Conversion tables for metric and imperial units

Factors for Converting Units of Length

This page contains a number of useful equivalences and conversion factors which can be used to equate the many units of measurement. Some are from a previous era and are rarely used to-day, but are interesting and useful if consulting old publications. When measuring, the metric system is the easiest to use since all smaller divisions are factors of 10. Workable lengths are nearly always quoted in millimetres (mm), eg. the standard kitchen unit or appliance is 600mm.
The calculator below can be dragged around with your mouse for working out your conversion.
Remember, to avoid mistakes measure twice and cut once! Although your error may not be as important as the one made when grinding the mirror for the Hubble Space Telescope. The measurement was small involving a few micrometres, but the cosequences were great, requiring additional optics to correct the focus and the resolution is still not what was intended.

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Useful Length Relationships

1 inch (in,") = 1/12th of a foot = 1/36th of a yard
1 link = 7.92 inches
1 foot (ft, ')= 12 inches = 1/3rd of a yard = 1/5280th of a mile = 0.3048 metre
1 rod = 1 pole = 1 perch = 25 links = 5 ½ yards = 5.0292 metres
1 chain = 4 rods = 4 perch = 22 yards = 20.117 metres
1 furlong (furrow long) = 10 chains = 220 yards = 201.17 metres
8 furlongs = 1 mile
1 mile = 63360 inches = 5280 feet = 1760 yards = 80 chains = 1.6093 kilometre

The chain was a measuring instrument used in surveying, made up of 100 links, each 7.92 inches long making 66 feet or 22 yards - the length of a cricket pitch. It is sometimes called Gunter's chain after the man who introduced it.
The league is an old international measurement but has different values depending on the country. It was the distance between marker stones placed on roads. The English league is 3 statute miles; the French and Spanish league varies between localities, but is about 2.5 and 4.2 miles respectively. The metric league is 4 kilometres.

To give an indication of the possible origins of linear measurements:-
3 barleycorns* = 1 inch
2¼ inches = 1 nail
3 inches = 1 palm
4 inches = 1 hand (used for the height of horses)
9 inches = 1 span
18 inches = 1 cubit
2½ feet = 1 military pace
5 feet = 1 geometrical pace.

* The barleycorn is used to define the increments in British shoe measurements. The difference between each size is one barleycorn or a third of an inch.

Nautical Measure - The fathom is a nautical and mining measurement based on the space to which a man can extend his arms.
one fathom = 6 feet.
1 shackle (shot) = 15 fathoms = 90 feet = 27.432 metres.
The shackle is used for the measurement of ropes and anchor chains on board ship. In Britain a shackle used to be 12½ fathoms (75 feet), but it was changed in 1949 to 15 fathoms, the same as the U.S. value where it is called a shot.
120 fathoms = 1 cable's length.
1 nautical mile (international) = 2,026.66 yards = 1.852 kilometres.
3 nautical miles = 1 league (nautical)
60 nautical miles = 69.121 statute miles = 1 degree
360 degrees = the earth's circumference.

The Knot (kt) is a unit of speed and equals one nautical mile per hour. It is used in aviation, meterology and at sea.

Circular Measure - Used in astronomical and geographical calculations
60 seconds = 1 minute (')
60 minutes = 1 degree (°)
30 degrees = 1 sign (zodiac)
60 degrees = 1 sevtant
90 degrees = 1 quadrant
360 degrees = 1 circle.

Length Conversion Factors

from to multiply by

inches (in) millimetres (mm) 25.4

millimetres (mm) inches (in) 0.03937

inches (in) centimetres(cm) 2.54

centimetres(cm) inches (in) 0.3937

feet (ft) metres(m) 0.3048

metres (m) feet (ft) 3.2808

metres (m) yards (yd) 1.0936

yards (yd) metres (m) 0.9144

kilometres (km) miles 0.6214

miles kilometres (km) 1.60934

For a less accurate mental reckoning the following can be used:-

metres to yards add 1/10th

yards to metres subtract 1/10th

kilometres to miles multiply by 5 and divide by 8

miles to kilometres multiply by 8 and divide by 5

Imperial to Metric Length Equivalences

inches = cm cm = inches

1 = 2.54 1 = 0.39

2 = 5.08 2 = 0.79

3 = 7.62 3 = 1.18

4 = 10.16 4 = 1.58

5 = 12.7 5 = 1.97

6 = 15.24 6 = 2.36

7 = 17.78 7 = 2.76

8 = 20.32 8 = 3.15

9 = 22.86 9 = 3.54

10 = 25.4 10 = 3.94

11 = 27.94 15 = 5.91

12 = 30.48 20 = 7.87

15 = 38.1 25 = 9.84

20 = 50.8 30 = 11.81

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metres to yards	add 1/10th
yards to metres	subtract 1/10th
kilometres to miles	multiply by 5 and divide by 8
miles to kilometres	multiply by 8 and divide by 5

inches	=	cm	cm	=	inches
1	=	2.54	1	=	0.39
2	=	5.08	2	=	0.79
3	=	7.62	3	=	1.18
4	=	10.16	4	=	1.58
5	=	12.7	5	=	1.97
6	=	15.24	6	=	2.36
7	=	17.78	7	=	2.76
8	=	20.32	8	=	3.15
9	=	22.86	9	=	3.54
10	=	25.4	10	=	3.94
11	=	27.94	15	=	5.91
12	=	30.48	20	=	7.87
15	=	38.1	25	=	9.84
20	=	50.8	30	=	11.81