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Lesson 01 · Beginner level

What the Heck Is a
Hole Callout?

A couple of symbols, four holes, and one thing to remember: the holes have to be the right size and in the right place.

Meet this line on a drawing:

4X Ø.50 THRU

Looks like alphabet soup? Not for long. Let’s read it one piece at a time:

4X

Four identical features. In this example, four holes.

Ø.50

The Ø means diameter—how wide the hole is across its center. If the print uses inches, .50 means half an inch.

THRU

The hole goes all the way through the material.

Put it together

Four half-inch-wide holes, all the way through the part (if the drawing uses inches).

Illustrated flange drawing showing four through-holes and bolt-circle explanation
Teaching illustration: a flange with four through-holes. Not a production drawing or complete specification; always check the actual print.

Okay, so where do the holes go?

That's a different question. The hole callout tells you their size and depth; it doesn't fully define their locations by itself.

Our example has a Ø3.00 bolt circle: each hole center sits on an imaginary circle that's 3 inches in diameter. If the drawing also specifies four holes equally spaced around the circle, their nominal angular spacing is 90°.

Shop-floor translation: “Four half-inch holes through the part. Their centers belong on the specified bolt circle, at the angles the drawing calls out.” Easy enough. Now don't go drilling them wherever they look pretty.

Don't confuse nominal size with tolerance

One new word: nominal. That just means the size the drawing is aiming for. A tolerance tells you how far the actual size may vary.

The callout shows a nominal diameter. The allowed variation in hole diameter comes from the drawing's applicable size tolerance, which may be specified next to the dimension or in general notes/title-block tolerances.

But the hole can be the perfect size and still be in the wrong spot. The drawing must also tell you where the holes belong and how much their locations are allowed to vary. A bolt-circle dimension alone does not tell you every acceptable location limit.

Quick check: did it stick?

What does “4X Ø.50 THRU” tell you?

Keep the new guy off the scrap pile.

Next lesson: what to look at before you ever start making parts.

NEXT: CHECK THE PRINT →

Note: This lesson uses a simplified example. Interpret customer drawings using their designated drawing standard, datum scheme, feature controls and applicable specifications.