Metric vs Imperial:
The Complete Unit Conversion Guide (2026)

Why Two Systems Still Exist in 2026

The metric system (officially the International System of Units, or SI) was formalised in France in 1795 and adopted worldwide over the following two centuries. Today, only three countries officially use the imperial system as their primary standard: the United States, Liberia, and Myanmar.

But in practice, imperial lives on everywhere. The UK uses miles on road signs but kilograms at the supermarket. Aviation uses feet for altitude globally. Shipping uses nautical miles. Screen sizes are universally measured in inches. A "pint" means different volumes in the US vs the UK. Understanding both systems — and converting between them fluently — is a genuinely useful real-world skill.

📏 Length Conversions

The most common daily conversions involve distance and height. The key anchor: 1 inch = 2.54 cm exactly (defined by international treaty in 1959). Everything else derives from this.

From	To	Multiply by	Quick check
1 inch	centimetres	× 2.54	1 in = 2.54 cm
1 foot	metres	× 0.3048	1 ft ≈ 30.5 cm
1 yard	metres	× 0.9144	1 yd ≈ 91.4 cm
1 mile	kilometres	× 1.609344	5 miles ≈ 8 km
1 metre	feet	× 3.28084	1 m ≈ 3 ft 3 in
1 kilometre	miles	× 0.621371	10 km ≈ 6.2 miles
1 nautical mile	kilometres	× 1.852	1 nmi = 1.852 km

Memory trick for miles ↔ km: The Fibonacci sequence — 1, 1, 2, 3, 5, 8, 13, 21 — gives approximate mile–km pairs. 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km. Not exact, but close enough for mental maths.

⚖️ Weight & Mass Conversions

Weight causes the most confusion between countries. The US uses pounds and ounces for body weight. The UK uses stones and pounds. Most of the rest of the world uses kilograms. The anchor: 1 pound = 0.45359237 kg exactly.

From	To	Multiply by	Quick check
1 pound (lb)	kilograms	× 0.4536	10 lbs ≈ 4.5 kg
1 kilogram	pounds	× 2.2046	1 kg ≈ 2.2 lbs
1 ounce (oz)	grams	× 28.35	1 oz ≈ 28 g
1 stone (st)	kilograms	× 6.350	10 st ≈ 63.5 kg
1 US ton	kilograms	× 907.18	1 US ton ≈ 0.907 t
1 metric tonne	pounds	× 2204.6	1 t ≈ 2,205 lbs

🌡️ Temperature Conversions

Temperature is the trickiest because it requires addition as well as multiplication. There are four scales in common use.

Conversion	Formula	Example
°C → °F	(°C × 9/5) + 32	100 °C = 212 °F
°F → °C	(°F − 32) × 5/9	98.6 °F = 37 °C
°C → K	°C + 273.15	0 °C = 273.15 K
K → °C	K − 273.15	300 K = 26.85 °C
°F → K	(°F + 459.67) × 5/9	32 °F = 273.15 K
°C → °R	(°C + 273.15) × 9/5	0 °C = 491.67 °R

Useful anchors to memorise:

• Water freezes at 0 °C / 32 °F / 273.15 K
• Body temperature: 37 °C / 98.6 °F
• Water boils at 100 °C / 212 °F
• A warm summer day: 30 °C ≈ 86 °F
• –40 °C = –40 °F (the crossover point)

🧪 Volume Conversions

Volume is complicated by the fact that US and UK "pints" and "gallons" are different sizes. For cooking and everyday use, the key anchors are:

From	To	Factor	Note
1 US gallon	litres	× 3.785	Common for US fuel
1 imp. gallon	litres	× 4.546	UK fuel, 20% larger
1 US pint	millilitres	× 473	473 mL
1 UK pint	millilitres	× 568	568 mL — bigger!
1 US cup	millilitres	× 237	Standard recipe cup
1 fl oz (US)	millilitres	× 29.57	≈ 30 mL
1 tablespoon	millilitres	× 14.79	≈ 15 mL
1 teaspoon	millilitres	× 4.93	≈ 5 mL

💨 Speed Conversions

Speed conversions matter for travel (mph vs km/h), aviation (knots), and physics (m/s).

From	To	Factor
1 mph	km/h	× 1.60934
1 km/h	mph	× 0.62137
1 m/s	km/h	× 3.6
1 knot	km/h	× 1.852
1 knot	mph	× 1.15078
Mach 1	km/h	≈ 1,235 km/h at sea level

💾 Data Storage Conversions

Digital storage uses powers of 1,024 (binary) — but marketing often uses powers of 1,000 (decimal). This is why a "1 TB" hard drive shows as ~931 GiB in your operating system.

Unit	Bytes (binary)	Bytes (decimal)
1 Kilobyte (KB/KiB)	1,024 B	1,000 B
1 Megabyte (MB/MiB)	1,048,576 B	1,000,000 B
1 Gigabyte (GB/GiB)	1,073,741,824 B	1,000,000,000 B
1 Terabyte (TB/TiB)	1,099,511,627,776 B	1,000,000,000,000 B

🔵 Pressure & Energy Quick Reference

Pro Tips for Unit Conversion

• Always convert via base units. To convert furlongs to nautical miles, convert furlongs → metres → nautical miles. Never multiply two non-base factors directly — you'll introduce rounding errors.
• Temperature offsets matter. You can't just multiply to convert Celsius to Fahrenheit — you must add 32. This is the single most common unit conversion mistake.
• Dimensional analysis saves lives (sometimes literally). Write your units as fractions and cancel them. (5 mi) × (1.609 km / 1 mi) = 8.045 km. If units don't cancel correctly, you've set up the conversion wrong.
• Significant figures. Don't report more decimal places than your input justifies. "1 inch ≈ 2.5400000000 cm" is technically correct but useless noise.
• SI prefixes are multiplicative powers of 1,000. kilo = 10³, mega = 10⁶, giga = 10⁹, tera = 10¹², milli = 10⁻³, micro = 10⁻⁶, nano = 10⁻⁹.

Use the Tool, Not Your Memory

Memorising a handful of key anchors — 1 inch = 2.54 cm, 1 kg ≈ 2.2 lbs, °F = °C × 1.8 + 32 — gets you through most everyday situations. For everything else, there's no shame in using a converter.

Our free Unit Converter covers all 10 categories above with over 100 units, shows every conversion simultaneously, and lets you copy any result with a single click. No ads blocking the results, no signup, no paywalled "premium" units.