This calculator estimates a practical timber screw length using member thickness, target embedment, drive angle, and allowances. It then snaps to a common standard length.
RawLength = ( (T1 + HeadAllowance) + (EmbedTarget / AngleFactor) + TipAllowance + Clearance ) × SafetyFactor
BoundedLength = min(RawLength, (T1 + T2 − 2))
RecommendedLength = nearestStandard(BoundedLength)
AngleFactor reduces effective embedment at lower angles. The no-protrusion limit subtracts 2 mm to reduce the chance of a sharp tip showing through.
- Choose units and enter both member thickness values.
- Select a connection type and your planned drive angle.
- Set embedment as a percentage or a fixed target value.
- Enter head, tip, and clearance allowances as needed.
- Press Submit to view the recommended standard length.
- Download a CSV or PDF summary for your records.
| Use case | First member | Second member | Angle | Target embedment | Typical standard length |
|---|---|---|---|---|---|
| Deck board to joist | 28 mm | 45 mm | 90° | 65% | 60–70 mm |
| Ledger to rim | 38 mm | 89 mm | 90° | 75% | 120–150 mm |
| Blocking to stud (angled) | 38 mm | 89 mm | 60° | 70% | 150–180 mm |
| Sheathing to framing | 12 mm | 38 mm | 90° | 55% | 35–45 mm |
Selecting Embedment Depth
Embedment controls withdrawal resistance and joint stiffness. A practical target is 60–75% of the second member thickness for framing, while sheathing often works around 50–60% to avoid blow‑through. For a 38 mm first member into an 89 mm second member, 70% embedment targets about 62 mm of penetration after the interface. Increase the target when loads are cyclic, or when the second member is end‑grain.
Angle Effects on Holding Power
Driving at 90° delivers the full embedment, but toe‑screwing reduces effective penetration. At 60°, the cosine factor is about 0.87, meaning you need more length to reach the same normal embedment. At 45°, cosine is about 0.71, so a 70 mm screw behaves closer to 50 mm of straight embedment once head and tip allowances are removed. Use lower angles only where access demands it.
Head Allowance and Surface Finish
Countersinking changes required length but also changes clamping. Typical head allowance ranges from 2–5 mm for flush seating in softwood and 5–8 mm when a deeper recess is needed for plugs or filler. If a washer is used, add its thickness as clearance. For hardwood, pre‑drill and keep countersink modest to reduce splitting.
Standard Lengths and Stock Planning
Most timber screws are stocked in steps such as 50, 60, 70, 80, 90, 100, 120, 150, and 200 mm. Selecting the nearest standard avoids special orders and keeps crews consistent. If the nearest size lands near the no‑protrusion limit, choose the “down” size and increase diameter or add a second fastener. Keep at least two adjacent lengths on site for adjustments.
Field Checks and Safety Notes
Confirm the assembled thickness, not just nominal sizes. Moisture movement, planing, and shims can change total depth by 2–6 mm. Use a safety factor around 1.05–1.15 for typical carpentry and closer to 1.20 when holes are misaligned or surfaces are uneven. Always verify code fastener schedules for ledgers and structural connections. Mark a test screw, drive once, and measure tip exposure first.
1) Is longer always better for timber screws?
No. Length must achieve embedment without protruding. Excess length can split members, hit hidden services, or reduce fit quality. Use the nearest standard length that meets embedment and respects the no‑protrusion limit.
2) What safety factor should I choose?
Use 1.05–1.15 for typical carpentry. Choose 1.15–1.20 when surfaces are uneven, holes are slightly misaligned, or wood moisture varies. Higher factors can push lengths toward protrusion, so recheck limits.
3) How does toe‑screwing change length needs?
Angled driving reduces effective embedment into the second member. A lower drive angle needs a longer screw to reach the same penetration. If you must drive at 60° or less, verify embedment and consider pre‑drilling.
4) When should I use fixed embedment instead of percent?
Use fixed embedment when drawings, engineering notes, or manufacturer guidance specify a minimum penetration. Percent targets are helpful for general framing, but fixed values are better for repeatable production or critical connections.
5) Does countersinking affect holding strength?
Yes. More countersink increases required length and can reduce surface bearing if overdone. Keep countersink just deep enough for flush seating or plugs. For softwood, 2–5 mm is common; adjust for hardware and finish needs.
6) Should I trust the “nearest standard” every time?
Use it as a practical starting point. If the nearest size is close to the no‑protrusion limit, choose the next shorter size and compensate with diameter, spacing, or additional fasteners. Always verify against local code requirements.