In 1999, NASA lost the Mars Climate Orbiter because one team used metric units and another used imperial with no conversion between them. The $327 million spacecraft was destroyed. Unit conversion is not trivial — in technical, medical, and engineering contexts it carries real consequences.
The Two Major Systems
The metric system (SI) is used by 195 of 196 countries as the official standard. It is based on powers of ten, making arithmetic straightforward. The imperial system — used customarily in the United States — has no consistent mathematical relationships between units: a mile is 5,280 feet, a foot is 12 inches, a pound is 16 ounces. These must be memorised.
Critical Length Conversions
1 metre = 3.28084 feet = 39.3701 inches. 1 kilometre = 0.621371 miles. 1 inch = 2.54 centimetres (defined relationship). 1 foot = 30.48 centimetres. For quick estimation: one metre ≈ one yard, one kilometre ≈ 0.6 miles.
Weight Conversions
1 kilogram = 2.20462 pounds. 1 pound = 453.592 grams. 1 stone (UK body weight) = 14 pounds = 6.35 kilograms. Medical dosing should always use kilograms and milligrams regardless of how information is communicated to patients.
Temperature: The Non-Linear Conversion
Celsius to Fahrenheit: (°C × 9/5) + 32. Fahrenheit to Celsius: (°F − 32) × 5/9. Key references: 0°C = 32°F (freezing), 100°C = 212°F (boiling), 37°C = 98.6°F (body temperature). The Unit Converter applies the correct formula automatically and displays results as clear text: "1 km = 0.621371 mi."
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