Kilometres vs Miles — Metric vs Imperial Distance
Kilometres (km) and miles (mi) are both units of distance but belong to different measurement systems — metric and imperial respectively. The metric system is used by virtually all countries and all scientific disciplines worldwide. Miles remain in common use in the United States, the United Kingdom (for road distances), and a few other countries. For software developers handling location data, understanding both units and how to convert between them is essential.
Comparison Table
| Aspect | Kilometres | Miles |
|---|---|---|
| System | Metric (SI system) | Imperial/US customary |
| Conversion | 1 km = 0.621371 miles | 1 mile = 1.60934 km |
| Usage | Most of the world, all scientific contexts | United States, UK road signs, nautical/aviation (nautical mile) |
| Subdivisions | metres (1 km = 1000 m) | yards, feet (1 mile = 5280 feet = 1760 yards) |
| GPS / mapping | Underlying calculations in all GPS systems use metric | Display unit only; converted from metric internally |
| Running/sports | International racing (5K, 10K, marathon = 42.195 km) | US races often in miles (1 mile, 5K is still 3.1 miles) |
When to Use Kilometres
Use kilometres for any globally-facing application, scientific data, and geospatial calculations. The Haversine formula for geographic distance returns metres and is easily converted to km. All mapping APIs (Google Maps, Mapbox) use metric internally. Default to kilometres unless you have a specific reason to use miles.
When to Use Miles
Use miles when displaying distances to US users (GPS navigation apps, delivery estimates, driving directions) or when dealing with UK road distances. Detect the user's locale and offer unit preferences where possible.
Convert Between Kilometres and Miles
FAQ
- How accurate is the approximation 1 km ≈ 0.6 miles?
- 1 km = 0.621371 miles. The 0.6 approximation has about a 3% error — reasonable for rough estimates but insufficient for precise engineering or navigation. Use the precise factor 1.60934 km/mile for calculations.
- What is a nautical mile?
- A nautical mile (NM) is 1,852 metres (1.852 km) — defined as one arc-minute of latitude on the Earth's surface. It is the standard distance unit in aviation and maritime navigation because of its direct relationship to geographic coordinates.
- How do GPS systems handle units internally?
- GPS coordinates are pure numbers (degrees of latitude and longitude) with no inherent unit. Distance calculations from coordinates (Haversine, Vincenty formulas) work in metres. Any display in miles or kilometres is a conversion step applied at the presentation layer.