Metric to Imperial Unit Conversions: The Complete Practical Guide

The United States uses imperial units. Most of the world uses metric. Scientists everywhere use metric. British people use a confusing hybrid of both depending on the context. And anyone who moves between countries, reads international recipes, follows a US fitness plan, or works in science and engineering faces unit conversion as a regular practical problem.

This guide provides the formulas, conversion factors, and worked examples you actually need — not just a conversion table to memorise, but enough understanding to sanity-check any conversion you're unsure about.

Why Two Systems Still Exist

The metric system (Système International, or SI) was formalised in France in 1795 and is now maintained internationally by the International Bureau of Weights and Measures (BIPM). It is built on logical base-10 multiples: 10 millimetres in a centimetre, 100 centimetres in a metre, 1000 metres in a kilometre. The relationships are easy to remember and arithmetic is straightforward.

The imperial (US customary) system evolved from historical English units: 12 inches in a foot, 3 feet in a yard, 1760 yards in a mile. There's no logical base — the numbers reflect historical accident rather than design. The US standardised on these units in the early 19th century and infrastructure, industry, and culture became embedded around them.

The UK officially adopted metric in 1965 for most purposes, but road distances are still in miles, pints are still used for beer and milk, and body weight is commonly expressed in stone and pounds. This dual-system reality is why conversion ability remains genuinely useful.

Length and Distance

Core conversion factors

• 1 inch = 2.54 cm (exact, by definition)
• 1 foot = 30.48 cm = 0.3048 m
• 1 yard = 0.9144 m
• 1 mile = 1.60934 km
• 1 cm = 0.3937 inches
• 1 m = 39.37 inches = 3.281 feet = 1.094 yards
• 1 km = 0.6214 miles

Worked examples

• A person is 5 feet 11 inches tall. In cm: (5 × 30.48) + (11 × 2.54) = 152.4 + 27.94 = 180.34 cm
• A running route is 10 km. In miles: 10 × 0.6214 = 6.21 miles
• A room is 15 feet wide. In metres: 15 × 0.3048 = 4.57 m

Quick mental approximations

• Miles to km: multiply by 1.6 (exact factor 1.609)
• Km to miles: multiply by 0.6 (exact factor 0.621)
• Inches to cm: multiply by 2.5 (exact 2.54)
• Feet to metres: divide by 3.28

Weight and Mass

Core conversion factors

• 1 pound (lb) = 453.592 grams = 0.4536 kg
• 1 kilogram = 2.2046 pounds
• 1 ounce = 28.35 grams
• 1 stone = 14 pounds = 6.350 kg
• 1 US ton = 2000 lb = 907.2 kg
• 1 metric tonne = 1000 kg = 2204.6 lb

Worked examples

• A person weighs 75 kg. In pounds: 75 × 2.2046 = 165.3 lb = 11 stone 11.3 lb
• A recipe needs 8 oz of butter. In grams: 8 × 28.35 = 226.8 g
• A package weighs 3.5 lb. In kg: 3.5 × 0.4536 = 1.59 kg

Stone to kg conversion

Stone is used for body weight in the UK and Ireland. To convert: multiply stone by 14 (to get total pounds), then multiply by 0.4536.

• 10 stone = 10 × 14 = 140 lb × 0.4536 = 63.5 kg
• 12 stone 6 = (12 × 14) + 6 = 174 lb × 0.4536 = 78.9 kg

Temperature

The formulas

• °C to °F: (°C × 9/5) + 32, or equivalently: °C × 1.8 + 32
• °F to °C: (°F − 32) × 5/9, or equivalently: (°F − 32) ÷ 1.8

Worked examples

• Body temperature: 37°C = (37 × 1.8) + 32 = 66.6 + 32 = 98.6°F
• Room temperature: 72°F = (72 − 32) × 5/9 = 40 × 0.556 = 22.2°C
• Oven for baking: 180°C = (180 × 1.8) + 32 = 324 + 32 = 356°F (use 350°F in practice)
• Freezing point: 0°C = 32°F; Boiling point: 100°C = 212°F

Reference points to memorise

• −40°C = −40°F (the one temperature where both scales agree)
• 0°C = 32°F (water freezes)
• 20°C = 68°F (comfortable room temperature)
• 37°C = 98.6°F (body temperature)
• 100°C = 212°F (water boils at sea level)

Volume

Core conversion factors

• 1 US fluid ounce (fl oz) = 29.57 mL
• 1 US cup = 236.6 mL (8 fl oz)
• 1 US pint = 473.2 mL (16 fl oz)
• 1 UK pint = 568.3 mL (20 fl oz) — larger than US pint
• 1 US gallon = 3.785 L (4 US quarts)
• 1 UK gallon = 4.546 L
• 1 litre = 0.264 US gallons = 1.76 UK pints

Cooking conversions

• 1 teaspoon (US) = 4.93 mL
• 1 tablespoon (US) = 14.79 mL (3 teaspoons)
• 1 cup (US) = 16 tablespoons = 240 mL (approximately)

Speed

• 1 mph = 1.60934 km/h
• 1 km/h = 0.6214 mph
• 60 mph ≈ 96.6 km/h
• 100 km/h = 62.1 mph (useful for European speed limits)
• 30 mph = 48.3 km/h (common UK town speed limit)
• 70 mph = 112.7 km/h (UK motorway limit)

Area

• 1 sq inch = 6.452 cm²
• 1 sq foot = 929 cm² = 0.0929 m²
• 1 sq yard = 0.836 m²
• 1 acre = 4047 m² = 0.4047 hectare
• 1 sq mile = 2.590 km²
• 1 hectare = 10,000 m² = 2.471 acres
• 1 km² = 100 hectares = 247.1 acres

Real-World Conversion Scenarios

Following a US recipe in metric

A US recipe calls for 2 cups of flour, 1/2 cup of butter, and bake at 350°F for 30 minutes.

• 2 cups flour ≈ 2 × 240 mL = 480 mL (or by weight: 1 cup plain flour ≈ 120–130 g, so 2 cups ≈ 250 g)
• 1/2 cup butter ≈ 120 mL (or by weight: 1 cup butter = 227 g, so 1/2 cup ≈ 113 g)
• 350°F = (350 − 32) × 5/9 = 318 × 0.556 = 176.7°C → use 175°C or 180°C

Reading European road signs

You're driving in France. The speed limit is 130 km/h on the motorway, 80 km/h on secondary roads.

• 130 km/h = 130 × 0.6214 = 80.8 mph
• 80 km/h = 80 × 0.6214 = 49.7 mph

Understanding fuel economy

The US measures fuel economy in miles per gallon (mpg). The UK also uses mpg but with the larger Imperial gallon. Europe uses L/100km.

• US mpg to L/100km: 235.2 ÷ US mpg. So 30 mpg US = 235.2 ÷ 30 = 7.84 L/100km
• UK mpg to L/100km: 282.5 ÷ UK mpg. So 40 mpg UK = 282.5 ÷ 40 = 7.06 L/100km

Common Unit Conversion Mistakes

• Confusing US and UK volume measurements. A US pint, cup, gallon, and fluid ounce are all different from their UK equivalents. Specify which system you mean.
• Using mph-to-km/h as km/h-to-mph. These are reciprocal — mph × 1.6 gives km/h; km/h × 0.6 gives mph. Applying the wrong direction gives a 2.5× error.
• Forgetting the offset in temperature conversion. It's not just a multiplication — you must add or subtract 32. °C × 1.8 alone gives the wrong answer; the +32 is essential.
• Confusing mass and weight in everyday use. Technically, kg measures mass and pounds measure weight (force), but in everyday use they're treated as equivalent. This distinction matters in physics but not in cooking or body weight discussions.

Frequently Asked Questions

Convert any unit instantly with the free ToolVerse AI Unit Converter — handles length, weight, temperature, volume, speed, and area. Related: Percentage Calculator Guide for unit-based percentage calculations.