Like all good stories this one starts with clarification of terminology and notation formats.

Inches can be written in a few ways:

  1. Fractions, as X-Y/Z". X is whole inches. Y/Z is then a fraction of an inch in addition to the whole inches.
  2. Decimals, as 0.00000...", with as many decimal places as you can accurately measure to.
  3. Thou, as .000", which is similar to decimals but specifically refers to thousandths of an inch and is often written without the leading zero.

Here's how 'half an inch' would be written in each:

  1. Fraction = 1/2"
  2. Decimal = 0.5"
  3. Thou = .500"

Millimetres (and other metric measurements) are more uniform, always being written as decimals. There is a minor difference in how the decimal is written between Europe and everywhere else:

  • Europe uses a , as a decimal place
  • Everywhere else uses a .

So three and a half millimetres would be written:

  • In Europe as 3,5mm
  • Everywhere else as 3.5mm

What's the calculation from inches to metric?

Inches to mm

1 inch = 25.4mm

So, to convert inches to mm you first need the decimal version of how many inches you have, then you multiply it by 25.4.

For example, 1.5 inches = 1.5 x 25.4 = 38.1mm

If you need to convert a fraction of inches to mm, first find out what the fraction is in decimal form, and then use this in your calculation.

For example, what's 2-3/8? This represents 2 whole inches plus 3/8 of an inch. In the table of fractions we can see that 3/8 = 0.375, so added to the 2 we have 2.375 inches. Multiply this by 25.4 to get the answer, which is that 2-3/8" = 60.325mm.

Inches to cm

1 inch = 2.54cm

So, to convert inches to mm you first need the decimal version of how many inches you have, then you multiply it by 2.54.

For example, 15 inches = 15 x 2.54 = 38.1cm

Inches to m

1 inch = 0.0254m

So, to convert inches to mm you first need the decimal version of how many inches you have, then you multiply it by 0.0254.

For example, 150 inches = 150 x 0.0254 = 3.81m

What's the calculation from metric to inches?

mm to inches

1mm = 0.0393701 inches

So, to convert mm to inches you need to multiply how many mm you have by 0.0393701.

For example, 15mm = 15 x 0.0393701 = 0.59" (to 2 decimal places)

cm to inches

1cm = 0.393701 inches

So, to convert mm to inches you need to multiply how many cm you have by 0.393701.

For example, 15cm = 15 x 0.393701 = 5.91" (to 2 decimal places)

m to inches

1m = 39.37 inches

So, to convert m to inches you need to multiply how many m you have by 39.37.

For example, 1.5m = 1.5 x 39.37 = 59.06" (to 2 decimal places)

Inch Fractions to mm

Here are the most common inch fractions converted to decimal inches and to mm. The smallest fraction we've given is 32nds - anything smaller than this tends to be represented as thou.

Inch /32Inch /16Inch /8Inch /4Inch /2Whole InchesIn wordsInch Decimalmm (millimetres)cm (centimetres)m (metres)
1/32"     one thirty second0.03125"0.79mm0.08cm0.0008m
2/32"1/16"    one sixteenth0.0625"1.59mm0.16cm0.0016m
3/32"     three thirty seconds0.09375"2.38mm0.24cm0.0024m
4/32"2/16"1/8"   one eighth0.125"3.18mm0.32cm0.0032m
5/32"     five thirty seconds0.15625"3.97mm0.4cm0.004m
6/32"3/16"    three sixteenths0.1875"4.76mm0.48cm0.0048m
7/32"     seven thirty seconds0.21875"5.56mm0.56cm0.0056m
8/32"4/16"2/8"1/4"  one quarter0.25"6.35mm0.64cm0.0064m
9/32"     nine thirty seconds0.28125"7.14mm0.71cm0.0071m
10/32"5/16"    five sixteenths0.3125"7.94mm0.79cm0.0079m
11/32"     eleven thirty seconds0.34375"8.73mm0.87cm0.0087m
12/32"6/16"3/8"   three eighths0.375"9.53mm0.95cm0.0095m
13/32"     thirteen thirty seconds0.40625"10.32mm1.03cm0.0103m
14/32"7/16"    seven sixteenths0.4375"11.11mm1.11cm0.0111m
15/32"     fifteen thirty seconds0.46875"11.91mm1.19cm0.0119m
16/32"8/16"4/8"2/4"1/2" one half0.5"12.7mm1.27cm0.0127m
17/32"     seventeen thirty seconds0.53125"13.49mm1.35cm0.0135m
18/32"9/16"    nine sixteenths0.5625"14.29mm1.43cm0.0143m
19/32"     nineteen thirty seconds0.59375"15.08mm1.51cm0.0151m
20/32"10/16"5/8"   five eighths0.625"15.88mm1.59cm0.0159m
21/32"     twenty one thirty seconds0.65625"16.67mm1.67cm0.0167m
22/32"11/16"    eleven sixteenths0.6875"17.46mm1.75cm0.0175m
23/32"     twenty three thirty seconds0.71875"18.26mm1.83cm0.0183m
24/32"12/16"6/8"3/4"  three quarters0.75"19.05mm1.91cm0.0191m
25/32"     twenty five thirty seconds0.78125"19.84mm1.98cm0.0198m
26/32"13/16"    thirteen sixteenths0.8125"20.64mm2.06cm0.0206m
27/32"     twenty seven thirty seconds0.84375"21.43mm2.14cm0.0214m
28/32"14/16"7/8"   seven eighths0.875"22.23mm2.22cm0.0222m
29/32"     twenty nine thirty seconds0.90625"23.02mm2.3cm0.023m
30/32"15/16"    fifteen sixteenths0.9375"23.81mm2.38cm0.0238m
31/32"     thirty one thirty seconds0.96875"24.61mm2.46cm0.0246m

Thou (Thousandths) of an inch to mm

"Thou" is used for smaller measurements under one inch when fractions don't offer the size required - tolerances, sheet thicknesses, machining parameters, threads. All sorts of small stuff. It can of course be expressed as a fraction, X/1000, but is written as a decimal. The leading 0 is usually omitted, so the measurement starts with a decimal. Here's how thou convert to metric:

Inch thouIn wordsmm (millimetres)cm (centimetres)
.001one thou0.03mm0.003cm
.002two thou0.05mm0.005cm
.005five thou0.13mm0.013cm
.010ten thou0.25mm0.025cm
.015fifteen thou0.38mm0.038cm
.020twenty thou0.51mm0.051cm
.025twenty five thou0.64mm0.064cm
.030thirty thou0.76mm0.076cm
.035thirty five thou0.89mm0.089cm
.040forty thou1.02mm0.102cm
.045forty five thou1.14mm0.114cm
.050fifty thou1.27mm0.127cm
.055fifty five thou1.4mm0.14cm
.060sixty thou1.52mm0.152cm
.065sixty five thou1.65mm0.165cm
.070seventy thou1.78mm0.178cm
.075seventy five thou1.91mm0.191cm
.080eighty thou2.03mm0.203cm
.085eighty five thou2.16mm0.216cm
.090ninety thou2.29mm0.229cm
.095ninety five thou2.41mm0.241cm
.100one hundred thou2.54mm0.254cm
.110one hundred and ten thou2.79mm0.279cm
.120one hundred and twenty thou3.05mm0.305cm
.130one hundred and thirty thou3.3mm0.33cm
.140one hundred and forty thou3.56mm0.356cm
.150one hundred and fifty thou3.81mm0.381cm
.160one hundred and sixty thou4.06mm0.406cm
.170one hundred and seventy thou4.32mm0.432cm
.180one hundred and eighty thou4.57mm0.457cm
.190one hundred and ninety thou4.83mm0.483cm
.200two hundred thou5.08mm0.508cm
.250two hundred and fifty thou6.35mm0.635cm
.300three hundred thou7.62mm0.762cm
.400four hundred thou10.16mm1.016cm
.500five hundred thou12.7mm1.27cm
.600six hundred thou15.24mm1.524cm
.700seven hundred thou17.78mm1.778cm
.800eight hundred thou20.32mm2.032cm
.900nine hundred thou22.86mm2.286cm

Whole Inches to metric

Larger sheet sizes are often quoted in inches. Below is a chart showing the conversion between common inch and metre sizes.

InchesFeetmm (millimetres)cm (centimetres)m (metres)

mm and metric to Inches

Despite individual ones being tiny, millimetres are used for measuring everything from the very small all the way up to large sheets. Here's how mm convert to cm, m, inches and feet.

mmcmmInch FractionThouInch DecimalFeet
1mm0.1cm0.001m .0390.04 
2mm0.2cm0.002m .0790.08 
4mm0.4cm0.004m .1570.16 
5mm0.5cm0.005m .1970.2 
6mm0.6cm0.006m .2360.24 
7mm0.7cm0.007m .2760.28 
8mm0.8cm0.008m .3150.31 
9mm0.9cm0.009m .3540.35 
10mm1cm0.01m .3940.39 
15mm1.5cm0.015m .5910.59 
20mm2cm0.02m .7870.79 
25.4mm2.54cm0.025m1" 10.083
30mm3cm0.03m  1.180.098
38.1mm3.81cm0.038m1-1/2" 1.50.125
40mm4cm0.04m  1.570.131
50mm5cm0.05m  1.970.164
50.8mm5.08cm0.051m2" 20.167
100mm10cm0.1m  3.940.328
200mm20cm0.2m  7.870.656
300mm30cm0.3m  11.810.984
400mm40cm0.4m  15.751.312
500mm50cm0.5m  19.691.64
600mm60cm0.6m  23.621.969
1000mm100cm1m  39.373.281
1200mm120cm1.2m  47.243.937
2000mm200cm2m  78.746.562
2400mm240cm2.4m  94.497.874
3000mm300cm3m  118.119.843