Unit Conversion: The Complete Guide to Metric, Imperial and Everything Between

Unit conversion is one of those practical skills that comes up constantly, in cooking, travel, engineering, science, business, and everyday life. The metric and imperial systems cover the same physical quantities in completely different units, and confusing them has caused everything from recipe disasters to the famous Mars Climate Orbiter crash in 1999 (a $125 million satellite lost because one team used metric units and another used imperial).

This guide covers all the major unit categories with quick reference tables, formulas, and the conversion tricks worth memorising.

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A Brief History: Metric vs Imperial

The metric system (now formalised as SI, Système International d'Unités) was developed in France in the 1790s and adopted internationally over the following two centuries. It is base-10, which makes conversions between units straightforward: 1 metre = 100 centimetres = 1,000 millimetres.

The imperial system evolved from a patchwork of historical English measurements, feet, inches, pounds, gallons, with no consistent mathematical relationship between units. 1 foot = 12 inches. 1 yard = 3 feet. 1 mile = 5,280 feet. There is no logic to it; it is entirely historical convention.

Today, virtually every country uses metric. The United States is the most notable exception, officially using US Customary units (a derivative of imperial) for everyday measurement. The UK is a hybrid, officially metric but with strong imperial persistence in road signs (miles), body weight (stone), and draught beer (pints).

Length Conversions

Base unit: metre (m)

Key conversions to memorise:

• 1 inch = 2.54 cm
• 1 foot = 30.48 cm
• 1 metre ≈ 3.28 feet
• 1 kilometre ≈ 0.621 miles (or: miles × 1.609 = km)
• 1 mile ≈ 1.609 km

Quick trick for km to miles: Multiply by 0.6 (or more precisely, 0.621). For miles to km, multiply by 1.6.

Weight Conversions

Base unit: kilogram (kg)

Key conversions to memorise:

• 1 kg ≈ 2.205 lb
• 1 lb ≈ 0.454 kg
• 1 stone = 14 lb = 6.35 kg
• 1 metric tonne = 1,000 kg ≈ 2,205 lb

Watch out: A metric tonne (1,000 kg) is not the same as a US short ton (907 kg) or a UK long ton (1,016 kg). In international trade contexts, always specify which ton you mean.

Temperature Conversions

Temperature is unique, you cannot simply multiply by a fixed factor because the scales have different zero points.

Quick reference temperatures:

Memory trick for Fahrenheit: A rough approximation, double the Celsius and add 30. So 20°C ≈ (20 × 2) + 30 = 70°F. The actual answer is 68°F, so this is close enough for quick estimates.

Volume Conversions

Volume is where the metric/imperial divide causes the most confusion, especially because the UK and US use different gallon sizes.

Base unit: litre (L)

The UK vs US gallon problem: A UK gallon is about 20% larger than a US gallon. This matters significantly for:

• Fuel economy (mpg is different in the UK and US)
• Recipes (a US recipe calling for 1 gallon of paint will give a different result if you use a UK gallon)
• Industrial quantities (an American company ordering a "gallon" means 3.785 L; a British company means 4.546 L)

Always specify UK or US when gallon quantities matter.

Area Conversions

Base unit: square metre (m²)

Useful comparisons:

• 1 hectare = 2.47 acres
• 1 acre ≈ 0.4 hectares (roughly a standard football pitch)
• A typical UK semi-detached house plot: 200–300 m²
• A football pitch: approximately 7,000 m²

Speed Conversions

Base unit: metres per second (m/s)

Quick conversions:

• mph × 1.609 = km/h
• km/h × 0.621 = mph
• 60 mph = 96.6 km/h (commonly approximated as 100 km/h)
• 100 km/h = 62.1 mph

Data Storage Conversions

Note on binary vs decimal: Storage manufacturers use decimal (1 KB = 1,000 bytes). Operating systems traditionally use binary (1 KiB = 1,024 bytes). This is why a "500 GB" hard drive shows as approximately 465 GB in Windows. Our converter uses decimal (SI standard).

Common Conversion Mistakes

Using US gallons for UK recipes. A US cooking website specifying "1 gallon of water" means 3.785 litres. A UK cook using a UK gallon (4.546 L) is adding significantly more liquid.

Confusing metric tonne, short ton, and long ton. Metric tonne: 1,000 kg. US short ton: 907 kg. UK long ton: 1,016 kg. In commodity trading and logistics, the difference is significant at scale.

Not accounting for DST in time zone conversions. Time zone offset conversions are straightforward, but Daylight Saving Time shifts the offset by one hour, and different countries change their clocks on different dates (or not at all).

Treating stone as a metric unit. Stone (st) is British imperial, 1 stone = 14 lb = 6.35 kg. It is widely used in the UK for human body weight but almost nowhere else in the world.

Frequently Asked Questions

How do I convert km/h to mph? Multiply by 0.621. Or divide by 1.609. So 100 km/h = 62.1 mph. A quick mental approximation: multiply by 0.6 for a close estimate.

How do I convert Celsius to Fahrenheit? Multiply by 1.8 (or 9/5), then add 32. Example: 25°C = (25 × 1.8) + 32 = 77°F.

How many kg in a stone? 1 stone = 6.35 kg. To convert stone to kg, multiply by 6.35. To convert kg to stone, divide by 6.35.

How many litres in a US gallon? 1 US gallon = 3.785 litres. 1 UK gallon = 4.546 litres.

Is a mile longer than a kilometre? Yes. 1 mile = 1.609 km. A kilometre is approximately 62% of a mile.

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