Unit Conversion Basics: Metric vs Imperial
The world uses two main measurement systems: the metric system (used in almost every country) and the imperial system (still dominant in the United States, with some legacy use in the UK). Knowing how to convert between them is useful for travel, cooking, fitness, construction, and science — basically every part of daily life.
This guide covers the six most common conversion categories, shows the exact factors you need, and gives you mental shortcuts so you do not have to reach for a calculator every time. When you do need one, the Unit Converter on this site handles all of these automatically.
✨Key takeaways
- Every conversion is just multiplication — except temperature, which needs an offset.
- 1 inch = 2.54 cm, 1 kg = 2.2046 lb, 1 mile = 1.609 km. Memorise three, derive the rest.
- °F = °C × 9/5 + 32, and °C = (°F − 32) × 5/9.
- Mental shortcut: multiply metric by a rough integer and adjust — e.g. km × 0.6 ≈ miles.
Length
Exact factor: 1 inch = 2.54 cm exactly. Everything else follows.
12 inches = 1 foot = 30.48 cm. 3 feet = 1 yard = 0.9144 m. 5,280 feet = 1 mile = 1,609.344 m = 1.609 km.
Worked example: convert 5 feet 10 inches to centimetres. 5 ft = 152.4 cm, 10 in = 25.4 cm, total = 177.8 cm.
Mental shortcut: km × 0.62 ≈ miles. So 100 km ≈ 62 miles — useful for European road signs.
Weight (mass)
1 kilogram = 2.2046 pounds. 1 pound = 0.4536 kg. 1 ounce = 28.35 g.
Worked example: a 160 lb person in kg. 160 ÷ 2.2046 ≈ 72.6 kg.
Mental shortcut: kg × 2, then add 10%. 70 kg → 140 + 14 = 154 lb. Close enough for most situations.
Worth noting: the stone is still used in the UK for body weight. 1 stone = 14 lb = 6.35 kg. So "12 stone 6" = 174 lb ≈ 79 kg.
Temperature (the one that needs an offset)
Every other conversion in this guide is a simple multiplication. Temperature is special because the two scales do not share a zero.
°F → °C: subtract 32, multiply by 5/9. Example: 77°F → (77 − 32) × 5/9 = 25°C.
°C → °F: multiply by 9/5, add 32. Example: 20°C → 20 × 1.8 + 32 = 68°F.
Mental shortcut for going °C → °F: double it and add 30. 20°C → 70°F (actual: 68°F). Good enough to decide what to wear.
Kelvin is simply °C + 273.15. Used in science, never in weather forecasts.
Volume
1 US gallon = 3.785 litres. 1 US cup = 236.6 ml. 1 US fluid ounce = 29.57 ml.
The UK gallon (imperial gallon) is larger at 4.546 litres. Be careful with older British recipes and UK fuel prices.
Worked example: 1 gallon of petrol in litres. 1 US gal = 3.785 L. So $4/gal is roughly $1.06/L.
Mental shortcut: litres × 0.26 ≈ US gallons. 50 L of fuel ≈ 13 gallons.
Area
1 square metre = 10.764 square feet. 1 acre = 4,046.86 square metres = 0.4047 hectares.
Worked example: convert a 1,500 sq ft apartment to square metres. 1500 ÷ 10.764 ≈ 139.4 m².
Remember: area scales with the square of length. If you double a length, you quadruple the area. Missing this trips people up when buying tiles or paint.
Speed
1 mph = 1.609 km/h. 1 km/h = 0.621 mph.
Worked example: a 70 mph US highway speed in km/h. 70 × 1.609 ≈ 112.6 km/h.
Mental shortcut: km/h × 0.6 ≈ mph. So 100 km/h ≈ 60 mph, 60 km/h ≈ 36 mph.
Knots (nautical miles per hour) still show up in aviation and sailing. 1 knot ≈ 1.852 km/h ≈ 1.151 mph.
Putting it all together
A running mental model: for everyday conversions, most people only need length, weight, and temperature. Memorise 2.54, 2.2, and the "double-and-add-30" trick, and you will handle 90% of conversions without a phone.
For the other 10% — recipes across systems, machine-shop tolerances, overseas property listings — use the Unit Converter and save yourself the mental load.
Try the calculators referenced in this guide
Put the maths into practice — every calculator is free and runs entirely in your browser.
Frequently Asked Questions
Which countries still use the imperial system?
Primarily the United States. Liberia and Myanmar still use some imperial units. The UK uses a mix — officially metric, but miles, pints, and stones are still widespread in daily life.
What is the easiest way to convert a recipe?
Weigh the ingredients in grams whenever possible. Kitchen scales are cheap and a recipe converted by weight is far more forgiving than one converted by volume, especially for baking.
Are there any cases where mental conversion is dangerous?
Medication dosing, engineering tolerances, and aviation fuel — always use precise conversion factors. The 1983 "Gimli Glider" 767 ran out of fuel partly because of a kg/lb confusion. When stakes are high, use a calculator.
Why does the UK use a different gallon?
The imperial gallon was standardised in 1824 based on the volume of 10 lb of water. The US gallon derives from the older "Wine Gallon" of 231 cubic inches. They have simply never been the same size.
Continue Reading
How to Calculate Your BMI Accurately (With Worked Examples)
The full BMI formula explained, worked examples in both metric and imperial units, the official healthy ranges, and what BMI actually misses.
Read guide →Understanding Compound Interest: A Complete Guide
The compound interest formula demystified — with clear examples showing how starting early, compounding frequency, and regular contributions change outcomes.
Read guide →Mortgage Calculator Explained: Principal, Interest, Taxes & Insurance
Every part of a monthly mortgage payment — PITI, PMI, HOA, and the amortisation schedule — decoded with real numbers and a step-by-step worked example.
Read guide →