Screw/Bolt Diameters

Borrowing a bit from our friends at Bolt Depot, their chart showing US Machine Screw Diameters is helpful, but often I’m designing with Metric units (or a unit-less system that outputs in millimeters) and I need to convert Imperial units to mm. (I tend to do a lot of work using OpenSCAD and Inkscape for 3D printing.)

The chart below allows me to specify screws and bolts and then design holes that will work. For instance, I used a lot of #4 screws, and the chart tells me I need a hole diameter of approximately 2.794mm. Handy!

Size Thread Diameter
Decimal Nearest Fractional Metric
#0 0.06″ 1/16″ 1.524mm
#1 0.07″ 5/64″ 1.778mm
#2 0.08″ 3/32″ 2.032mm
#3 0.09″ 7/64″ 2.286mm
#4 0.11″ 7/64″ 2.794mm
#5 0.12″ 1/8″ 3.048mm
#6 0.13″ 9/64″ 3.302mm
#8 0.16″ 5/32″ 4.046mm
#10 0.19″ 3/16″ 4.826mm
#12 0.21″ 7/32″ 5.334mm
#14 0.24″ 1/4″ 6.096mm

See Also: Millimeters, Inches, Fraction, Decimals


Millimeters, Inches, Fraction, Decimals

inches mm
1/16 0.0625 1.5875
1/10 0.1 2.54
1/8 0.125 3.175
3/16 0.1875 4.7625
1/4 0.25 6.35
5/16 0.3125 7.9375
3/8 0.375 9.525
7/16 0.4375 11.1125
1/2 0.5 12.7
9/16 0.5625 14.2875
5/8 0.625 15.875
11/16 0.6875 17.4625
3/4 0.75 19.05
13/16 0.8125 20.6375
7/8 0.875 22.225
15/16 0.9375 23.8125
~ 1.0 24.5

Handy chart for conversion between stupid Imperial units (fractions and decimals) and smart Metric units.

See Also: Mixing Imperial and Metric and Screw/Bolt Diameters

Get to this quickly using


Mixing Imperial and Metric

Converting Imperial to Metric

In an ideal world, we’d all be using the Metric system, but the world is far from ideal, and math is confusing, so we’re sort of stuck.

Most of the design work I do for objects (laser cut or 3D printed things) is done in millimeters. This makes sense, and is oh so easy compared to trying to use those damn inches! Sadly, I grew up with Imperial Units, and while I can roughly estimate what 12 inches is, I still have problems visualizing something like 100 millimeters. I can usually remember that it’s about 4 inches, but it’s still difficult to put my hands in space 100 millimeters apart without first converting it in my mind to 4 inches.. Dammit!

Converting Imperial to Metric

So I end up converting values. Typically using Google’s conversion utility (because I’m in front of a computer 98% of the time I’m designing something.)

So if I wanted to find the Metric equivalent of 69/500″ (also known as 0.1380″) which is the diameter of a 6-32 bolt, I can find out. Oddly enough, when it comes to the small numbers, visualizing things is the opposite! It’s easy to visualize 3.5mm but impossible for me to imagine what 69/500″ looks like. Whatever!

Here’s a simple chart of bolt sizes I’m posting just for my own use.

5-40 0.1250″ 1/8″ 3.175mm
6-32 0.1380″ 69/500″ 3.5052mm
8-32 0.1640 41/250″ 4.1656mm

Update: A larger chart!

While I would love to use Metric hardware more often, it’s expensive! I usually just buy random bits from the local hardware stores, and the Metric stuff is in short supply, and cost more. I find it quite annoying. I don’t know if prices online are any better, a quick search seemed to suggest not.

(And don’t even get me started on pixels!)