Tool Belt CalcMeasure
Screw Size Chart
Screw gauges with diameters and pilot hole sizes.
| Gauge | Major dia (in) | Major dia (mm) | Pilot, softwood (in) | Pilot, hardwood (in) |
|---|---|---|---|---|
| #2 | 0.0860 | 2.18 | 0.060 | 0.073 |
| #4 | 0.1120 | 2.84 | 0.078 | 0.095 |
| #6 | 0.1380 | 3.51 | 0.097 | 0.117 |
| #8 | 0.1640 | 4.17 | 0.115 | 0.139 |
| #10 | 0.1900 | 4.83 | 0.133 | 0.162 |
| #12 | 0.2160 | 5.49 | 0.151 | 0.184 |
| #14 | 0.2420 | 6.15 | 0.169 | 0.206 |
| #16 | 0.2680 | 6.81 | 0.188 | 0.228 |
Pilot sizes are a rule of thumb — roughly 70% of the major diameter in softwood and 85% in hardwood, so the thread has material to bite while the shank does not split the piece. Dense hardwoods and end grain want the larger figure; always test in an offcut.
Screw gauge is a linear scale — each step adds thirteen thousandths of an inch to the major diameter — so the whole chart is computed. Pilot sizes follow as a proportion of that diameter, which is what the rule of thumb actually is.
Why use this tool?
Computed from the definition
Major diameter is 0.060 plus 0.013 per gauge, so every row is exact.
Pilot holes both ways
Softwood and hardwood, derived as proportions of the major diameter.
Inches and millimetres
For matching against either drill set.
Test in an offcut
Dense hardwood and end grain both want the larger pilot.
How this screw size chart works
Screw gauge is linear: major diameter in inches is 0.060 plus 0.013 times the gauge number. A #8 screw is therefore 0.060 + 0.104 = 0.164 inches. Every row here comes from that expression.
Pilot holes are given as roughly seventy per cent of major diameter in softwood and eighty-five per cent in hardwood. That leaves the thread material to bite while giving the shank enough room not to split the piece.
Those proportions are guidance, not specification. Dense hardwoods, end grain and screwing near an edge all want the larger figure, and the only reliable check is a test hole in an offcut of the actual material.
How to use it
Step 1: Find your screw gauge
Printed on the box — #6, #8, #10 and so on.
Step 2: Read the major diameter
The outside diameter across the threads.
Step 3: Pick a pilot
Softwood or hardwood column, then the nearest drill above it.
Step 4: Test first
Especially near an edge or in end grain, where splitting is likeliest.
Example usage
- The most common screw
- A #8 is 0.164 in major diameter — 4.17 mm. Softwood pilot about 0.115 in, hardwood about 0.139.
- Bigger fasteners
- A #12 is 0.216 in, over thirty per cent thicker than a #8, which is why it needs a noticeably larger pilot.
- Small work
- A #4 is only 0.112 in. In hardwood that wants roughly a 0.095 in pilot — skipping it will split thin stock.
Frequently asked questions
What size is a #8 screw?
0.164 inches major diameter, or 4.17 mm. Gauge is linear: 0.060 plus 0.013 per gauge number.
What size pilot hole do I need?
Roughly 70% of the screw's major diameter in softwood and 85% in hardwood. For a #8 that is about 0.115 and 0.139 inches respectively.
How is screw gauge calculated?
Major diameter equals 0.060 plus 0.013 times the gauge. That linear relationship generates the whole scale, which is why this chart is computed rather than looked up.
Do I always need a pilot hole?
In hardwood, near an edge, or in end grain, yes. Softwood in the middle of a wide board often takes a screw without one, though a pilot still reduces splitting and driving torque.
What about self-drilling screws?
They cut their own pilot and generally do not need one in softwood. In hardwood a pilot still helps, and near an edge it is still worth drilling.
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