How to Choose Drill Bit Size? | Match the Bit to the Job

The wrong drill bit size turns a five-minute job into a ruined board and a trip back to the hardware store. Pilot holes that are too small split the wood. Clearance holes that are too tight bind the screw. Wall anchors that don’t fit either strip the drywall or won’t grip at all. The fix for each of these problems follows the same logic — match the bit to the screw’s shaft, not its threads — with a few adjustments for material and purpose. Here is the exact method, plus the chart you need to get it right the first time.

The One Rule That Covers 90% of Home Drilling

The most common mistake is choosing a bit that matches the screw’s outer thread diameter. That makes the pilot hole too wide, and the threads have nothing to bite into. The correct pilot-hole bit is roughly the same thickness as the screw’s shaft — the solid center column between the threads.

There is a fast visual test that takes two seconds. Hold the drill bit directly in front of the screw. The ideal view shows the threads clearly but hides the shaft behind the bit. If you see both the shaft and the threads, the bit is too small — size up. If you see neither, the bit is too large — size down.

For soft woods like pine, choose a bit slightly smaller than the shaft. For hard woods like oak, choose a bit equal to the shaft or a hair larger. That adjustment stops the wood from splitting while still giving the threads enough material to grip.

Pilot Hole, Clearance Hole, or Anchor: Each One Is Different

A pilot hole is drilled into the piece where the threads will grab. A clearance hole is drilled through the top piece so the screw passes freely. A wall anchor hole is sized to the anchor’s body, not the screw at all. Mixing these three up is the fastest route to a failed installation.

Pilot Holes for Wood Screws

Pre-drilling prevents splitting and makes driving the screw easier without stripping the head. Drill Bit Warehouse recommends holding the bit in front of the screw as described above, and adjusting for material hardness. In manual machining, a pilot hole is also required before drilling any hole larger than 3.2 mm in ferrous metal — the center web of a twist drill doesn’t cut efficiently, and the pilot keeps the hole from wandering.

Clearance Holes

This hole is the same diameter as the screw’s outer threads, or slightly larger, so the screw passes through without catching. Use a drill bit size chart to find the right bit for the screw’s major diameter. The Reddit r/DIY guide on matching screws to bits notes that clearance holes use the screw’s full outer diameter plus a small allowance for fit.

Wall Anchors

The bit size equals the anchor body — the plastic or metal sleeve — minus the flange. Test it: the anchor should push into the hole with thumb pressure only. If you have to hammer it, the hole is too small. If it slides in loosely, the hole is too large and the anchor won’t grip when expanded.

Drill Bit Size Chart: The Reference You Actually Need

CustomPartNet’s comprehensive size chart covers all four systems side by side. Keep that page bookmarked — it converts between fractional, metric, gauge, and letter sizes in one glance. Most home drills use a 3/8-inch or 1/2-inch chuck, so check that the bit shank fits before you buy.

How to Read the Numbers on a Drill Bit

Every quality drill bit has a number engraved on the shank. “4 HSS” means the bit is 4 mm in diameter and made of high-speed steel — the standard for general-purpose drilling. Metric numbers are straightforward: 5 mm equals approximately 3/16 inch, 6 mm is just under 1/4 inch. Gauge numbers are trickier: a #4 bit is 0.2090 inches, and a #5 bit is 0.2055 inches. Gauge number size decreases as the number goes up — #80 is the smallest common size at 0.0135 inches.

If the engraving is worn off, use a pair of calipers or a micrometer to measure the bit’s diameter at the shank. A physical drill bit gauge, available at any hardware store, also works as a quick reference. For readers tackling tougher materials, our tested roundup of drill bits for hardened steel covers the specific bits that handle high-carbon and tool-steel work without dulling on the first pass.

Material-Specific Adjustments That Save the Hole

Three Common Mistakes That Waste Time and Materials

Choosing based on thread diameter is the most frequent error. A bit that matches the threads creates a hole too wide for the threads to grip, and the screw spins freely instead of pulling tight. Always reference the shaft, not the threads, for pilot holes.

Forcing wall anchors into undersized holes crushes the anchor and ruins the drywall. If the anchor won’t push in with a thumb, re-drill one step larger. If it slides in loosely, the anchor won’t hold — step down one size and re-drill the correct hole.

Drilling blind holes without marking depth leads to holes that go deeper than planned, breaking through the far side or damaging material underneath. Wrap a piece of tape around the bit at the desired depth before drilling.

Checklist: Choose the Right Drill Bit Every Time

Check the marking on the shank. If it is missing, measure with calipers. Identify whether you need a pilot hole, clearance hole, or anchor hole. For pilot holes, hold the bit in front of the screw — you want to see the threads but not the shaft. Adjust for material hardness. For clearance holes, use a bit larger than the screw’s outer diameter. For wall anchors, measure the anchor body minus the flange. Verify the bit shank fits your drill’s chuck. Mark depth on the bit for blind holes. Drill a test hole in scrap if you are unsure. Use the chart above to convert between measurement systems when the bit doesn’t match the screw’s spec.