Do I really need a pilot hole?

For hardwood: yes, almost always. Driving a screw into oak or maple without a pilot will split the wood 80%+ of the time, especially near the edge. For softwood: pilot if the screw is #8 or larger, near an edge, or driving close to another screw. For #6 and smaller in pine/fir, you can usually drive without a pilot. Skipping pilots also strips screw heads (the torque required to drive without a pilot is 2-3x higher). When in doubt: pilot. Takes 5 extra seconds and saves you from a split board.

What pilot hole for a #8 wood screw?

Hardwood: 7/64" (0.109"). Softwood: 3/32" (0.094"). For really hard wood (white oak, hickory, ipe), bump up to 1/8" (0.125") to avoid splitting. The pilot doesn't need to be the full screw length — drilling 70-80% of the screw's thread length is plenty. The unbroken wood at the bottom gives the screw something to grab and pull tight.

What's the right tap drill for 1/4-20 UNC?

#7 drill (0.201"). Mathematical optimum for 75% thread engagement is 0.20" exactly; #7 is the closest standard drill. If you don't have a #7, 13/64" (0.203") works fine — slightly less engagement (~73%) but easier to tap. 5mm metric (0.197") is also acceptable. Don't use 7/32" (0.219") for 1/4-20 — that's too large, gives only ~60% engagement and weak threads.

What's the difference between #7 drill and 7/32" drill?

#7 is from the 'number drill' set (1-80, where #1 is largest at 0.228"). 7/32" is from the fractional set (1/64" increments). #7 = 0.201", 7/32" = 0.219". They're not the same — about 0.018" apart. Tap-drill applications need precision, so use the actually-correct number; for hobby drilling where exact fit doesn't matter, fractional is fine. Number drills, letter drills (A-Z, 0.234"-0.413"), and fractional all overlap in size range; pros own all three sets.

What size hole for a 1/4" through-bolt?

Clearance hole (the bolt passes through, doesn't thread): 17/64" or 9/32" — slightly larger than 1/4" so the bolt slides freely. The exact size depends on tolerance class: 'close fit' = 17/64" (0.266"), 'normal fit' = 9/32" (0.281"), 'free fit' = 19/64" (0.297"). Most DIY work uses normal fit. Threaded hole (where the bolt threads in): use a tap drill (see above). Don't drill the same size as the bolt — that's neither clearance nor tap, and the bolt won't fit either way.

What bit do I need for a Tapcon (concrete screw)?

Tapcons are sized by the screw diameter, but the bit size is specified separately on the package. 3/16" Tapcons (most common DIY size) use a 5/32" carbide masonry bit. 1/4" Tapcons use a 3/16" carbide bit. Always check the package — manufacturer-specific (Confast vs Tapcon brand vs generic concrete screws may differ slightly). Drill 1/4" deeper than the screw length to allow concrete dust to clear; vacuum or blow out the dust before driving the screw or it won't seat fully. Use a hammer drill or rotary hammer; standard cordless drills struggle with concrete past 1/4" depth.

Home & Life · Free tool

Drill Bit Size Lookup

Find pilot and anchor hole bit sizes for wood screws, machine screws, and wall anchors online. Instant results, free to use without registration.

Updated June 2026

ApplicationScrew / anchor size

Recommended bit

3/32"

0.0938 in · 2.38 mm

Alternatives

1/8

Notes

Softwood: 3/32" - Hardwood: 7/64"

Softwood grips threads easily so smaller pilot is fine. Hardwood needs a larger pilot to prevent splitting — go up one bit size if the screw still binds.

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What it does

Look up the right drill bit size for your application: pilot holes for wood screws (different sizes for softwood vs hardwood), anchor holes for drywall plastic anchors and toggle bolts, tap-drill holes for machine threads (UNC and UNF), masonry holes for concrete anchors, and clearance holes for through-bolts. Output covers fractional inches (1/16, 3/32, 1/8...), decimal inches (0.0625, 0.0938...), and metric (1.5mm, 2mm, 2.5mm...). Includes the practical softwood/hardwood adjustment that most generic charts skip.

Why pilot holes matter: driving a screw without a pilot in hardwood (oak, maple, walnut) splits the wood almost every time. Even softwood (pine, fir, cedar) splits if the screw is large or the hole is near an edge. Pilot holes also reduce torque required, preventing screw heads from stripping out (especially important with cheap screws). The general rule: pilot hole = ~70% of the screw’s thread diameter for softwood, ~85-90% for hardwood. A #8 wood screw (about 0.164" thread diameter) gets a 7/64" pilot in hardwood (~0.109", 66%) and 3/32" in softwood (0.094", 57%). Many tradespeople drill an additional counterbore for the screw head if it needs to sit flush.

Tap-drill holes for machine threads use a different rule: the tap drill diameter equals the major thread diameter minus the thread pitch. For UNC 1/4-20 (most common 1/4" coarse thread): tap drill = 1/4− (1 / 20) = 0.25 − 0.05 = 0.20", which rounds to #7 drill (0.201"). For metric M6 × 1.0: tap drill = 6 − 1 = 5mm. The math aims for ~75% thread engagement, which is the strength sweet spot. Higher engagement (smaller drill) makes tapping harder and breaks taps; lower engagement (larger drill) weakens the thread but is easier to tap. For aluminum or plastic, drop to 60-65% thread engagement (slightly larger pilot) because soft materials grip tightly anyway. For stainless steel, stick to 75% — never go above because tapping stainless is already difficult.

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How to use it

Pick application: wood screw pilot, drywall anchor, concrete anchor, machine thread tap drill, clearance hole, dowel hole.
For wood screws: enter screw size (#4, #6, #8, #10, #12 etc.) and wood type (softwood like pine/cedar, hardwood like oak/maple).
For machine threads: enter thread size (1/4-20 UNC, 5/16-18 UNC, M6 × 1.0 etc.) and the tool gives the tap-drill bit.
For drywall anchors: enter anchor type (plastic expansion, self-drilling, toggle bolt) and weight rating; tool returns the anchor body and hole sizes.
Read bit size in fractional inches, decimal, and millimeters. Pick whichever you have on hand — drill bits in the same decimal range are functionally identical.
For special cases: aluminum/plastic tap drills go 1 size larger than the chart (less engagement is fine in soft material); stainless taps stick to chart (going larger weakens threads in already-difficult-to-tap material).

When to use this tool

Driving wood screws — always pilot in hardwood; pilot in softwood for screws #8 and larger.
Tapping threads in metal or plastic — wrong tap drill ruins the tap (too small) or weakens threads (too large).
Installing drywall anchors — too-big hole means the anchor spins; too-small means it cracks the drywall during install.
Installing concrete anchors (Tapcons, sleeve anchors, wedge anchors) — manufacturer specifies exact bit size; getting it wrong means the anchor won't grip or won't fit.

When not to use it

Drilling pilot holes in MDF or particleboard — those don't split, so pilot holes are optional except near edges; size by feel.
Through-holes that don't need precision (rough framing, decking) — '3/16-ish' is fine; specialty bits aren't needed.
Drilling for self-tapping or self-drilling screws — those have built-in cutting tips and don't need pilots.
Plastic that's brittle (acrylic, polycarbonate) — those need stepped or specialty bits to prevent cracking; standard wood/metal bits chip the material.

Common use cases

Pre-decision sanity-check on inputs and outputs
Educational use — demonstrating the underlying concept
Onboarding a colleague who needs the same calculation/conversion
Verifying a number or output before passing it on

Frequently asked questions

Do I really need a pilot hole?: For hardwood: yes, almost always. Driving a screw into oak or maple without a pilot will split the wood 80%+ of the time, especially near the edge. For softwood: pilot if the screw is #8 or larger, near an edge, or driving close to another screw. For #6 and smaller in pine/fir, you can usually drive without a pilot. Skipping pilots also strips screw heads (the torque required to drive without a pilot is 2-3x higher). When in doubt: pilot. Takes 5 extra seconds and saves you from a split board.
What pilot hole for a #8 wood screw?: Hardwood: 7/64" (0.109"). Softwood: 3/32" (0.094"). For really hard wood (white oak, hickory, ipe), bump up to 1/8" (0.125") to avoid splitting. The pilot doesn't need to be the full screw length — drilling 70-80% of the screw's thread length is plenty. The unbroken wood at the bottom gives the screw something to grab and pull tight.
What's the right tap drill for 1/4-20 UNC?: #7 drill (0.201"). Mathematical optimum for 75% thread engagement is 0.20" exactly; #7 is the closest standard drill. If you don't have a #7, 13/64" (0.203") works fine — slightly less engagement (~73%) but easier to tap. 5mm metric (0.197") is also acceptable. Don't use 7/32" (0.219") for 1/4-20 — that's too large, gives only ~60% engagement and weak threads.
What's the difference between #7 drill and 7/32" drill?: #7 is from the 'number drill' set (1-80, where #1 is largest at 0.228"). 7/32" is from the fractional set (1/64" increments). #7 = 0.201", 7/32" = 0.219". They're not the same — about 0.018" apart. Tap-drill applications need precision, so use the actually-correct number; for hobby drilling where exact fit doesn't matter, fractional is fine. Number drills, letter drills (A-Z, 0.234"-0.413"), and fractional all overlap in size range; pros own all three sets.
What size hole for a 1/4" through-bolt?: Clearance hole (the bolt passes through, doesn't thread): 17/64" or 9/32" — slightly larger than 1/4" so the bolt slides freely. The exact size depends on tolerance class: 'close fit' = 17/64" (0.266"), 'normal fit' = 9/32" (0.281"), 'free fit' = 19/64" (0.297"). Most DIY work uses normal fit. Threaded hole (where the bolt threads in): use a tap drill (see above). Don't drill the same size as the bolt — that's neither clearance nor tap, and the bolt won't fit either way.
What bit do I need for a Tapcon (concrete screw)?: Tapcons are sized by the screw diameter, but the bit size is specified separately on the package. 3/16" Tapcons (most common DIY size) use a 5/32" carbide masonry bit. 1/4" Tapcons use a 3/16" carbide bit. Always check the package — manufacturer-specific (Confast vs Tapcon brand vs generic concrete screws may differ slightly). Drill 1/4" deeper than the screw length to allow concrete dust to clear; vacuum or blow out the dust before driving the screw or it won't seat fully. Use a hammer drill or rotary hammer; standard cordless drills struggle with concrete past 1/4" depth.

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