The Complete Guide to Length and Distance Conversion — Metres, Feet, Inches, Miles and More

Length and distance conversion is one of the most commonly performed unit conversions in the world. From checking your height in a different unit system to converting a road distance for navigation, calculating fabric for a sewing project, or specifying dimensions on an engineering drawing — the ability to convert accurately between metres, feet, inches, miles, kilometres, and other units of length is a fundamental everyday skill. This complete guide covers every major unit of length, all the key conversion factors, and worked examples for every common conversion scenario.

The Two Major Systems of Length Measurement

Like most unit conversions, length conversion is complicated primarily by the coexistence of two measurement systems — the metric system used by most of the world and the imperial system used primarily in the United States with partial use in the United Kingdom.

Metric Length Units

The metric system uses the metre as its base unit of length. All other metric length units are derived by applying SI prefixes — multiplying or dividing by powers of 10. This makes metric length conversion straightforward — just move the decimal point.

The metric length hierarchy:

Unit	Symbol	Relationship to Metre
Kilometre	km	1,000 metres
Hectometre	hm	100 metres
Decametre	dam	10 metres
Metre	m	Base unit
Decimetre	dm	0.1 metres
Centimetre	cm	0.01 metres
Millimetre	mm	0.001 metres
Micrometre	μm	0.000001 metres
Nanometre	nm	0.000000001 metres

The metre itself is defined by the speed of light — specifically as the distance light travels in a vacuum in 1/299,792,458 of a second. This makes it the most precisely defined unit of length ever established.

Imperial Length Units

The imperial system uses a set of units with historical origins that have no consistent mathematical relationship to each other. Converting between imperial units requires knowing specific irregular conversion factors.

Imperial length units:

Unit	Abbreviation	Relationship
Inch	in or “	Base reference
Foot	ft or ’	12 inches
Yard	yd	3 feet = 36 inches
Mile	mi	1,760 yards = 5,280 feet
Nautical mile	nmi	1,852 metres (international standard)
Furlong	fur	220 yards (horse racing)
Chain	ch	66 feet (surveying)
Fathom	fm	6 feet (nautical depth)
League	lea	3 miles (historical)

The Most Important Length Conversion Factors

Metric to Imperial

From	To	Multiply by
Metres	Feet	3.28084
Metres	Inches	39.3701
Metres	Yards	1.09361
Centimetres	Inches	0.393701
Millimetres	Inches	0.0393701
Kilometres	Miles	0.621371
Kilometres	Feet	3,280.84
Kilometres	Yards	1,093.61

Imperial to Metric

From	To	Multiply by
Feet	Metres	0.3048
Feet	Centimetres	30.48
Feet	Millimetres	304.8
Inches	Centimetres	2.54
Inches	Millimetres	25.4
Inches	Metres	0.0254
Yards	Metres	0.9144
Miles	Kilometres	1.60934
Miles	Metres	1,609.34
Nautical miles	Kilometres	1.852
Nautical miles	Miles	1.15078

Within Metric

From	To	Multiply by
Kilometres	Metres	1,000
Metres	Centimetres	100
Metres	Millimetres	1,000
Centimetres	Millimetres	10
Millimetres	Centimetres	0.1
Centimetres	Metres	0.01
Metres	Kilometres	0.001

Within Imperial

From	To	Multiply by
Miles	Feet	5,280
Miles	Yards	1,760
Miles	Inches	63,360
Yards	Feet	3
Yards	Inches	36
Feet	Inches	12
Inches	Feet	0.08333
Feet	Yards	0.33333

The Key Anchor Conversions to Memorise

There are a small number of exact or near-exact conversions that are worth memorising because they come up constantly:

1 inch = exactly 2.54 centimetres (this is the legal definition, exact) 1 foot = exactly 30.48 centimetres (derived from the inch definition) 1 mile ≈ 1.609 kilometres (precise: 1.60934 km) 1 kilometre ≈ 0.621 miles (precise: 0.621371 miles) 1 metre ≈ 3.28 feet (precise: 3.28084 feet) 1 nautical mile = exactly 1.852 kilometres (international definition)

Step-by-Step Worked Examples

Example 1 — Converting Height: 5 feet 11 inches to Centimetres

Step 1 — Convert to total inches: (5 × 12) + 11 = 71 inches Step 2 — Convert inches to cm: 71 × 2.54 = 180.34 cm

So 5’11” = approximately 180 cm.

Example 2 — Converting 100 Metres to Feet (Athletics)

100 m × 3.28084 = 328.084 feet

The 100 metre sprint is therefore approximately 328 feet or just under 110 yards.

Example 3 — Converting 26.2 Miles (Marathon) to Kilometres

26.2 mi × 1.60934 = 42.165 km

This is why the standard marathon distance is 42.195 km (the official distance is 42.195 km, established in 1921).

Example 4 — Converting 50 mm to Inches (Engineering)

50 mm × 0.0393701 = 1.969 inches

Or: 50 ÷ 25.4 = 1.969 inches

Example 5 — Converting 500 Feet to Metres

500 ft × 0.3048 = 152.4 metres

Example 6 — Converting 15 km to Miles (Road Distance)

15 km × 0.621371 = 9.321 miles

Example 7 — Converting 6 Nautical Miles to Kilometres

6 nmi × 1.852 = 11.112 km

Quick Reference Conversion Tables

Centimetres and Inches — Side by Side

cm	inches		inches	cm
1 cm	0.394”		1”	2.54 cm
2 cm	0.787”		2”	5.08 cm
5 cm	1.969”		3”	7.62 cm
10 cm	3.937”		4”	10.16 cm
15 cm	5.906”		5”	12.70 cm
20 cm	7.874”		6”	15.24 cm
25 cm	9.843”		8”	20.32 cm
30 cm	11.811”		10”	25.40 cm
50 cm	19.685”		12”	30.48 cm
100 cm	39.370”		18”	45.72 cm

Kilometres and Miles — Road Distance Reference

km	miles		miles	km
1 km	0.621 mi		1 mi	1.609 km
5 km	3.107 mi		5 mi	8.047 km
10 km	6.214 mi		10 mi	16.093 km
20 km	12.427 mi		20 mi	32.187 km
50 km	31.069 mi		50 mi	80.467 km
100 km	62.137 mi		100 mi	160.934 km
500 km	310.686 mi		500 mi	804.672 km
1,000 km	621.371 mi		1,000 mi	1,609.34 km

Height Conversion Table — Feet and Inches to Centimetres

This is one of the most commonly searched conversions worldwide:

Feet & Inches	Total Inches	Centimetres
4’10”	58”	147.3 cm
4’11”	59”	149.9 cm
5’0”	60”	152.4 cm
5’1”	61”	154.9 cm
5’2”	62”	157.5 cm
5’3”	63”	160.0 cm
5’4”	64”	162.6 cm
5’5”	65”	165.1 cm
5’6”	66”	167.6 cm
5’7”	67”	170.2 cm
5’8”	68”	172.7 cm
5’9”	69”	175.3 cm
5’10”	70”	177.8 cm
5’11”	71”	180.3 cm
6’0”	72”	182.9 cm
6’1”	73”	185.4 cm
6’2”	74”	187.9 cm
6’3”	75”	190.5 cm
6’4”	76”	193.0 cm
6’5”	77”	195.6 cm

Millimetres and Inches — Engineering Reference

This table is essential for engineers, machinists, and craftspeople working between metric and imperial specifications:

mm	inches (decimal)	inches (fraction approx)
1 mm	0.0394”	3/64”
2 mm	0.0787”	5/64”
3 mm	0.1181”	1/8”
4 mm	0.1575”	5/32”
5 mm	0.1969”	13/64”
6 mm	0.2362”	15/64”
8 mm	0.3150”	5/16”
10 mm	0.3937”	25/64”
12 mm	0.4724”	15/32”
15 mm	0.5906”	37/64”
20 mm	0.7874”	25/32”
25 mm	0.9843”	63/64”
25.4 mm	1.0000”	1” exactly
30 mm	1.1811”	1 3/16”
50 mm	1.9685”	1 31/32”
100 mm	3.9370”	3 15/16”

Length Conversion in Different Fields

Construction and Architecture

Construction professionals work with length units constantly and often need to convert between metric and imperial. Building plans in the UK and Australia are typically in millimetres. American construction uses feet and inches. International projects may have drawings in both systems.

Key construction length references:

• Standard door height: 2,040 mm (UK) / 6’8” = 2,032 mm (US)
• Standard ceiling height: 2,400 mm (Australia/UK) / 8 ft = 2,438 mm (US)
• Standard brick length: 215 mm (UK metric) / approximately 8.5 inches
• Standard timber stud spacing: 400 mm or 600 mm (metric) / 16” or 24” (imperial)
• A sheet of plywood: 1,200 × 2,400 mm (metric) / 4 ft × 8 ft (imperial) — these are the same sheet

Textiles and Fashion

Fabric is sold by the metre in most countries and by the yard in the United States. Since 1 yard = 0.9144 metres, a yard and a metre are close in size but not the same. Buying fabric in the wrong unit can result in not having enough material for a project.

Common textile conversions:

• 1 yard = 91.44 cm = 0.9144 m
• 1 metre = 1.0936 yards = 39.37 inches
• Clothing measurements are typically in centimetres (international) or inches (US/UK)

Running and Athletics

Running distances are specified in both metric and imperial depending on the country and the event:

Race	Metric	Imperial
Sprint	100 m	109.4 yards
Short sprint	200 m	218.7 yards
Quarter mile	400 m	437.4 yards
Half mile	800 m	874.9 yards
Mile	1,609 m	1 mile
5K	5,000 m	3.107 miles
10K	10,000 m	6.214 miles
Half marathon	21,097 m	13.109 miles
Marathon	42,195 m	26.219 miles

Navigation and Aviation

Navigation uses a mixture of units depending on the context. Nautical miles and knots are used universally in maritime and aviation navigation. One nautical mile equals exactly 1.852 kilometres or 1.15078 statute miles. The nautical mile is defined as one minute of arc of latitude along any meridian — making it naturally aligned with geographic coordinates.

Altitude in aviation is measured in feet in most of the world (including in countries that use metric for all other purposes) due to the global adoption of ICAO (International Civil Aviation Organisation) standards. A cruising altitude of 35,000 feet is approximately 10,668 metres or 10.67 kilometres.

Screen and Display Technology

Screen sizes are specified in inches diagonally in most countries regardless of whether the country uses metric or imperial. A 55-inch television has a diagonal measurement of 55 inches = 139.7 cm = 1.397 metres.

Display resolution is specified in pixels per inch (PPI) or dots per inch (DPI). A smartphone screen with 460 PPI has approximately 181 pixels per centimetre.

Astronomy

Astronomical distances are so vast that standard length units become impractical. Astronomy uses specialised units:

• Astronomical Unit (AU): The average distance from the Earth to the Sun = approximately 149,597,870.7 km = 92,955,807 miles
• Light year: The distance light travels in one year = approximately 9.461 × 10¹² km = 5.879 × 10¹² miles
• Parsec: Approximately 3.086 × 10¹³ km = 3.26 light years — used for stellar and galactic distances

The nearest star to our Sun (Proxima Centauri) is approximately 4.24 light years = 40.14 trillion km = 268,000 AU away.

Common Length Conversion Mistakes

Forgetting there are 12 inches in a foot: When converting a height like 5’9” to centimetres, the most common error is converting 5 feet and 9 inches separately and adding without accounting for the base. The correct approach is to convert to total inches first (5 × 12 + 9 = 69 inches) then multiply by 2.54.

Confusing miles and nautical miles: A nautical mile (1.852 km) is longer than a statute mile (1.609 km). Speed quoted in knots (nautical miles per hour) is therefore faster than the same number in miles per hour. A ship travelling at 20 knots is covering 37.04 km/h, while a car travelling at 20 mph covers only 32.19 km/h.

Using the wrong kilometre to mile conversion direction: The conversion factor 1.60934 always goes from miles to kilometres (multiply miles by 1.60934 to get km). The reverse — kilometres to miles — uses 0.62137 (multiply km by 0.62137 to get miles). Applying the wrong factor gives a result 2.6x too large or too small.

Rounding millimetres to inches too early: In precision engineering, rounding 25 mm to 1 inch introduces a 1.6% error (25 mm = 0.9843 inches, not 1 inch). For engineering work always use the full conversion: 1 inch = exactly 25.4 mm.

A Quick Mental Conversion Method

Kilometres to miles (rough): Multiply by 0.6 (actual: 0.621). So 50 km ≈ 30 miles (actual: 31.1 miles). Close enough for road navigation.

Miles to kilometres (rough): Multiply by 1.6 (actual: 1.609). So 30 miles ≈ 48 km (actual: 48.3 km).

Centimetres to inches (rough): Divide by 2.5 (actual: divide by 2.54). So 30 cm ≈ 12 inches (actual: 11.81 inches). Very close.

Metres to feet (rough): Multiply by 3 and add 10% (actual: multiply by 3.28084). So 10 m → 30 + 3 = 33 feet (actual: 32.8 feet). Close enough for everyday use.

Frequently Asked Questions

Q: What is the exact relationship between inches and centimetres? 1 inch is defined as exactly 2.54 centimetres. This is an exact legal definition agreed internationally in 1959 by the United States, United Kingdom, Australia, Canada, New Zealand, and South Africa. There is no approximation — it is exactly 2.54 cm, not approximately. This means that all imperial length units can be converted to metric with complete precision using this single exact definition.

Q: How long is a metre compared to a yard? A metre (100 cm) is slightly longer than a yard (91.44 cm). A metre is approximately 9.36% longer than a yard. In practical terms if you have 10 metres of fabric you have approximately 10.94 yards. The two units are close enough that they are sometimes confused but different enough that the error matters in precise work.

Q: Why do Americans still use miles and feet? The United States has used customary units (a version of the imperial system) since before independence. Attempts to metricate in the 1970s were unsuccessful due to the enormous cost of converting infrastructure, road signs, and cultural familiarity. The US remains one of only three countries in the world — alongside Liberia and Myanmar — that have not officially adopted the metric system for everyday use, though the US scientific and medical communities use metric universally.

Q: What is a micron and where is it used? A micron is an informal name for a micrometre (μm) — one millionth of a metre or one thousandth of a millimetre. Microns are used to measure: the diameter of human hair (approximately 50–70 μm), particulate matter in air quality (PM2.5 means particles smaller than 2.5 μm), tolerances in precision manufacturing, wavelengths of infrared light, and biological cell sizes (a human red blood cell is approximately 6–8 μm in diameter).

Q: What is the longest and shortest official SI length unit? The SI system technically has no upper or lower limit — you can always add a larger or smaller prefix. In practice the largest named prefix is yotta (Y, 10²⁴) and the smallest is yocto (y, 10⁻²⁴). So one yottametre = 10²⁴ metres (larger than the observable universe at approximately 8.8 × 10²⁶ metres) and one yoctometre = 10⁻²⁴ metres (smaller than a proton at approximately 10⁻¹⁵ metres).

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This article is for educational and informational purposes. For precision engineering, construction, and navigation applications always verify conversions using authoritative references and confirm with qualified professionals.