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How to Check a Chamfer or Countersink
Guide · Updated October 27, 2026
Countersink and chamfer sizes are hard to measure directly because the edge you need to measure isn't really there. The shop answer is a precision ball, a height reading and one formula. Here's how, with a gage block to keep the height reading honest.
Check your height reading
Build a check stack near the expected height with the free Gage Block Calculator. Free, no login.
The formula
D = 2 × tan(α) × (r ÷ sin α + r − h), where α is half the countersink angle (41° for 82°), r is the ball radius and h is the height of the ball top above the surface. If the ball top sits below the surface, h is negative.
Worked examples
| Countersink | Ball dia. | Ball top above surface | Countersink dia. |
|---|---|---|---|
| 82° | 0.500 in | 0.100 in | 0.9233 in |
| 90° | 0.500 in | 0.100 in | 1.0071 in |
| 100° | 0.500 in | 0.100 in | 1.1354 in |
Step by step
- Clean the hole and ball: Wipe the countersink and a precision ball (or bearing ball of known size).
- Seat the ball: Drop the ball into the countersink. It must touch the cone, not the hole edge.
- Measure the height: Zero a height gage or indicator on the surface next to the hole, then read the top of the ball.
- Check the gauge: Confirm the reading against a gage block of similar height.
- Calculate the diameter: D = 2 × tan(α) × (r ÷ sin α + r − h), with α = half the countersink angle.
Tips
- Pick a ball that seats on the cone and stands proud of the surface. Too small and it drops into the hole.
- Measure in a few spots around the hole to catch an oval or tilted countersink.
- Check the indicator itself with gage blocks first: see test indicator and height gage.
FAQ
How do you measure a countersink diameter?
Drop in a precision ball of known size, measure how far its top sits above the surface, and use D = 2 × tan(α) × (r ÷ sin α + r − h), where α is half the countersink angle, r the ball radius and h the height above the surface.
What is the countersink diameter for an 82° countersink with a 0.500 in ball 0.100 in above the surface?
About 0.9233 in, from D = 2 × tan 41° × (0.25 ÷ sin 41° + 0.25 − 0.100).
How do I check an outside chamfer?
For a rough check, use a chamfer gauge or an optical comparator. For an accurate check, measure the chamfer leg length on two faces with a height gage or calipers and compare with the drawing.
How do gage blocks help?
Zero the height gage or indicator on the surface and check its reading with a gage block close to the expected height, so the ball height is trustworthy. Build any check size free at gageblockcalc.com.
Can I use a pin instead of a ball?
For a V-groove or an outside chamfer edge, yes. For a round countersink, use a ball, because it seats on the cone evenly all the way round.
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General shop information only — follow your gage manufacturer's instructions and your quality system.