Drill Bit Size Calculator

Drill Bit Size Calculator for wood screws and tapped threads. Get the pilot, clearance and countersink sizes by wood density, or the tap drill for a UNC, UNF or metric thread size.

Sizing Pilot and Tap Holes With the Drill Bit Size Calculator

The Drill Bit Size Calculator picks the bit for two different jobs. In wood it sizes the pilot hole, the clearance hole and the countersink for a flat-head wood screw. In metal or hard plastic it finds the tap drill for a UNC, UNF or metric thread.

Woodworkers use the screw mode for face frames, cabinet backs and any joint where one board is screwed to another. The tap mode covers jigs, fixtures and hardware, where a thread has to be cut or formed in the hole. Both modes round to the bits you own, whether fractional, number and letter, or metric.

Why Published Pilot Charts Disagree

Look up a pilot hole for a #8 screw and the answers run from 5/64 in to 1/8 in. Some charts list a 5/32 in clearance hole, which is smaller than the 0.164 in shank it has to pass. Most of the spread comes from charts that split wood into just softwood and hardwood, with no rule behind the line.

The calculator works from the thread root instead. The NDS lead hole rules from the American Wood Council size a withdrawal pilot at about 90% of the root diameter when specific gravity G is above 0.60, and about 70% when G is between 0.50 and 0.60. Wood at G 0.50 or below needs no lead hole for withdrawal at all under NDS.

$$d_p=0.90D_r(G>0.60)$$

$$d_p=0.70D_r(G\le 0.60)$$

The root diameters come from NDS Appendix Table L3, 0.131 in for a #8. In northern red oak at G 0.63, 90% of that is 0.118 in, so the nearest bit is 1/8 in. Black walnut at G 0.55 falls in the lower band, so the same screw takes a 3/32 in pilot even though walnut is sold as a hardwood.

That density line is the part most charts miss. Species come preset with their Wood Handbook specific gravity, from 0.32 for western redcedar to 0.68 for white oak, and any other wood takes its own G.

NDS lists roots only from #6 up. For #4 and #5 the calculator uses the Wood Handbook's note that the root averages about two-thirds of the shank, and the alert says so. Calipers on the actual screw give a better number, and the Measured Screw option takes shank and root directly.

Clearance Hole and Countersink for the Top Board

The first card gives the clearance bit, the smallest bit in your set that is at least as wide as the shank. For a #8 that is 11/64 in, 0.008 in over the 0.164 in shank. Drill it through the top board only, so the threads slide past and the head pulls the joint closed.

The second card sizes the countersink from the flat-head dimensions in ASME B18.6.1, which gives these heads an 82 degree cone. Its largest #8 head is 0.332 in across, and its smallest is 0.292 in, so cutting to the larger width seats every head in the box flush.

$$A=2.04D-0.003$$

$$h=\frac{A-D}{2\tan 41^{\circ }}$$

Here A is the countersink width and h the cone depth, 0.096 in for a #8. When the cone is deeper than the top board is thick, the alert suggests a smaller screw or a counterbore instead.

How the Drill Bit Size Calculator Rates Bite and Pull-Out

The third card measures how far the screw reaches past the joint line into the base board. NDS sets a minimum penetration of six shank diameters for wood screw design values, 0.984 in for a #8. A 2 in screw through a 3/4 in top board gives 1-1/4 in, which clears it, with 1/2 in of wood left under the tip of a 1-3/4 in base.

The fourth card estimates pull-out with the Wood Handbook equation for wood screws in side grain. G is specific gravity, D the shank diameter and L the threaded length that sits in the base board.

How much of the screw is threaded depends on the Thread Type setting. Rolled-thread screws follow the ASME B18.6.1 rule of four shank diameters or two-thirds of the length, whichever is longer, and short ones are threaded all the way. Traditional cut-thread screws with a tapered shank use two-thirds of the length.

$$P=15,700G^{2}DL$$

The default #8 in red oak comes to 1,277 lb, and the same screw in black walnut to 974 lb. These are average ultimate test loads, not safe working loads, and real holding also varies with moisture content, grain and how the hole was drilled.

The Wood Handbook puts end-grain withdrawal at about 75% of side grain when splitting is avoided, and the card shows that figure. The Handbook also calls end-grain results erratic, so do not count on it for a joint that carries weight.

The equation holds for the gauge range the Handbook tested at each length, such as #7 to #16 for a 2 in screw. Outside that range, the alert warns that real loads are likely lower. If the screw would poke through the far face, the tool flags it and gives the longest length that fits.

Tap Drill Math Behind the Major Minus Pitch Rule

Search results for this keyword lean heavily toward tapping, and the quick rule you see everywhere is major diameter minus pitch. That rule is a close cousin of the full formula. FM Carbide removes a chosen percentage of thread height, with a factor of 1.299 times the pitch for cut taps.

$$d_t=D-1.299\times \frac{\%}{100}\times p$$

Here D is the major diameter and p the pitch, 1 divided by threads per inch for inch threads. Major minus pitch works out to about 77% thread, which is why it lands near the 75% charts.

For 1/4-20 at 75%, the target is 0.2013 in, and the nearest bit is a #7 at 0.201 in, giving 75.4% thread. FM Carbide suggests 65% to 75% for cut taps, and the third card turns amber outside that band.

Form taps push metal into the thread instead of cutting it away, so they need a bigger hole. The factor drops to 0.68, which is the 0.0068 per TPI figure Jarvis Cutting Tools prints for its forming taps, along with a 65% starting point.

$$d_t=D-0.68\times \frac{\%}{100}\times p$$

A 1/4-20 form tap at 65% calls for a #1 drill at 0.228 in, the same answer Harvey Performance gives. The fourth card always shows the other tap type's drill, because using a cut-tap drill with a form tap is a common way to break one.

The first two cards list the next bit up and down with the thread each leaves. Past 75%, torque climbs quickly for little gain in strength, so the looser bit is often the better choice in hard material or blind holes.

Input Limits in Both Modes

Screw mode takes screws 1/4 to 8 in long, a top board up to 6 in and a base board up to 12 in. Shank diameter runs 0.05 to 0.5 in, the root must be smaller than the shank, and G must fall between 0.20 and 1.20. The screw also has to reach at least 1/16 in past the top board.

Tap mode takes 20% to 100% thread, inch threads from 0.06 to 0.875 in at 8 to 80 TPI, and metric threads from 1.6 to 20 mm at 0.2 to 2.5 mm pitch. If the target hole falls outside the bit set you picked, the tool asks you to switch sets.

Hole Mistakes That Ruin a Joint

Drilling the pilot through both boards is the classic one. With threads gripping the top board too, the head cannot draw the boards together, and the joint stays open no matter how hard you drive the screw.

Trusting a softwood or hardwood label is the second. Cherry and walnut are hardwoods, yet they sit in the lower NDS band, so a hardwood chart value can leave the pilot oversized in them.

The third is driving a wood screw with a hammer. The Wood Handbook says screws should always be turned in, since hammering tears the fibers and damages the threads the pull-out figure depends on.

For related layouts, the Pocket Hole Calculator handles Kreg screw lengths and drill collar settings. The Lap Joint Calculator gives the longest screw that fits a half-lap, and the Dowel Spacing Calculator lays out holes for dowel joints.

Straight Answers on Screw and Tap Holes

What size pilot hole does a #8 wood screw need?

It depends on the wood's density, not just whether it is called hardwood. With the NDS root of 0.131 in, dense wood above G 0.60, such as hard maple or red oak, takes a 1/8 in bit. Lighter wood from G 0.50 to 0.60, such as walnut, takes 3/32 in.

At G 0.50 or below, NDS does not require a lead hole for withdrawal. A pilot still helps near board ends and edges, where a screw driven into bare wood is most likely to split it.

What drill do I use to tap 1/4-20?

For a cut tap, a #7 drill at 0.201 in, which leaves about 75% thread. A 13/64 in bit at 0.203 in is the closest fractional size and leaves about 72%.

For a form tap, move up to a #1 drill at 0.228 in for about 65% thread. The cut-tap drill is too small for a form tap and raises the torque sharply.

Is the tap drill just the major diameter minus the pitch?

It is a close shortcut that gives about 77% thread on a cut tap. For M6 x 1 it gives a 5.0 mm drill, and for 1/4-20 it gives 0.200 in, both very near the 75% sizes.

The shortcut does not work for form taps, which need about 65% thread and a noticeably larger hole. It also cannot show how far a standard bit lands from the target, which the thread percentage card does.

Can I drive a wood screw without a pilot hole?

In light woods you can, as far as withdrawal strength goes. NDS does not require a lead hole at G 0.50 or below, and small screws in pine or cedar usually drive fine.

Dense hardwoods are different. Screws there tend to split the board or snap, so drill the pilot, and a little soap on the threads makes driving easier, which the Wood Handbook says has little effect on withdrawal strength.

How wide should a countersink be for a #8 screw?

Cut it 0.332 in wide at the surface, the largest #8 flat head that ASME B18.6.1 allows. The cone ends up about 0.096 in deep on an 82 degree countersink bit.

Smaller heads in the box, down to 0.292 in, then sit just below flush rather than proud. A 90 degree metric countersink will not seat inch screws cleanly, so check the angle on your bit.