Length Converter
Convert between metric and imperial length units. See all conversions at once.
Metric vs Imperial
The International System of Units (SI) — metric — is used by ~95% of the world's population and is the standard in all scientific contexts. The U.S., Liberia, and Myanmar are the three countries that still officially use the imperial / U.S. customary system (feet, yards, miles) in daily life — though the U.S. is the only large economy in this group.
The Mars Climate Orbiter disaster, 1999
NASA's Mars Climate Orbiter crashed into Mars in September 1999 because one engineering team used imperial units (pound-seconds) while another used metric (newton-seconds) for thrust calculations. The 4.45× difference between the two pushed the orbiter too close to Mars during atmospheric insertion. Total loss: $327.6 million. The investigation report (official NASA) concluded that unit conversion errors were the proximate cause. This is the most famous reminder ever to always specify units in calculations.
Useful Reference Points
| Conversion | Exact / Approximate Value |
|---|---|
| 1 inch | 2.54 cm (exact by definition since 1959) |
| 1 foot | 12 inches = 30.48 cm |
| 1 yard | 3 feet = 0.9144 m |
| 1 mile | 1.609344 km (5,280 feet) |
| 1 nautical mile | 1.852 km (1 minute of latitude) |
| 1 meter | distance light travels in 1/299,792,458 second (official SI definition since 1983) |
| 1 light year | ~9.461 × 10¹² km (the distance light travels in a year) |
| 1 marathon | 42.195 km = 26.219 miles |