Unit Conversion: The Most Common Mistakes and How to Avoid Them

The Mars Climate Orbiter accident (1999)

NASA lost a $327 million probe because one team used imperial units (pound-force seconds) and another used SI (newton-seconds) without converting. The spacecraft entered the Martian atmosphere at the wrong angle and was destroyed. It is the most expensive unit conversion error in history.

Temperature: the conversion that isn't multiplication

Unlike almost all other units, temperature is not converted by multiplying by a factor. °C to °F: F = C × 9/5 + 32. °F to °C: C = (F − 32) × 5/9. The zero points of Fahrenheit and Celsius do not coincide. Hence 0°C = 32°F and 100°C = 212°F. Kelvin is multiplicative from absolute zero: K = C + 273.15.

Common length confusions

• •1 statute mile = 1,609.34 m ≠ 1 nautical mile (1,852 m)
• •1 foot = 30.48 cm; 1 inch = 2.54 cm
• •1 yard = 0.9144 m (3 feet)
• •1 light year = 9.461 × 10¹² km (not a unit of time)

Mass: the problem of the 'pound'

In the imperial system there are several 'pounds': the avoirdupois pound (the common one, 453.59 g), the troy pound (used in precious metals, 373.24 g) and the pound-force (not a unit of mass but of force). Gold is quoted in troy ounces (31.10 g), not avoirdupois ounces (28.35 g). The difference is ~10%.