🔁 Conversion
Metric vs Imperial: A Practical Guide to Everyday Units
Most of the world uses metric; a few countries stick with imperial. Learn the key conversions for length, weight and temperature you actually meet in daily life — and how to switch between them without the mental gymnastics.
Updated 20 August 2026 · 2 min read
Two measurement systems still share the modern world. Metric — metres, kilograms, Celsius — is used by almost every country and all of science. Imperial (and the closely related US customary system) — feet, pounds, Fahrenheit — hangs on in the United States and, informally, in the UK. Most of us end up straddling both, and the friction shows up in the small moments: a recipe in cups, a road sign in miles, a weather app in the "wrong" degrees.
Length: the everyday conversions
Length is where the two systems collide most often — heights, distances, screen sizes, DIY measurements. The anchors worth memorising:
- 1 inch = 2.54 cm (exactly). This is the one to know — everything else in length builds from it.
- 1 foot = 30.48 cm, so a person 6 ft tall is about 183 cm.
- 1 mile ≈ 1.609 km, and 1 km ≈ 0.621 miles.
For heights specifically, converting between centimetres and feet-and-inches is fiddly to do in your head because it mixes two units — the height converter handles it cleanly. For everything else, the length converter covers millimetres up to miles.
Weight and mass
The workhorse conversion here is kilograms to pounds:
The UK adds a wrinkle: body weight is often given in stone, where 1 stone = 14 lb. So 70 kg is about 11 stone. The weight converter moves between grams, kilograms, pounds, ounces and stone without you having to chain the steps together.
Temperature: the one that catches people out
Temperature isn't a simple multiply-and-go, because the two scales start from different zero points. That's why there's an addition step in the formula:
A couple of reference points anchor the rest: water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F), and body temperature is about 37 °C (98.6 °F). Because the formula is easy to fumble, the temperature converter is the safe bet — and our guide on temperature scales explains where Fahrenheit, Celsius and Kelvin come from.
When to just use a tool
Rough conversions are great for a gut feel — is that 30 °C hot? (yes) — but for anything that has to be right, a recipe, a shipping weight, a medical dose, the small errors in mental maths matter. Keep the anchors above for estimating, and reach for the exact converter when precision counts.
Frequently asked questions
- What's the easiest way to convert kg to pounds in my head?
- Double the kilograms, then add about 10%. So 40 kg → 80, plus 8 → roughly 88 lb (the exact figure is 88.2). It's close enough for everyday estimates; use a converter when you need the precise number.
- Why can't I convert Celsius to Fahrenheit by just multiplying?
- Because the two scales have different zero points — 0 °C is the freezing point of water, but 0 °F is much colder. You have to scale by 9/5 and then add 32 to line the zero points up, which is why the formula has an addition step.