Comprehensive Imperial Screw Size Gauge

An screw sizing gauge for all standard imperial diameters screws to help classify both flat and round head screws
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updated October 20, 2024

Description

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A screw sizing gauge for all standard imperial size screws (at least all the ones in the McMaster-Carr Catalog) as shown below.

The sides are angled and slotted to make it very easy to align and determine the length of screws/bolts with either a round head/flat bottom or with a flat head/chamfered bottom.

Determining Fastener Length

This model makes it easy to determine the length of your fasteners, thanks to the angled and slotted sides.  Angled side helps hold screws in place for measurement, and the slots make it easy to determine the length of the screw either visually or by placing a fingernail between the slots as shown in the photos.

Measuring Flat-Bottomed Screws

To measure screws with a flat bottom (including those with a hex, round, or socket head) use the left size of the gauge and place the flat bottom of the screw on top of the gauge and the shaft of the screw down through the channel.

Measuring Flat-Topped Screws

To measure screws with a flat top and an angled bottom, use the right side of the gauge, placing the flat top of the screw against the flat portion of the gauge, with the head in the cutout.

Supported Thread Sizes

The table below lists all of the supported thread sizes.

Thread SizeDiameter
#20.086"
#30.099"
#40.112"
#50.125"
#60.138"
#70.151"
#80.164
#90.177"
3/16"0.1875"
#100.190"
#120.216"
#140.242"
1/4"0.25"
#160.268"
#180.294"
5/16"0.3125"
#200.320"
#240.372"
3/8"0.375"
1/2"0.5"
5/8"0.625"
3/4"0.75"

Note: Depending on your printer's calibration and the filament you use the holes may be very slightly undersized, if so it's helpful to screw a fastener of known size into the relevant hole and then use a drill/driver to spin the screw boring out the hole.

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