Metric vs Imperial: The Complete Guide to Unit Conversion

The metric system (SI) is used by almost every country in the world for science, medicine, and daily life. The imperial system — still dominant in the United States and used alongside metric in the UK — uses units like miles, pounds, and Fahrenheit. Understanding how to convert between them is an essential practical skill for travel, cooking, health, and international business.

Length Conversions

The fundamental relationship to memorise: 1 inch = 2.54 centimetres. Everything else follows from this:

• 1 foot = 12 inches = 30.48 cm
• 1 yard = 3 feet = 91.44 cm
• 1 mile = 1,609.34 metres = 1.609 km
• 1 metre = 3.281 feet = 39.37 inches
• 1 kilometre = 0.6214 miles

Quick approximation: 1 km ≈ 0.62 miles, or roughly 5/8 of a mile. To convert km to miles mentally, multiply by 0.6. To go the other way, divide by 0.6 (or multiply by 1.6).

Weight Conversions

• 1 pound = 0.4536 kg
• 1 kilogram = 2.205 pounds
• 1 stone = 14 pounds = 6.35 kg
• 1 ounce = 28.35 grams
• 1 US ton = 2,000 pounds = 907 kg
• 1 metric tonne = 1,000 kg = 2,205 pounds

Quick approximation: to convert kg to pounds, double the kilograms and add 10%. For example, 70 kg × 2 = 140, plus 10% = 154 lbs. The exact answer is 154.3 lbs — close enough for everyday use.

Temperature Conversions

Three scales are in common use: Celsius (°C), Fahrenheit (°F), and Kelvin (K). The formulas:

• °C to °F: multiply by 9/5, then add 32 — e.g. 25°C = (25 × 1.8) + 32 = 77°F
• °F to °C: subtract 32, then multiply by 5/9 — e.g. 98.6°F = (98.6 − 32) × 5/9 = 37°C
• °C to Kelvin: add 273.15 — e.g. 0°C = 273.15 K
• Kelvin to °C: subtract 273.15

Useful reference points: water freezes at 0°C/32°F, body temperature is 37°C/98.6°F, a hot summer day is 35°C/95°F, and water boils at 100°C/212°F.

Volume Conversions

• 1 US fluid ounce = 29.57 mL
• 1 US cup = 8 fl oz = 236.6 mL
• 1 US pint = 2 cups = 473.2 mL
• 1 US gallon = 3.785 litres
• 1 UK gallon = 4.546 litres
• 1 litre = 1.057 US quarts = 0.264 US gallons

Area Conversions

• 1 square foot = 0.0929 square metres
• 1 square metre = 10.764 square feet
• 1 acre = 4,047 square metres = 0.4047 hectares
• 1 hectare = 10,000 square metres = 2.471 acres
• 1 square mile = 2.59 square kilometres

Speed Conversions

• 1 mph = 1.609 km/h
• 1 km/h = 0.621 mph
• 1 knot = 1.852 km/h = 1.151 mph
• 60 mph = 96.56 km/h (close to 100 km/h)

Why the US Still Uses Imperial

The United States officially adopted the metric system in 1975 (the Metric Conversion Act), but the transition was voluntary and never fully implemented for everyday life. Deep cultural familiarity, the cost of changing infrastructure, and industry inertia have kept imperial measurements dominant for everyday use — even as science, medicine, and the military routinely use metric.

Convert Any Unit Instantly

Our unit converters cover length, weight, temperature, volume, area, speed, pressure, energy, and more. Each converter is free, instant, and works on any device — no account or download needed.