Screw length for joining two boards
Enter both board thicknesses. This gives the length to buy, the thread that ends up in the second board, and whether the screw you have comes out the far side.
= 0.75"
= 0.75"
Buy
1 1/4"screw
Ideal length
1 1/4"
Thread in the far board
1/2"
Longest that still clears
1 3/8"
Where to buy this
- 1 1/4" #8 wood screwsHome DepotLowe'sAmazon
- 5/64" drill bitHome DepotLowe'sAmazon
The length and gauge are the answer; head style, drive and coating are yours — a cabinet screw, a deck screw and a drywall screw of the same size are not interchangeable.
Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.
The 1 1/4" screw in this joint
- Enough thread in the far board — passes67% of the way in — the rule wants at least 50%.
- Tip stops short of the far face — passes0.25" of wood left past the tip.
The screw you picked is reported as it is rather than quietly swapped for one that passes — if a check is red, the length above is the one that would not be.
Holes for a #8
Clearance hole 11/64" through the first board, so the threads bite nothing there and the head can pull the joint tight. Pilot 5/64" into the second.
Both come from the same table the pilot hole calculator reads, so the two pages cannot disagree about the same screw.
Every stocked length that works here
- 1 1/4"nearest the ideal
- 1/2" of thread
About picking a screw length
- How long should a screw be for 3/4" plywood into 3/4" plywood?
- 1 1/4". The rule is that the screw passes through the first board doing no work, then buries about two thirds of the second — two thirds of 3/4" is 1/2", and 3/4" plus 1/2" is 1 1/4". That is why 1 1/4" pocket screws are the size you buy by the box for cabinet work.
- Why two thirds and not all the way?
- Because the last third is where the risk is and almost none of the holding. Withdrawal strength is proportional to how much thread is in the wood, so going from two thirds to seven eighths buys you about a quarter more hold and a real chance of the tip breaking the far face — which on a cabinet side or a finished top is damage you cannot undo. This page treats an eighth of an inch of wood past the tip as the minimum, and fails the check rather than shortening your screw for you.
- Does the first board's thickness really not matter to holding power?
- Not to withdrawal, no. The shank passing through it is smooth on a traditional wood screw, and even on a fully threaded screw the first board is being clamped by the head rather than gripped by the thread — that is the whole point of drilling a clearance hole through it. What the first board's thickness changes is the length you have to buy, which is why it is the first field.
- What if no stocked length fits?
- Then the joint needs a different fastener, and the page says so instead of rounding. Thin material with nothing suitable between "too short to hold" and "through the other side" is the normal case for a pocket screw, a through bolt, or simply glue and a clamp. It is a real answer and better than a screw that does one of those two things.
You might also need
- Pilot hole calculatorThe right drill bit for a wood screw, by gauge number and by softwood or hardwood.
- Screw and nail holding strengthWhat a screw, nail or lag holds before it pulls out, by species and by how deep the thread goes.
- Pocket hole screw calculatorScrew length and thread by material thickness, and how far apart to space the holes along an edge joint.