Useful Imperial fraction to metric conversions

I’ve been having to do a lot of fiddly metric conversions for recent projects. The problem is that conventions for small (sub inch) imperial measurements are often an incoherent mess; a mix of fractions and decimal values. Woodworkers tend to use fractions for easy halving and use of a tape measure, whereas machinists tend to use decimals, but I’ve seen all sorts of combinations.

If I'm working on a model that's scaled down from imperial measurements, I naturally try to work in metric. But converting imperial fractions into metric involves two numeric steps, which is a minor nuisance.

So to save time I slapped together a simple table comparing three linear measurements: imperial fractions, imperial decimal, and metric.

Measurements in bold are common major fractions. Those in blue are useful fractions that are close to whole unit millimetre measurements. There are some goofy and uncommon fractions here, mainly for completeness.

Here’s my table in case it’s useful to anyone else.

Imperial fractions to imperial decimals to metric

Fraction Decimal Metric Approx

1/128" 0.0078" 0.198mm

0.0098" 0.25mm (0.25mm)

1/64" 0.0156" 0.397mm

0.020" 0.508mm (~0.5mm)

3/128" 0.0234" 0.595mm

0.0295" 0.75mm

1/32" 0.0313" 0.793mm

5/128" 0.039" 0.99mm (~1mm)

3/64" 0.0469" 1.19mm

1/16" 0.0625" 1.59mm

0.072" 1.829mm

5/64" 0.0781" 1.984mm (~2mm)

0.081" 2.06mm

3/32" 0.094" 2.38mm

7/64" 0.1094" 2.778mm

0.114" 2.896mm

0.118" 3.0mm (3.0mm)

1/8" 0.125" 3.175mm

9/64" 0.141" 3.572mm

5/32" 0.1563" 3.97mm (~4mm)

11/64" 0.1719" 4.365mm

3/16" 0.1875" 4.76mm

0.1969" 5.0mm (5.0mm)

13/64" 0.203" 5.16mm

7/32" 0.219" 5.56mm

15/64" 0.234" 5.953mm (~6mm)

1/4" 0.25" 6.35mm

17/64" 0.266" 6.747mm

0.276" 7.00mm (7.0mm)

9/32" 0.2813" 7.14mm

19/64" 0.2969" 7.54mm

5/16" 0.313" 7.94mm (~8mm)

21/64" 0.328" 8.334mm

11/32" 0.344" 8.73mm

0.354" 9.00mm (9.0mm)

23/64" 0.359" 9.128mm

3/8" 0.375" 9.525mm

25/64" 0.391" 9.92mm (~10mm)

13/32" 0.406" 10.32mm

27/64" 0.4219" 10.72mm

7/16" 0.4375" 11.11mm

29/64" 0.453" 11.51mm

15/32" 0.469" 11.91mm (~12mm)

31/64" 0.484" 12.3mm

1/2" 0.5" 12.7mm

17/32" 0.53" 13.49mm

9/16" 0.5625" 14.29mm

19/32" 0.5938" 15.08mm (~15mm)

5/8" 0.625" 15.875mm

21/32" 0.656" 16.67mm

11/16" 0.6875" 17.46mm

23/32" 0.719" 18.256mm

3/4" 0.75" 19.05mm (~19mm)

25/32" 0.781" 19.84mm

0.79" 20mm (20mm)

13/16" 0.8125" 20.64mm

7/8" 0.875" 22.23mm

29/32" 0.906" 23.02mm

15/16" 0.9375" 23.81mm

1" 1.0" 25.4mm

1 1/4" 1.25" 31.75mm

1 1/2” 1.5" 38.1mm

1 3/4" 1.75" 44.45mm

2" 2.0" 50.8mm

Holier than thou

Next, some thou (1/1000 inch, sometimes called mils) conversions, since Americans like to use that system for stuff like sheet styrene.

Note that “thou” is pronounced like the first half of the word “thousand,” not like the archaic second-person singular pronoun.

Thou mm approx mm

1 thou 0.0254mm (~0.25mm)

2 thou 0.0508mm (~0.05mm)

3 thou 0.0762mm

4 thou 0.1016mm (~0.1mm)

5 thou 0.127mm

6 thou 0.1524mm (~0.15mm)

7 thou 0.1778mm

8 thou 0.2032mm (~0.2mm)

9 thou 0.2286mm

10 thou 0.254mm (~0.25mm)

20 thou 0.508mm (~0.5mm)

30 thou 0.762mm (~0.75mm)

40 thou 1.016mm (~1mm)

60 thou 1.524mm (~1.5mm)

80 thou 2.0574mm (~2mm)

120 thou 3.048mm (~3mm)

Wire sizes

What about the size of electrical wires? Well, here’s another table.

AWG Dia " Dia mm Metric, area mm²

1 AWG 0.289" 7.34 mm 42.4 mm²

4.51 mm 16 mm²

6 AWG 0.162" 4.11 mm 13.3 mm²

3.57 mm 10 mm²

8 AWG 0.128" 3.25 mm 8.37 mm²

2.76 mm 6 mm²

10 AWG 0.102" 2.59 mm 5.26 mm²

2.26 mm 4 mm²

12 AWG 0.081" 2.06 mm 3.31 mm²

1.78 mm 2.5 mm²

14 AWG 0.062" 1.57 mm 2.08 mm²

1.38 mm 1.5 mm²

16 AWG 0.051" 1.30 mm 1.31 mm²

1.13 mm 1.0 mm²

20 AWG 0.032" 0.81 mm 0.518 mm²

0.798 mm 0.5 mm²

22 AWG 0.0253" 0.643 mm 0.326 mm²

24 AWG 0.0201" 0.511 mm 0.205 mm²

26 AWG 0.0159" 0.404 mm 0.129 mm²

28 AWG 0.0126" 0.320 mm 0.081 mm²

30 AWG 0.01" 0.254 mm 0.0509 mm²

But wait! What is this random AWG thing anyway?

AWG

Well, the US (and Canada) use a sensible, logical, and easy to understand convention (it can’t charitably be called a system) called American Wire Gauge (AWG). The value is determined by the number of passes through an obsolete wire drawing die, with the sizes going up logarithmically as the diameter of the wire (the number of draw passes) decreases. That or it refers to the size of an average monkey’s ass divided by the phase of the moon.

In practice people just memorize a handful of AWG sizes and what they’re normally used for. For example, 14 AWG carries up to 15 amps at 110/120VAC and 12 AWG carries up to 20 amps. Ethernet cables often use 23 or 24 AWG wires. Basically thick wires have small numbers; thin wires have large numbers.

Gauges are fun. There are like 50 different things called “gauge” which all measure different things in incompatible ways. AWG is just the best known gauge, since it’s used for electrical wires and jewellery piercings.

Metric

The rest of the world uses the metric IEC 60228 standard for wires, which is simply the cross-sectional area of the wire in square millimetres. This system is straightforward, useful, and meaningful because the cross-section of a wire determines both how big and heavy it is, and how much electrical resistance it offers. The drawback is that non engineers might think it refers to the diameter of the wire, or something easy to visualize like that.

Anyway. Here’s a chart of common AWG sizes and their equivalent diameters in inches and millimetres, and common metric wire sizes in square millimetres and their equivalent diameters in millimetres.

Stranded wire is larger in diameter than solid (because you have to calculate the area of each strand to determine the total wire area). And of course insulated wire is thicker again.

I mainly added diameters because I find it useful to know the physical size of a wire when designing a 3D model that has wires routed through. (well, minus the insulation of course)