Metric vs Imperial: The Complete Unit Conversion Guide
Most of the world uses metric. The United States uses imperial. The internet serves content from both. The result: anyone who follows American recipes, fitness programmes, or product listings regularly encounters numbers that don't match the measurement system they grew up with.
This guide covers the conversions you'll actually need — not every obscure unit, just the ones that come up in daily life.
Weight: KG and LBS
The key relationship: 1 kg = 2.20462 lbs. For quick mental arithmetic, multiply by 2.2.
Common conversions:
- 50 kg = 110 lbs
- 70 kg = 154 lbs
- 80 kg = 176 lbs
- 100 kg = 220 lbs
When you need this: Reading US fitness programmes that prescribe "135 lbs on the bar" (61 kg). Checking your weight on an American health app. Shopping for imported food products where serving sizes are in ounces. Converting a recipe that requires "1 lb of chicken" to grams (453 g).
Note: the UK still uses stone informally for body weight. 1 stone = 14 lbs = 6.35 kg. So "12 stone 4" = (12 × 14) + 4 = 172 lbs = 78 kg.
Distance: KM and Miles
The key relationship: 1 km = 0.621 miles. Or: 1 mile = 1.609 km.
Common conversions:
- 5 km = 3.1 miles (common 5K race)
- 10 km = 6.2 miles
- 21.1 km = 13.1 miles (half marathon)
- 42.2 km = 26.2 miles (marathon)
- 100 km = 62.1 miles
When you need this: Following a running programme written in miles. Reading UK road signs (in miles) while used to km. Understanding American driving distances quoted in miles.
Length and Height: CM and Inches/Feet
The key relationship: 1 inch = 2.54 cm. So 1 foot (12 inches) = 30.48 cm.
Common height conversions:
- 5'0" = 152.4 cm
- 5'6" = 167.6 cm
- 5'10" = 177.8 cm
- 6'0" = 182.9 cm
- 6'2" = 187.9 cm
When you need this: Clothing size charts (waist and inseam in inches). Screen sizes (always in inches diagonally). Height on US visa forms. Furniture dimensions from American manufacturers.
Temperature: Celsius and Fahrenheit
The formulas:
- °C to °F: multiply by 1.8, then add 32
- °F to °C: subtract 32, then multiply by 5/9
Key reference points:
- 0°C = 32°F (water freezes)
- 20°C = 68°F (comfortable room temperature)
- 37°C = 98.6°F (normal body temperature)
- 100°C = 212°F (water boils)
- 180°C = 356°F (moderate oven — common in baking)
- 200°C = 392°F ≈ 400°F (hot oven)
When you need this: American recipes specifying oven temperatures in Fahrenheit. Weather forecasts when travelling to the US. Medical references to fever thresholds.
Volume: Cups, Tablespoons, and ML
US cooking uses cups and tablespoons. Everyone else uses ml and grams.
- 1 cup = 240 ml (rounded; exact is 236.6 ml)
- ½ cup = 120 ml
- ¼ cup = 60 ml
- 1 tablespoon = 15 ml
- 1 teaspoon = 5 ml
- 1 US fluid ounce = 29.6 ml
When you need this: Following any American recipe. Scaling recipes between cup-based and metric systems. Understanding nutrition labels that list serving sizes in fluid ounces.
⚠️ Baking note: Cups are volume measurements, not weight. A cup of flour weighs approximately 125 g. A cup of sugar weighs approximately 200 g. A cup of water weighs exactly 240 g. For precise baking results, convert cups to grams using ingredient-specific conversion charts — don't assume equal weights.
Quick Mental Shortcuts
For when you need a rough conversion without a calculator:
- Km to miles: divide by 1.6 (or multiply by 0.6)
- Kg to lbs: multiply by 2.2
- Cm to inches: divide by 2.5
- °C to °F (rough): double it and add 30 (accurate within a couple of degrees for most everyday temperatures)
- Litres to US gallons: divide by 3.785 (roughly: quarter it)
Frequently Asked Questions
Which countries use the imperial system?
The United States is the only major country that has not officially adopted the metric system for everyday use. The UK officially uses metric but retains imperial units informally for road distances (miles), draught beer (pints), and personal height and weight in everyday conversation. Most other countries use metric exclusively.
How do I convert kg to lbs?
Multiply kilograms by 2.20462. So 70 kg = 70 × 2.20462 = 154.3 lbs. For a quick approximation, multiply by 2.2.
How do I convert Celsius to Fahrenheit?
Multiply the Celsius temperature by 1.8 (or 9/5) and then add 32. So 25°C = (25 × 1.8) + 32 = 77°F. To go the other direction, subtract 32 and multiply by 5/9.
How many ml in a cup?
1 US cup = 236.6 ml (usually rounded to 240 ml for recipes). 1 tablespoon = approximately 15 ml. 1 teaspoon = approximately 5 ml. Note: Australian and UK tablespoons differ slightly from US measurements.