Metric vs Imperial: A Practical Comparison (with Conversion Tables)

Most of the planet measures in metric. The United States, Liberia and Myanmar still lean on imperial. If you cook, travel, shop online or read a science paper, you'll hit both — often in the same paragraph.

Why Two Systems Still Exist

Metric (the SI system) was designed in 1790s France around decimals — every unit scales by powers of ten. Imperial grew from British trade weights and Roman measures over a thousand years; the units are practical but inconsistent. The US adopted the British system before the metric switch happened in the UK, and the cost of recalibrating every road sign, recipe book and factory has kept it there ever since.

Length

• 1 inch = 2.54 cm (exact)
• 1 foot = 30.48 cm
• 1 yard = 0.9144 m
• 1 mile = 1.609 km
• Quick rule: miles × 1.6 = km, or km × 0.62 = miles.

Weight

• 1 ounce = 28.35 g
• 1 pound = 453.59 g
• 1 stone = 6.35 kg (UK bodyweight unit)
• Quick rule: kg × 2.2 = lb, or lb ÷ 2.2 = kg.

For more units (mg, metric tons, short tons, long tons), use the Weight Converter.

Volume

Volume is where US and UK imperial diverge:

• 1 US fluid ounce = 29.57 mL · 1 UK fluid ounce = 28.41 mL
• 1 US pint = 473 mL · 1 UK pint = 568 mL
• 1 US gallon = 3.785 L · 1 UK gallon = 4.546 L

Temperature

Celsius and Fahrenheit don't scale linearly the way length and weight do — they have different zero points. The formula:

• °F = (°C × 9/5) + 32
• °C = (°F − 32) × 5/9
• Mental shortcut: double °C and add 30 for a rough °F (works between 5 °C and 30 °C).

Which to Use, When

• Science, medicine, engineering — metric, always.
• Cooking — match the recipe's origin; converting volumes between US and UK is easy to get wrong.
• Travel — learn the local one; conversion apps drain battery and patience.

Frequently Asked Questions