Length units: Metric (SI) vs Imperial / US Customary
Length is the most fundamental physical dimension used in navigation, construction, manufacturing, and everyday life. Most of the world standardizes on the International System of Units (SI), while the US and parts of the UK still use customary/imperial units (feet, inches, miles) in common contexts.
Quick conversions (most searched)
- 1 inch = 2.54 cm (exact)
- 1 foot = 30.48 cm (exact)
- 1 yard = 0.9144 m (exact)
- 1 mile = 1.609344 km (exact)
Key Conversion Standards
- Human Height: Globally, height is often measured in centimeters (e.g., 180cm), but the US standard of feet and inches (e.g., 5'11") remains popular in aviation and media.
- Aviation: Surprisingly, even metric-using countries often use Feet for altitude due to historical conventions in aerospace engineering.
- Screen Sizes: Televisions and smartphones are almost universally marketed in Inches (diagonal), regardless of the country.
Common Conversion Factors
| Unit | Metric Equivalent | Usage |
|---|---|---|
| 1 Inch | 2.54 cm | Screens, Pipes, Paper sizes |
| 1 Foot | 30.48 cm | Height, Aviation Altitude |
| 1 Mile | 1.609 km | Road distances (US/UK) |
Accuracy & rounding
For most use cases, 2–4 decimals are enough. If you’re chaining multiple conversions (e.g., for CAD or manufacturing), avoid rounding until the final step to prevent cumulative error.
Standards note
Many imperial–metric relationships are defined exactly (e.g., inch to cm). When a value is “exact”, any rounding you see is purely display formatting.
Frequently Asked Questions
- How many feet are in a meter?
- 1 meter = 3.28084 feet. For a quick estimate, multiply meters by ~3.3.
- What is 5'10" in cm?
- 5′10″ is about 177.8 cm (≈178 cm). Calculation: (5 × 30.48) + (10 × 2.54) = 177.8 cm.
- Why are miles still used?
- Miles remain standard in the US and for many UK road contexts due to legacy infrastructure, signage, and vehicle instrumentation. In technical work, SI units are often preferred for consistency.
- Is an inch exactly 2.54 cm?
- Yes. 1 inch is defined as exactly 2.54 centimeters.