Guide to Holes on Blueprints [Types, Symbols & Features]

blind holes blueprint example
3 blind holes specified

Blind holes are holes that get drilled or milled to a specified depth but do not go all the way through the part.

thru hole blueprint example
Thru hole example

Thru holes

Thru holes get drilled all the way through a part.

They can be specified as “thru” as shown in the example or the blueprint can make it clear visually that the hole goes all the way through the part.

cutaway example of blind and thru holes
3 blind holes and 1 thru hole

Blind holes vs thru holes

This picture of a part cut in half shows the difference between a thru hole and a blind hole. The three holes on the left are blind holes which do not break through to the other surface.

The thru hole on the right goes all the way through the part.

Symbols used to dimension holes

Diameter Blueprint GD&T Symbol o with line through it
Diameter symbol


The diameter of the hole is the size. The diameter is the distance across the hole.

Radius Blueprint GD&T Symbol r
Radius symbol


Less often, holes are specified with a radial dimension instead of a diameter. The radius is the distance from the center of the hole to the edge of the hole. This is equal to 1/2 of the diameter.

Depth Blueprint GD&T Symbol line with arrow pointing down
Depth symbol


The depth of the hole is the distance that it is drilled to. 

Features associated with holes

Counterbore Blueprint GD&T Symbol u shape
Counterbore symbol

Holes often get counterbores that allow a fastener such as a bolt head to be recessed. 

Holes with a counterbore will specify the size and depth of the hole as well as the size and depth of the counterbore.

Countersink Blueprint GD&T Symbol two lines pointing down
Countersink symbol

Countersinks get added to holes as either a way to specify a chamfer for protective purposes or to allow a fastener such as a screw head to be recessed.

Holes with a countersink will specify the size and depth of the hole as well as the size and angle of the countersink.

Spotface Blueprint GD&T Symbol SF in a u
Spotface symbol

Spotfaces are functionally no different than a counterbore. They are machined on a part to allow a fastener to sit flush, though not always completely recessed.

Holes with a spotface will specify the size and depth of the holes as well as the size and depth of the spotface.

Note: spotfaces are not always specified with the symbol shown. At times, they will only use the counterbore symbol and other times they will use the SF notation only.

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