Hole sizes on engineering drawings are dimensioned using a combination of diameter callouts, depth notations, and special symbols that tell a machinist exactly what to make. The method depends on the type of hole: a simple drilled hole gets a straightforward diameter dimension, while a threaded hole requires a multi-part callout specifying thread size, pitch, and fit. Here’s how each type works.
Simple Drilled Holes
A basic round hole is dimensioned with its diameter, indicated by the symbol ⌀ (the Greek letter phi) followed by the size. On a drawing, you’ll typically see a leader line pointing to the hole with a note like ⌀ .500 or ⌀ 12.00, depending on whether the drawing uses inch or metric units. If the hole doesn’t go all the way through the part, the depth is added using the ⊻ symbol (a downward-pointing arrow) followed by the depth value. A through hole, one that passes completely through the material, is labeled with the abbreviation “THRU” or simply left without a depth note, since the drawing views make it obvious.
Inch-based drawings express hole diameters as three-place decimals (like .375 or .500), while metric drawings use millimeters with the diameter symbol (⌀ 10.0). In inch notation, dimensions under one inch typically drop the leading zero, so you’d see .750 rather than 0.750. Metric dimensions always include the leading zero when the value is less than one millimeter.
Threaded Hole Callouts
Threaded holes carry more information because a machinist needs to know the thread diameter, how fine the threads are, what standard they follow, and how tight the fit should be. The callout follows a specific sequence, and it differs between inch and metric systems.
Inch (Unified National) Threads
An inch thread callout reads in this order: nominal size, threads per inch, thread series, and class of fit. For example, .500 - 13UNC - 1A tells you the major diameter is .500 inches, there are 13 threads per inch, the thread series is UNC (Unified National Coarse), and the class of fit is 1A. The major diameter can be written as a fraction or a three-place decimal. Smaller sizes (below 1/4 inch) use numbered designations like #10 or #8 instead of decimals.
The thread series tells you the spacing between threads. UNC means coarse threads (fewer, larger threads per inch), UNF means fine threads (more, smaller threads), and UNEF means extra-fine. The class of fit, expressed as a number and letter like 2B, indicates how much play exists between the bolt and the hole. Classes 1 through 3 go from loose to tight, and the letter designates whether it’s an external thread (A) or internal thread (B). If the thread is left-handed, meaning it tightens by turning counterclockwise, “LH” appears at the end: .4375 - 20UNF - 2A, LH. Right-hand threads are the default and don’t need a label.
Metric Threads
Metric thread callouts start with the letter M, followed by the diameter in millimeters, a multiplication sign, and the pitch (the distance between threads) in millimeters. So M16 x 1.5 means a metric thread with a 16 mm diameter and a 1.5 mm pitch. This is simpler than the inch system because the pitch is stated directly rather than expressed as threads per inch. If the pitch is omitted, the standard (coarse) pitch for that diameter is assumed.
Counterbored and Countersunk Holes
Many holes are designed to let a fastener head sit flush with or below the surface. These require additional dimensions beyond the basic hole diameter.
A counterbored hole has a flat-bottomed recess at the top to accept a socket head cap screw or similar fastener. The callout lists the through-hole diameter first, then the counterbore diameter (marked with the ⌴ symbol), and the counterbore depth. A typical note might read: ⌀ .375 THRU ⌴ ⌀ .625 ⊻ .250, meaning a .375 through hole with a .625 diameter counterbore that’s .250 deep.
A countersunk hole has an angled recess instead of a flat one, designed for flat-head screws. The callout specifies the through-hole diameter, the countersink diameter (marked with the ⌵ symbol), and the included angle of the cone, which is almost always 82° or 90°.
Dimensioning Hole Patterns
When multiple holes are arranged in a pattern, each hole doesn’t get its own individual location dimension. Instead, the drawing defines the pattern geometry and labels it efficiently.
A bolt circle, one of the most common patterns, is a set of equally spaced holes arranged in a ring. The drawing specifies the bolt circle diameter (the circle on which all hole centers sit), the angular spacing between holes as basic dimensions, and the size of the holes themselves. A note like 6X ⌀ .375 THRU on a bolt circle tells the machinist there are six holes of that size evenly distributed around the circle. The “6X” prefix means “six times” and eliminates the need to call out each hole separately.
Rectangular hole patterns use coordinate dimensions instead. The drawing provides X and Y distances from a reference point (usually a datum or an edge) to each hole, or to the first hole with consistent spacing to the rest. When precision matters, these locations are given as basic dimensions inside rectangular boxes, paired with a geometric tolerance (position tolerance) that defines how far each hole center can deviate from its ideal location.
Tolerances on Hole Sizes
Every hole dimension on a production drawing includes a tolerance, either stated directly or inherited from a title block. A tolerance tells the machinist the acceptable range. You might see ⌀ .500 ± .002, meaning the finished hole can be anywhere from .498 to .502 and still pass inspection.
For tighter control, limit dimensions specify the minimum and maximum directly: ⌀ .498 / .502. The larger number is placed on top in most drafting standards. Holes that need to accept press-fit pins or bearings get very tight tolerances, sometimes just a few ten-thousandths of an inch, while clearance holes for bolts can be more generous.
Holes can also reference a standard fit system. In metric drawings, a hole might be dimensioned as ⌀ 25 H7, where “H7” is a code that defines a specific tolerance band without the drafter needing to calculate the numbers. The machinist or inspector looks up the actual limits in a reference table.