Ring Size Calculator

Ring Size Calculator: convert ring size instantly. Full formula, unit table, and worked example.

A length converter switches between metric units (millimetres, centimetres, metres, kilometres) and imperial/US customary units (inches, feet, yards, miles), as well as nautical miles, light-years, and astronomical units for scientific applications. The US is one of only three countries in the world that has not adopted the metric system as its primary measurement system — meaning Americans frequently encounter both systems in everyday life, construction, science, and international trade.

Length conversions are needed for home improvement projects, international clothing and shoe sizes, engineering drawings, road distances, astronomy, and scientific calculations.

Enter the length value and select the source unit.
The calculator converts to all common units simultaneously.
For construction: note that US lumber uses nominal (named) dimensions that differ from actual dimensions — a 2×4 is actually 1.5 in × 3.5 in.

Key length conversion factors

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 = 1,760 yards = 1,609.344 m = 5,280 feet

1 nautical mile = 1,852 m (used in aviation and maritime navigation)

1 kilometre = 1,000 m ≈ 0.6214 miles | 1 metre = 3.2808 feet

Length conversion reference

Quick estimation benchmarks

1 metre ≈ 1 yard + 3.37 inches (a metre is slightly longer than a yard). 1 kilometre ≈ 0.62 miles (5 km ≈ 3.1 miles). 1 foot ≈ 30 cm. 1 inch ≈ 2.5 cm. Human height: 5 ft 9 in = 175 cm; 6 ft = 183 cm. Marathon distance: 26.219 miles = 42.195 km. Mount Everest: 29,031.7 ft = 8,849 m. The distance from Earth to the Moon averages 238,855 miles = 384,400 km.

Conversion tips and best practices

For quick miles-to-km conversion: multiply miles by 1.6 (actual factor 1.609 — close enough for navigation).
For quick cm-to-inches: divide by 2.54, or approximate as "divide by 2.5 then subtract 2%."
Clothing sizes differ between US and EU systems even when labelled in inches — always check manufacturer size charts for international purchases.
Aviation and maritime use nautical miles (1 NM = 1.152 statute miles); speed in knots = nautical miles per hour.

The Mars Climate Orbiter was lost in 1999 due to a unit mismatch: one team used pound-force·seconds, another used newton·seconds — a $327 million error caused by failure to convert units.
The circumference of the Earth at the equator is 24,901 miles (40,075 km).
One light-year = 5.879 trillion miles = 9.461 trillion kilometres — the distance light travels in one year.

Common mistakes to avoid

Confusing feet and metres — a 10-foot room ceiling is 3.05 m; a room described as "3 metres" is about 9.8 feet.
Using the wrong inch definition — the US survey inch (used for land measurement) and the international inch differ by 0.000002 inches, which becomes significant over large distances.
Forgetting that nautical miles and statute (land) miles are different — 1 NM = 1.152 miles, not 1 mile.

The inch is defined as exactly 2.54 cm by international agreement (1959). US land surveys historically used the US survey foot, which is being phased out in favour of the international foot. For legal land surveys, title documents, and engineering drawings, confirm which standard applies.