Countersink Depth Calculator
Find how deep to countersink a flat-head screw so it seats flush, from the head diameter and cone angle.
Reviewed by the ToolNestr Editorial Team — July 2026
How the countersink depth calculator works
A flat-head screw sits flush when it drops into a cone-shaped recess that matches its head. The countersink is that cone, and its depth is set by simple geometry. The recess must open at the surface to the full head diameter, and it narrows at the screw's included angle. The depth is half the head diameter divided by the tangent of half the included angle.
The included angle is the full cone angle of both the screw head and the countersink cutter. Inch flat-head wood and machine screws are 82°, metric screws are 90°, and some rivets and sheet-metal fasteners use 100° or 120°. The cutter must match the screw: an 82° head dropped into a 90° recess touches only at the rim, so it can spin, sit crooked, or crack the surface instead of spreading its load across the whole cone.
Cutting to the calculated depth leaves the head exactly flush. Going slightly deeper sinks the head below the surface for filling or plugging; going shallower leaves it proud. Because the surface diameter equals the head diameter, the tool also reports that width so you can watch the growing rim as you cut and stop when it reaches the head size. Values are given in both inches and millimeters.
Fastener head and countersink geometry follow ANSI/ASME B18.6.3 for machine screws and related flat-head fastener standards.
Measure the head
The flat-head screw's top diameter.
Match the angle
82° inch, 90° metric, or another standard.
Cut to depth
Stop when the rim reaches the head diameter.
The formula explained
Depth
= (head ÷ 2) ÷ tan(angle ÷ 2). A 0.316 in head at 82°: 0.158 ÷ tan(41°) = 0.182 in.
Half-angle
= angle ÷ 2 = 82 ÷ 2 = 41°, the angle of the cone wall from center.
Surface diameter
Equal to the head diameter, so the rim finishes flush with the material face.
Worked example
A metric M5 flat-head screw, head diameter 0.394 in (10 mm), 90° cutter.
Tips for cutting a clean countersink
Sneak up on the final depth
Cut a little shallow, test-fit the screw, and take a second light pass rather than plunging straight to the calculated depth. A countersink cut too deep cannot be undone, while one that is slightly shallow just needs another pass.
Use the correct cutter angle for the fastener
Keep separate 82° and 90° countersink bits on hand if you work with both inch and metric hardware. Swapping in the wrong angle produces a cone that only contacts the screw head at the rim, so it looks close but never actually seats flush.
Watch the rim diameter, not just the depth gauge
Because the finished countersink diameter should equal the screw head diameter, checking the growing rim with calipers as you cut is often more reliable than trusting a drill press depth stop alone, especially in softer woods where bit chatter can throw off the reading.
Real-world context
Cabinet and furniture work commonly uses #8 flat-head wood screws with a 0.316 in head at the standard 82° inch angle. Depth = (0.316 ÷ 2) ÷ tan(41°) = 0.182 in — deep enough that woodworkers often cut slightly past this to leave room for a wood plug or filler over the screw head, then sand it flush.
Slightly larger #10 screws, common in deck and structural framing hardware, have a 0.365 in head. At the same 82° angle, depth = (0.365 ÷ 2) ÷ tan(41°) = 0.210 in — about 0.03 in deeper than the #8 example above, which matters when setting a fixed depth stop for a batch of dozens of screws on a deck board.
Aerospace and sheet-metal rivets often use a 100° included angle rather than 82° or 90°. A 0.5 in diameter rivet head at 100° needs depth = (0.5 ÷ 2) ÷ tan(50°) = 0.210 in — nearly identical to the #10 wood screw depth above despite a much larger head, because the wider 100° angle produces a shallower cone for a given diameter than the narrower 82° angle would.
Common misconceptions
"Any countersink bit works as long as it's roughly the right size." The angle matters more than the size. A countersink cut at the wrong included angle only contacts the screw head at the rim of the cone, not along its full face, which concentrates stress at that edge and can crack brittle materials like hardwood or fiberglass.
"A deeper countersink always gives a stronger, cleaner joint." Not once you pass flush. Cutting well past the calculated depth removes extra material and weakens the surrounding surface for no benefit — the screw head is already fully seated at the flush depth, so any additional depth is purely wasted material removal.
Related calculators
Frequently asked questions
How deep should a countersink be?
For a flat-head screw to sit flush, the countersink depth equals half the head diameter divided by the tangent of half the included angle. For a 0.316-inch head at 82°, that is (0.316 ÷ 2) ÷ tan(41°) = about 0.182 inch deep.
What is the countersink included angle?
It is the full cone angle of the countersink and the matching screw head. Standard inch flat-head wood and machine screws use 82°, metric screws use 90°, and some rivets and aerospace fasteners use 100°, 120°, or others. The countersink must match the screw.
How wide should the countersink be at the surface?
The countersink diameter at the surface should equal the screw head diameter, so the head rim finishes flush with the material. Cutting it wider sinks the head below the surface; cutting it narrower leaves the head proud.
Why does the angle have to match the screw?
A flat-head screw only seats evenly if the cone it sits in has the same included angle. An 82° screw in a 90° countersink touches only at the rim and can spin or crack the surface; matching the angle spreads the load across the whole cone.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.