In 1999 NASA lost the Mars Climate Orbiter — worth £327 million — because one engineering team used metric units and another used imperial. The mismatch went undetected. The probe was destroyed entering the Martian atmosphere. Unit conversion errors are not minor academic inconveniences. Here's how to get them right every time.
Why Conversions Go Wrong
- Assuming everyone is using the same measurement system
- Multiplying when you should divide (or vice versa)
- Forgetting to square or cube conversion factors for area and volume
- Confusing similar unit names (UK pint vs US pint; UK ton vs metric tonne)
- Not sanity-checking that the answer seems physically reasonable
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The Safest Method: Dimensional Analysis
Multiply by fractions equal to 1, arranged so unwanted units cancel. Convert 60 mph to m/s: 60 miles/hr × (1.609 km/mile) × (1000 m/km) × (1 hr/3600 s) = 26.8 m/s. Units cancel across numerator and denominator. If they cancel correctly, the setup is right.
Key Conversions to Know
Length: 1 inch = 2.54 cm | 1 foot = 30.48 cm | 1 mile = 1.609 km
Weight: 1 kg = 2.205 lb | 1 stone = 6.35 kg | 1 ounce = 28.35 g
Temperature: °C to °F: (°C × 9/5) + 32 | °F to °C: (°F − 32) × 5/9
Volume: 1 litre = 1.76 UK pints | 1 UK gallon = 4.546 litres
The Area and Volume Trap — Critical
Always square the linear factor for area; cube it for volume. 1 foot = 0.3048 m → 1 ft² = 0.3048² = 0.0929 m² (NOT 0.3048 m²). 1 m = 100 cm → 1 m² = 10,000 cm² (NOT 100 cm²). Getting this wrong produces errors 10–100× the correct value.
UK-Specific Hazards
- UK pint = 568 ml; US pint = 473 ml (nearly 20% difference)
- UK long ton = 1,016 kg; US short ton = 907 kg; metric tonne = 1,000 kg
- Miles per gallon figures assume UK gallons — US MPG figures look worse due to smaller US gallon
The Sanity Check
Always ask: does this answer seem physically right? 5 miles to km should be around 8 — if you got 3, recalculate. A normal bedroom in m² should be 12–15 m² — if you got 50, something's wrong. This basic check catches most errors before they become expensive.
Further reading: NIST publishes authoritative unit conversion tables for all scientific and engineering units. Access NIST's official unit conversion reference.