is not going away; knowing the right factor simply prevents most mistakes before they happen.

Why does temperature conversion work differently?

For length, zero means zero in every unit: 0 metres and 0 inches both describe no length at all. Temperature scales, however, behave differently. Celsius puts its zero at the freezing point of water, Fahrenheit uses another reference, and kelvin starts at absolute zero. Therefore you convert temperature with a shifted formula rather than a single factor.

• °C → °F: multiply by 1.8, then add 32; 25 °C becomes 77 °F.
• °F → °C: subtract 32, then divide by 1.8; 212 °F becomes 100 °C.
• °C → K: add 273.15; 0 °C becomes 273.15 K.

The International Bureau of Weights and Measures (BIPM) sets out the Celsius and kelvin relationship in its SI Brochure. Also note that kelvin takes no degree sign; the correct form is "300 K". Finally, the two common scales meet at −40, where −40 °C equals exactly −40 °F.

The tool still calculates inputs below absolute zero (0 K), but it shows a warning next to the result, because such a temperature cannot exist physically. In practice that warning tells you that you probably picked the wrong source unit.

US gallon, UK gallon or cup: which volume definition applies?

Volume confuses people more than any other category, because units with the same name have different sizes in different countries. A US gallon is 231 cubic inches, or about 3.785 litres. The UK Weights and Measures Act 1985 defines the imperial gallon as exactly 4.54609 litres. The gap is close to 20 percent, so fuel comparisons and imported product labels go wrong quickly when the two get mixed.

Cups come in two flavours too:

• US legal cup (240 ml): the US nutrition labeling rule (21 CFR 101.9) counts one cup as 240 ml. This is also the cup the tool uses.
• US customary cup (about 236.6 ml): the traditional definition in the NIST tables, common in older recipes.

The difference between the two cups is about 1.4 percent; irrelevant for a cake, relevant for a production recipe. Fluid ounces also differ: the tool lists the US fluid ounce, while a UK fluid ounce is one twentieth of a UK pint and therefore slightly smaller.

Finally, keep mass and volume apart. How many grams a cup of flour weighs depends on the density of the flour; a converter only moves between units of the same quantity and cannot turn volume into weight.

How do stone, pounds and kilograms relate for body weight?

In the UK and Ireland many people still state their body weight in stone and pounds. One stone equals 14 pounds under the Weights and Measures Act 1985. One pound, in turn, equals exactly 0.45359237 kilograms. As a result, one stone comes to 6.35029318 kilograms.

• 11 stone is 154 pounds, or 69.85 kg.
• 12 stone 6 pounds is 174 pounds, or about 78.93 kg.
• 80 kg is about 176.37 pounds, or 12 stone 8.37 pounds.

Because the tool works with one unit at a time, you convert a mixed value such as 12 st 6 lb in two steps: first convert 12 stone to pounds (168 lb), then add the 6 pounds and convert 174 lb to kilograms. The same approach applies to heights such as 5 ft 11 in.

If you are converting height and weight for a health check, the BMI calculator already accepts both metric and imperial input, so you can skip the manual step there.

How does a unit converter help e-commerce listings and product feeds?

For a shop that sells abroad, product size is one of the first things buyers check after the price. For example, a customer in the US struggles to picture a 50 cm bag, but 19.685 inches places it instantly. In our e-commerce consulting projects, writing size charts in the target market's units is one of the first fixes we make.

Units matter in data feeds as well, not only on product pages:

• Google Merchant Center: the shipping weight attribute accepts lb, oz, g and kg, and shipping dimensions use cm or inches.
• Structured data: the schema.org unitCode property expects UN/CEFACT codes, such as CMT for centimetre, INH for inch, KGM for kilogram and LBR for pound.
• Shipping and customs paperwork: consistent units for weight and dimensions spare you back and forth with carriers.

How you present measurements next to product images also shapes conversion; I cover that in the article on e-commerce visual language and UX. For the markup itself, the schema generator also helps you build product structured data.

How many decimal places should you keep?

Picking the right unit matters as much as writing the result with sensible precision. In Auto mode the tool rounds according to the size of the number; for a fixed format you use the decimal buttons. Here is the rule of thumb I follow:

• 0 decimals: everyday values such as body weight, room temperature or plot size.
• 2 decimals: product labels, invoices and shipping forms.
• 4 or 6 decimals: technical drawings, production tolerances and scientific work.

Above all, round at the very end. If you round first and multiply afterwards, the error grows: round 1 mile to 1.6 km, multiply by 100 and you get 160 km, while the correct figure is 160.9344 km. For chained conversions, enter the original value in the tool instead of rounding at every step.

Also remember that the tool reports a calculation, not the precision of your measurement. For instance, writing a distance you measured roughly with a tape to six decimals suggests an accuracy that does not exist. If you need a percentage difference between two converted values, the percentage calculator takes it from there.

What can a unit mix-up actually cost?

The best-known example of a unit mix-up is NASA's Mars Climate Orbiter. According to NASA's mission page, ground software worked in English units while the spacecraft software expected metric units. As a result, that mismatch threw off the trajectory, and the orbiter came too close to Mars and disappeared on 23 September 1999.

Your project may not be a spacecraft, but the same error happens on a small scale every day:

• A product page lists centimetres where it means inches, and the customer receives something smaller than expected.
• Someone reads 350 °F in an American recipe as 350 °C, while the right setting is about 177 °C.
• A buyer mixes up gallon definitions and budgets the wrong fuel cost.

The shared fix is simple: put the unit next to every number and treat unit choice as part of the design. That is exactly what UX writing asks for; copy that leaves nothing to guesswork. On a multilingual site, think about units together with language and market, as I explain in the multilingual website SEO guide.