Length Conversion Reference

The exact definitions (since 1959)

1 yard

0.9144 m exactly — everything follows from this

1 inch

25.4 mm exactly

1 foot

0.3048 m exactly

1 mile

1609.344 m exactly

1 metre

39.3701 in / 3.28084 ft

1 kilometre

0.621371 miles

1 nautical mile

1852 m — a DIFFERENT unit, not offered here

Within each system

1 foot

12 inches

1 yard

3 feet = 36 inches

1 mile

5280 feet = 1760 yards = 63360 inches

1 metre

100 cm = 1000 mm

1 kilometre

1000 m

Rough conversions for doing it in your head

Metres to feet

x 3.3 (a little over 3)

Feet to metres

x 0.3

Km to miles

x 0.6, or halve and add a tenth

Miles to km

x 1.6

Inches to cm

x 2.5 (slightly under the true 2.54)

⚠ For anything that matters

use the exact figures above

Where unit mistakes actually bite

• Ordering material — check whether the supplier quotes metric or imperial
• Feet and inches written as a decimal by mistake — 5.6 ft is not 5 ft 6 in
• Nautical miles used where statute miles were meant, or the reverse
• A drawing scaled in mm read as cm — a factor of ten, easy to miss
• Rounding a mile to 1609 over a long distance
• Mixing units within one calculation instead of converting first
• Trusting a converter that offers weight and length in one dropdown

5 ft 6 in is not 5.6 feet

This is the single most common length error and it hides well because the number looks plausible. Six inches is half a foot, so 5 ft 6 in is 5.5 feet. Convert the inches to a fraction of a foot before doing anything else, or work entirely in inches and convert once at the end.