Length Converter

About Length Converter

How This Length Converter Works

The converter uses meters as an internal base unit. Every input value is first converted to meters using a known factor, then converted from meters to the target unit. This two-step approach means the tool only needs to store one factor per unit rather than a separate factor for every possible unit pair.

Worked example - converting 5 feet 8 inches to centimeters: First, convert 5 feet 8 inches to inches: (5 x 12) + 8 = 68 inches. Next, multiply by the inch-to-meter factor: 68 x 0.0254 = 1.7272 meters. Finally, convert to centimeters: 1.7272 x 100 = 172.72 cm. You can verify this by entering 68 in the converter with "Inches" as the source and "Centimeters" as the target.

Another example - converting a 10K race distance to miles: 10 km x 1,000 = 10,000 meters. Then 10,000 / 1,609.344 = 6.2137 miles. A 10K is just over 6.2 miles.

Length Unit Conversion Factors

Every metric-to-imperial length conversion traces back to a single definition: 1 inch equals exactly 25.4 millimeters. This was established by the International Yard and Pound Agreement of 1 July 1959, signed by the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. The agreement defined the international yard as exactly 0.9144 meters, which makes the inch exactly 25.4 mm, the foot exactly 304.8 mm, and the mile exactly 1,609,344 mm (source: US National Bureau of Standards, Federal Register 1959).

From	To	Multiply By	Exact?
Inches	Centimeters	2.54	Yes (by definition)
Feet	Meters	0.3048	Yes (by definition)
Yards	Meters	0.9144	Yes (by definition)
Miles	Kilometers	1.609344	Yes (by definition)
Centimeters	Inches	0.393701	Approximation of 1/2.54
Meters	Feet	3.28084	Approximation of 1/0.3048
Kilometers	Miles	0.621371	Approximation of 1/1.609344

Where Do These Definitions Come From?

The meter itself is defined by the speed of light. Since 1983, the meter has been the distance light travels in a vacuum in exactly 1/299,792,458 of a second. This definition was reworded (but not changed) as part of the 2019 SI redefinition, which tied all seven base SI units to fundamental physical constants (source: NIST). Because both the meter and the inch are defined by fixed physical constants and exact ratios, conversion factors between metric and imperial length units are exact, not approximations.

Before 1959, the US and UK actually used slightly different inch lengths. The US survey foot (used for land measurement) was defined as exactly 1200/3937 meters, giving an inch of about 25.40005 mm. The difference was tiny - about 2 parts per million - but it mattered for large-scale land surveys. The US officially retired the survey foot on 1 January 2023, making the international foot the sole US standard (source: NIST Special Publication 811).

All Eight Units Compared

Unit	Symbol	Millimeters	System	Typical Use
Millimeter	mm	1	Metric	Engineering tolerances, rainfall, paper thickness
Centimeter	cm	10	Metric	Body measurements, clothing, screen sizes (diagonal)
Meter	m	1,000	Metric (SI base)	Room dimensions, building heights, swimming pools
Kilometer	km	1,000,000	Metric	Road distances, running races
Inch	in	25.4	Imperial	Screen sizes, screw sizes, pipe diameters
Foot	ft	304.8	Imperial	Height, room dimensions, elevation
Yard	yd	914.4	Imperial	Fabric, American football fields, golf
Mile	mi	1,609,344	Imperial	Road distances (US/UK), running

Common Conversions Quick Reference

From	To	Quick Mental Estimate
1 inch	centimeters	~2.5 cm (exact: 2.54)
1 foot	centimeters	~30 cm (exact: 30.48)
1 meter	feet	~3 ft 3 in (exact: 3.281 ft)
1 yard	meters	~0.9 m (exact: 0.9144)
1 km	miles	~0.6 mi (exact: 0.6214)
1 mile	kilometers	~1.6 km (exact: 1.6093)
100 m	yards	~109 yd (exact: 109.36)

Mental Maths Shortcuts for Length Conversion

When you need a rough conversion without a calculator, these tricks help:

Km to miles: Multiply by 5 and divide by 8. For example, 80 km x 5 = 400, then 400 / 8 = 50 miles. The actual answer is 49.71 miles, so this is accurate to within 1%. You can also use consecutive Fibonacci numbers (5, 8, 13, 21, 34, 55, 89) - each number in miles is roughly the previous number in kilometers. So 8 miles is about 13 km, 13 miles is about 21 km, and so on.

Inches to cm: Multiply by 2.5 and add 1% for better accuracy. For 12 inches: 12 x 2.5 = 30, plus 1% (0.3) = 30.3 cm. The exact answer is 30.48 cm.

Meters to feet: Multiply by 3 and add 10%. For 10 meters: 10 x 3 = 30, plus 10% (3) = 33 feet. The exact answer is 32.81 feet - close enough for a quick estimate.

Feet to meters: Multiply by 3 and divide by 10. For 100 feet: 100 x 3 = 300, then 300 / 10 = 30 meters. Exact answer: 30.48 meters.

Standard Race Distances in Both Systems

Runners frequently need to convert between kilometers and miles. Here are the standard race distances used worldwide:

Race	Official Distance	In Miles	In Kilometers
5K	5 km	3.107 mi	5.000 km
10K	10 km	6.214 mi	10.000 km
Half Marathon	21.0975 km	13.109 mi	21.098 km
Marathon	42.195 km	26.219 mi	42.195 km
Mile	1 mile	1.000 mi	1.609 km
100 yards	100 yd	N/A	0.09144 km (91.44 m)

The marathon distance of 42.195 km (26 miles 385 yards) was standardized by the International Association of Athletics Federations (now World Athletics) in 1921. The odd distance comes from the 1908 London Olympics, where the course was extended so the finish line would be in front of the royal box at the White City Stadium.

Height Conversion Reference

Converting height between feet/inches and centimeters is one of the most common everyday conversions, especially when filling in forms or comparing heights internationally. Here are the most looked-up heights:

Feet & Inches	Inches Total	Centimeters	Meters
5'0"	60	152.40	1.524
5'4"	64	162.56	1.626
5'6"	66	167.64	1.676
5'8"	68	172.72	1.727
5'10"	70	177.80	1.778
6'0"	72	182.88	1.829
6'2"	74	187.96	1.880

To convert any height in feet and inches to centimeters: multiply the feet by 12 to get total inches, add the remaining inches, then multiply by 2.54. For the reverse, divide centimeters by 2.54 to get total inches, then divide by 12 for feet with the remainder as inches. The CM to Inches Converter handles this automatically.

Common Conversion Mistakes to Avoid

Confusing square and linear units. Converting a 10 ft x 10 ft room to meters is not 3.048 m x 3.048 m = 9.29 square meters, unless you want the area. The room dimensions are 3.048 m x 3.048 m, but the area conversion factor is 0.3048 squared (0.0929), not 0.3048. Always convert each dimension separately before multiplying.

Mixing up nautical and statute miles. A nautical mile (1,852 meters) is about 15% longer than a statute mile (1,609.344 meters). Maritime and aviation charts use nautical miles, while road distances use statute miles. The KM to Miles Converter handles both types.

Rounding too early. When chaining conversions (e.g., inches to cm to meters), rounding at each step compounds the error. Convert directly from the source to the target unit using a single multiplication instead.

When Length Conversion Matters

Field	Common Conversions	Why
Construction	Feet to meters, inches to mm	US plans in imperial, materials often spec'd in metric
Running / cycling	Km to miles	Race distances in km, but US runners think in miles
Sewing / fabric	Yards to meters	US patterns in yards, fabric sold in meters elsewhere
Screen sizes	Inches to cm	Marketed in inches globally, but specifications use cm
Aviation	Feet to meters	Altitude in feet, but runway lengths sometimes in meters
Science	Everything to metric (SI)	Metric is the standard for all scientific measurement
Real estate	Square feet to square meters	UK listings use sq ft, EU listings use sq m

Metric vs Imperial by Country

Only three countries have not officially adopted the metric system: the United States, Liberia, and Myanmar. Liberia and Myanmar are both in the process of transitioning to metric. The UK officially went metric in 1995 but still uses miles on road signs, feet and inches for personal height, and stone for body weight. Canada adopted metric in 1970 but construction and real estate still commonly use feet and inches.

System	Countries	Status
Metric only	Most of the world (EU, Asia, Africa, South America, Australia)	Official and everyday use
US customary	United States	Everyday use; metric for science, medicine, and military
Mixed	United Kingdom	Metric official since 1995, but miles for roads, feet/inches for height, stone for weight
Mixed	Canada	Metric official since 1970, but feet/inches still common for height and construction
Transitioning	Liberia, Myanmar	Metric increasingly used, full transition underway

Screen Size Conversions

TV and monitor sizes are marketed in inches worldwide, measured diagonally. When comparing screens or checking if a TV fits a space, you often need the actual width and height in centimeters. The diagonal measurement alone does not tell you how wide the screen is - that depends on the aspect ratio.

For a 55-inch TV with a 16:9 aspect ratio, the width is about 121.7 cm (47.9 inches) and the height is about 68.5 cm (27.0 inches). For a 27-inch monitor at 16:9, the width is about 59.8 cm (23.5 inches) and the height is about 33.6 cm (13.2 inches). Laptop screens at 14 inches (16:10 ratio) measure roughly 30.0 cm wide by 18.7 cm tall.

When shopping internationally, keep in mind that some regions list screen dimensions in centimeters rather than inches. A "140 cm TV" is a 55-inch TV. The conversion is always the same: divide centimeters by 2.54 to get inches.

For dedicated kilometer and mile conversions including nautical miles, the KM to Miles Converter includes the Fibonacci estimation trick. The CM to Inches Converter is useful for quick screen size and height lookups. For converting weight units between metric and imperial, the Weight Converter works the same way as this tool. All calculations run entirely in your browser with no data sent to any server.

Sources

• NIST - SI Units of Length
• BIPM - SI Brochure (Definition of the Metre)
• NIST - US Survey Foot Deprecation
• World Athletics - Marathon Distance Standard
• NIST Special Publication 811 - Guide for SI Units
• CIA World Factbook - Weights and Measures by Country