How to Convert Units (Length, Weight, and Temperature)

Convert units without the mental math

Some conversions you can do in your head: a foot is 12 inches, a kilometre is 1,000 metres. But the moment a recipe lists 200 grams of butter, a flight gives the temperature in Celsius, or a furniture page measures a shelf in centimetres, the arithmetic gets fiddly and easy to fumble. That is exactly when a converter earns its keep.

This guide walks through the three conversions people reach for most — length, weight, and temperature — using The Toolbox converters. Each one runs entirely in your browser, so nothing you type is uploaded, there is no sign-up, and you get an answer the instant you type a number. The same approach extends to area and volume too. If you would rather memorise a few common values, the unit conversion reference collects the ones worth knowing by heart.

How to convert units online

The pattern is the same for every measurement type: pick the tool, choose your two units, type a value, read the result. Here is each one in detail.

Convert length (metres, feet, inches, miles)

1. Open the length converter. It loads with a "from" unit and a "to" unit already selected.
2. Set the from unit to what you have — say, centimetres for a measurement off a tape.
3. Set the to unit to what you want — inches, for example.
4. Type your number into the "from" field. The converted value appears immediately; you do not press a button or submit anything.
5. To go the other direction, swap the two units (or type into the "to" field instead). A 30 cm shelf reads as roughly 11.8 inches.

This covers millimetres up through kilometres and miles, so it handles both everyday measuring and map distances.

Convert weight (grams, kilograms, pounds, ounces)

1. Open the weight converter.
2. Choose the from unit — grams if you are reading a metric recipe.
3. Choose the to unit — ounces or pounds for US measurements.
4. Type the value. 200 grams shows as about 7.05 ounces; 1 kilogram is roughly 2.2 pounds.
5. Reverse it the same way to turn pounds back into kilograms when you need metric.

Strictly speaking these are units of mass rather than weight, but for kitchen and shipping use the everyday labels work fine.

Convert temperature (Celsius, Fahrenheit, Kelvin)

1. Open the temperature converter.
2. Set the from unit to your source scale — Celsius from a weather app, say.
3. Set the to unit to Fahrenheit.
4. Enter the temperature. 20 C becomes 68 F; an oven at 180 C is about 356 F.
5. Switch the units to convert the other way, or add Kelvin when you need the scientific scale (0 C is 273.15 K).

Temperature is the one to use a tool for rather than doing in your head, because it is not a simple multiply — Fahrenheit involves both a scale factor and a 32-degree offset, which is where mental math usually goes wrong.

Going further: area and volume

Once you are comfortable with the three above, the rest work identically. Use the area converter for square metres, square feet, acres, and hectares — handy for floor plans or land sizes. Use the volume converter for litres, gallons, cups, and millilitres, which is what you want for fuel, drinks, and larger recipe quantities.

Tips

• Copy the result, don't retype it. Re-typing a long decimal is where errors sneep in. Select the output and copy it so the value stays exact.
• Decide how precise you actually need to be. A converter may show several decimal places, but a recipe rarely needs more than one. Round to a sensible figure for the task rather than copying every digit.
• Watch US vs UK gallons and cups. A US gallon and an Imperial gallon are different sizes, and US cups differ from metric cups. If your converter lists both, pick the one that matches your source.
• Mind the direction. The most common mistake is converting the wrong way. Double-check which unit is "from" and which is "to" before trusting the number — especially with temperature, where 20 and 68 look nothing alike.
• Bookmark the one you use most. If you constantly convert recipes, keep the weight and temperature converters one click away.

FAQ

Do these converters work offline or send my data anywhere? The calculation runs locally in your browser using built-in conversion factors, so the numbers you type are not uploaded and there is no account to create. You do need an internet connection to load the page the first time, but the conversion itself happens on your device.

Why does converting back and forth sometimes give a slightly different number? That is rounding. If you convert 1 kg to pounds, round the result, then convert that rounded figure back, you can land a hair off the original. Keep more decimal places during intermediate steps, or convert directly from your true source value rather than chaining conversions.

What is the easiest way to convert an oven temperature for a recipe? Open the temperature converter, set "from" to the recipe's scale (often Celsius) and "to" to your oven's scale (often Fahrenheit), then type the number. Many ovens also round to the nearest 5 degrees, so a converted 356 F is fine to set as 355 F.

Can I convert between metric and Imperial in one step? Yes. Each tool lists both metric and Imperial units in the same dropdowns, so you can go straight from centimetres to inches or grams to ounces without an intermediate conversion.

Need a different measurement? The length, weight, and temperature converters sit alongside area, volume, and more in The Toolbox converter set.