How To Know What Size Pilot Hole To Drill | The 1/64 Inch

Many DIYers skip pre‑drilling because it feels like an unnecessary step. Grab a screw, drive it in, move on — that’s the instinct. One split board and a stripped screw head later, the lesson sticks.

A pilot hole solves both problems. It lets the screw pass through without forcing the wood fibers apart, and it keeps the threads biting where they should. The trick is picking the right drill bit — too small and you’ll still split the wood, too large and the screw won’t grip. The answer is simpler than it sounds.

The Two Factors That Determine Pilot Hole Size

Every screw has two key dimensions: the shank (the smooth part between the head and the threads) and the thread. The pilot hole needs to match the shank diameter, not the full thread width. That leaves the threads room to cut into the wood and hold tight.

The second factor is material hardness. Hardwood (oak, maple, walnut) demands a slightly larger hole than softwood (pine, fir, cedar). A hole that works in pine will often split oak because the wood can’t compress around the threads.

Bolt Depot publishes a pilot hole chart that covers common screw sizes for both wood types. The differences are small — often just 1/64 inch — but they matter.

Why Guessing Leads to Split Wood or Stripped Holes

Without a pilot hole, the screw acts like a wedge. Forcing a wide thread into dense hardwood pushes the wood fibers apart — that’s how splits start. A crack near the edge of a board can ruin the whole piece.

Too large a pilot hole has its own trouble. The threads barely catch the wood, so the screw spins freely when you try to tighten it. What you get is a loose joint that needs a bigger screw or wood filler to fix.

• Split wood: The hole is too small for the screw’s shank, or you skipped the pilot hole entirely. This is most common when driving screws near the edge of a board.
• Stripped screw (no grip): The hole is too large, so the threads don’t have enough material to grab. The screw turns but doesn’t pull the board tight.
• Hard to drive: The hole is the right size but you’re using softwood pilot specs on hardwood. The screw binds and your drill struggles or strips the head.
• Poor holding power: Even if the joint seems tight, a too‑large pilot hole reduces the screw’s pull‑out strength. Over time the connection can loosen.
• Broken screw heads: When the wood resists too much, the torque builds up and snaps the head off — especially with small #6 or #8 screws in hardwood.

Most of these issues vanish once you match the bit to the screw and the wood. The 1/64‑inch rule from All Points Fasteners is a solid starting point: choose a bit 1/64 inch larger than the screw’s shank for hardwood, or the exact shank size for softwood.

How To Find the Right #8 Screw Pilot Hole Size

The #8 screw is the most common size in general‑purpose woodworking — cabinet hinges, drawer slides, furniture assembly. Its shank measures roughly 0.128 inches. For softwood, a 3/32 inch bit (0.09375 inches) is slightly under the shank, allowing the threads to bite while the shank passes freely. For hardwood, step up to 1/8 inch (0.125 inches).

Lowes defines a pilot hole as a small‑diameter hole drilled before driving a screw to prevent splitting, and their pilot hole definition points out that you can also use a tapered bit designed to match the screw’s full profile — including the countersink. Those bits simplify the process: one bit drills the pilot and countersink at once.

If you’re drilling into hardwood with a #8 screw, start with 1/8 inch and test on a scrap piece. The screw should turn with noticeable resistance but not feel like it’s fighting you. If it’s too easy to drive, the hole is too large.

Metal Screws Require a Different Formula

Metal screws — sheet metal screws and tapping screws — don’t compress like wood. The pilot hole must allow the threads to cut into the metal without binding the shank. A common approach is to use a drill bit equal to the screw’s outer diameter minus the thread depth.

Fastenere’s chart gives specific sizes for sheet metal screws. For a #8 sheet metal screw, the recommended pilot hole is 0.1360 inches, which corresponds to a #29 drill bit. For a #10 screw, it’s 0.1610 inches (#20 bit).

For tapping threads in metal (creating threads for a machine screw), the formula gets more technical. The standard tap drill size for a 1/4‑20 UNC thread at 75% engagement is a #7 drill bit (0.2010 inches). For a 3/8‑16 UNC thread, it’s 5/16 inch. Metric threads follow a similar pattern:

1. Identify the thread size and pitch. Example: M6 x 1.0 — the “6” is the outer diameter in mm, the “1.0” is the pitch.
2. Apply the 75% thread engagement formula. For M6 x 1.0, the tap drill is 5.2 mm. For M8 x 1.25, it’s 6.9 mm. For M10 x 1.5, it’s 8.7 mm.
3. Use a lubricant. Cutting fluid or even a few drops of oil on the tap reduces friction and gives cleaner threads.
4. Back out the tap regularly. Turn it forward half a turn, then back a quarter turn to break the chips and prevent binding.
5. Test with the bolt. The bolt should thread in with moderate resistance but not require a wrench to seat fully.

Never guess on metal pilot holes — the small margin for error can ruin the workpiece or break the tap. Refer to a tap drill chart for your specific thread.

Pilot Hole Tips That Save Time and Materials

Before drilling into your actual project, find a scrap piece of the same material. Test the pilot hole size and see how the screw drives. If it splits or strips, adjust the bit size before you commit.

Per the 1/64 inch rule from Allpointsfasteners, this rule of thumb works for most wood screws: hold the screw next to the bit — if the bit is the same diameter as the shank (the smooth part), you’re close enough. For hardwood, go one step larger (1/64 inch). For softwood, stay with the shank size or one step smaller.

Also check your drill’s chuck size. A compact 3/8‑inch drill can handle bits up to 3/8 inch shank, but some larger bits (like 1/2 inch and up) need a 1/2‑inch chuck. If you plan to drill pilot holes for big screws, make sure your drill can accommodate the bit.

The Bottom Line

A pilot hole is a small step that keeps your work from splitting and your screws from stripping. Match the bit to the screw’s shank, adjust for hardwood vs. softwood, and test on scrap before you commit. The tables and rules above cover the most common sizes — keep a printed chart in your toolbox for quick reference.

If you’re working on a critical project or an unfamiliar material, bring a sample screw to your local hardware store and ask the staff to help match a bit; they can often pull a pilot hole chart right at the counter.