Get a starting pilot hole and drilling depth for a threaded insert in wood, and check that enough wood is left under a blind hole. Covers the common inch and metric sizes in softwood and hardwood.
A threaded insert turns a soft wooden part into something you can bolt to again and again: knock-down furniture, jig fences, table-leg hangers, router-table accessories. The outside thread of the insert cuts its own path in the wood, so the pilot hole has to be big enough that the insert does not split the part, and small enough that the outer thread has wood to bite.
Hardwood is less forgiving than softwood, so the hardwood pilot is a size larger. The depth is the insert length plus a little extra so the insert can sit just below the surface without bottoming out.
| Insert | Softwood pilot | Hardwood pilot |
|---|---|---|
| 1/4-20 | 5/16 in | 11/32 in |
| 5/16-18 | 3/8 in | 13/32 in |
| 3/8-16 | 7/16 in | 15/32 in |
| M6 | 8 mm | 8.5 mm |
| M8 | 10 mm | 10.5 mm |
These are the calculator’s starting sizes for compact inserts. Insert outer diameters vary noticeably between brands and styles — knife-thread, hex-drive, flanged, and press-in — so if the packet gives a pilot size, use the packet. When in doubt, drill a test hole in an offcut of the same species.
Depth = insert length + 1/16 in (about 1.5 mm). The wall left under the hole is the stock thickness minus that depth. Less than about ⅛ in of wall in a blind hole is fragile: the insert can push a plug of wood out of the far face as it bottoms.
depth = L + 0.0625 in · wall = stock − depth
| Insert length | Drill depth | Wall in ¾ in stock | Wall in 1 in stock |
|---|---|---|---|
| 0.375 in | 0.4375 in | 0.3125 in | 0.5625 in |
| 0.5 in | 0.5625 in | 0.1875 in | 0.4375 in |
| 0.625 in | 0.6875 in | 0.0625 in | 0.3125 in |
A ¼-20 insert in a pine jig fence. ½ in long insert, ¾ in stock, softwood: 5/16 in pilot, 0.5625 in deep, leaving 0.1875 in of wall. That is fine for a blind hole.
An M8 insert in an oak table apron. 13 mm long insert, ¾ in stock, hardwood: 10.5 mm pilot, depth 0.5743 in (14.6 mm), wall 0.1757 in. Workable, but drill with a depth stop.
A 5/16-18 insert that is too long. ⅝ in insert in ¾ in hardwood: the depth comes to 0.6875 in and the wall to 0.0625 in. The calculator warns about it. Use a shorter insert, thicker stock, or a through hole with the insert installed from the show face.
The part split. The pilot was too small for that insert, or it was too close to an edge. Move the hole in from the edge, go up a size, or drill a larger hole and bed the insert in epoxy.
The insert spins without biting. The pilot is too big or the wood is very soft. Epoxy the insert in, or use a larger insert.
It went in crooked. Back it out, plug the hole with a glued dowel, and redrill. Forcing a crooked insert straight usually strips it.
Yes, but treat MDF and particleboard gently — use the softwood size or slightly larger, and keep away from edges. Plywood behaves like a hardwood with many glue lines.
Metric inserts are usually specified with metric drills. The depth is still reported in both inches and millimetres.
Not usually. A good pilot gives plenty of hold. Epoxy is a repair for an oversize hole or a very soft wood.
Keep at least the insert’s outer diameter of wood between the hole and the edge, more in end grain or brittle species.