What Is a Clearance Hole?
Bolts and screws only do their job when they can slide through one part freely and bite into another. The plain, unthreaded hole that lets them do that is called a clearance hole: a hole drilled slightly larger than the fastener's shaft, so the threads never touch the sides.
Get the size right and assembly feels effortless — the shaft slips through, the head pulls up flat, and the joint clamps tight. Too small and the bolt jams halfway. Too large and you sacrifice bearing area and alignment. Clearance holes show up everywhere parts are bolted together: furniture, brackets, steel plates, 3D-printed enclosures.
The Clearance Hole Formula
This calculator uses a simple average of the two dimensions that matter:
[
\text{Clearance Hole Diameter} = \frac{\text{Shaft Diameter} + \text{Head Diameter}}{2}
]
Where:
- Shaft Diameter is the outside diameter of the bolt or screw shank
- Head Diameter is the width across the widest part of the head
Why the average works: the hole must be larger than the shaft or nothing passes through, and smaller than the head or the head pulls straight through. Splitting the difference lands safely inside that window, giving comfortable slip fit for the shaft while leaving plenty of bearing surface for the head. Both measurements must be in the same units — the units cancel, so the answer comes out in whatever unit you started with.
Worked Example: An M10-Style Bolt
Suppose you're fastening two plates together with a bolt that has:
- Shaft Diameter: 10 mm
- Head Diameter: 16 mm
Step 1: Apply the formula
[
\text{Clearance Hole} = \frac{10 \text{ mm} + 16 \text{ mm}}{2}
]
Step 2: Calculate
[
\text{Clearance Hole} = \frac{26 \text{ mm}}{2} = 13 \text{ mm}
]
So you would drill a 13 mm clearance hole. That is 3 mm wider than the 10 mm shaft — easy sliding fit — yet still 3 mm narrower than the 16 mm head, which seats firmly against the plate. Try those same numbers in the calculator above to confirm.
Clearance Hole Reference Table
Common imperial bolt sizes, calculated with the same formula:
| Fastener Shaft | Head Diameter | Clearance Hole |
|---|---|---|
| 0.250 in (1/4") | 0.438 in | 0.344 in |
| 0.375 in (3/8") | 0.563 in | 0.469 in |
| 0.500 in (1/2") | 0.750 in | 0.625 in |
| 0.750 in (3/4") | 1.125 in | 0.938 in |
| 1.000 in (1") | 1.500 in | 1.250 in |
For quick mental math in inches: the clearance hole is always about 1.4× the shaft diameter whenever the head is roughly 1.75× the shaft, which covers most standard hex-head hardware.
Choosing a Real-World Drill Size
The formula gives you a target diameter; the drill aisle gives you fixed choices. Three practical rules:
- Pick the nearest standard drill bit at or above your calculated diameter — never below, or the bolt won't pass through.
- If the joint needs precise location between parts, go closer to the shaft size (a closer fit); if it needs easy assembly with sloppy tolerances, go toward the head size (a looser fit).
- For load-critical work, published standards like ASME B18.2.8 list exact close/normal/loose fit drills per bolt size — treat this calculator as your starting estimate, not a substitute for the drawing.
Quick Recap
- A clearance hole lets a fastener slip through unthreaded material so the threads grip only the far side.
- Formula: clearance hole = (shaft diameter + head diameter) ÷ 2, same units for both.
- The result sits safely between "too small to pass the shaft" and "too big to hold the head".
- Verify critical sizes against a published standard or engineering drawing.
Once your holes are drilled and the hardware chosen, the bolt length calculator helps you pick a fastener long enough to reach through both parts and engage the nut properly.