What Size Drill Bit Do I Need for a Tap or Screw?

Tap drill sizes for common inch threads

A tap cuts threads into a hole you have already drilled. If the hole is too small, the tap binds and can snap. If it is too large, the threads are shallow and strip easily.

The sizes below give a thread depth of roughly 75 percent, which is the usual balance of strength and ease of tapping. Where a fractional bit is close enough, it is listed too.

How the tap drill size is figured

You can work out the size yourself if your tap is not on a chart. Thread callouts such as 1/4-20 mean a 1/4 in outside diameter with 20 threads per inch.

The steps are short:

• Divide 1 by the threads per inch to get the pitch. For 1/4-20, that is 1 divided by 20, or 0.050 in.
• Subtract the pitch from the outside diameter. For 1/4-20, that is 0.250 minus 0.050, or 0.200 in.
• Pick the bit closest to that number without going under it. For 1/4-20, that is a #7 bit at 0.201 in.
• Check 3/8-16 the same way: 0.375 minus 0.0625 gives 0.3125 in, which is exactly a 5/16 in bit.

Metric taps

Metric threads work the same way, except the pitch is already given in millimeters. Subtract the pitch from the diameter. An M6 x 1.0 tap uses a 5.0 mm bit, because 6.0 minus 1.0 is 5.0.

Here are a few common ones:

• M3 x 0.5: 2.5 mm bit
• M4 x 0.7: 3.3 mm bit
• M5 x 0.8: 4.2 mm bit
• M6 x 1.0: 5.0 mm bit
• M8 x 1.25: 6.8 mm bit
• M10 x 1.5: 8.5 mm bit

Pilot and clearance holes for #8 and #10 screws

Screws need different holes than taps. A pilot hole guides the threads into the wood and keeps it from splitting. A clearance hole is larger, so the screw slides through the top piece and pulls it tight against the bottom piece.

These are typical wood screw sizes. Hardwood needs a larger pilot than softwood, and a quick test in scrap wood is the best way to confirm. Dense or brittle woods near an edge benefit from the larger size.

Machine screws and threaded holes

A #8 or #10 machine screw is a different case from a wood screw. If you are threading it into metal, use the tap drill from the first table: #29 for an 8-32 and #25 for a 10-24.

If the screw only has to pass through a part, drill a clearance hole slightly larger than the screw. About 11/64 in suits a #8 screw, and about 7/32 in suits a #10 screw. Use a snugger hole when the parts must line up closely, and a looser one when you need some wiggle room for alignment.

Tips and safety for clean holes

Mark the spot with a center punch so the bit does not wander. Start with a small pilot bit for larger holes, then step up. Drill straight and keep the drill square to the work.

Safety matters more with metal and with tapping:

• Wear safety glasses. Metal chips and broken tap pieces can fly.
• Clamp the workpiece. A bit that grabs can spin the part out of your hand.
• Wear hearing protection for long sessions or when drilling large holes in metal.
• Metal chips are hot and sharp. Brush them away rather than wiping with bare hands.
• Use cutting oil on steel and aluminum to cut heat and wear.

Tapping by hand, and matching bits to your drill

Turn taps by hand with a tap handle, not a drill. A drill spins too fast and cannot feel when a tap is about to bind. Turn a half turn forward, then back a quarter turn to break the chip, and repeat. Taps are brittle, and a broken one is very hard to remove.

Your drill only needs to make the hole. Check the chuck first. A 3/8 in keyless chuck, like the one on the AVID POWER CD316-20 on our best drill page, handles every tap drill in the first table, up to 5/16 in. For a 27/64 in hole, which is the 1/2-13 tap drill, you need a drill with a 1/2 in chuck, such as the DEWALT DCD777D1.

Number-size bits like #25 and #7 are rarely in general-purpose kits, so look for an index of fractional, letter and number sizes. Our best drill driver bit set page covers kits for driving screws. If you drill thin sheet metal, a best step drill bit page lists step bits that make clean, round holes in thin material, though they are not meant for tap drilling.

Frequently asked questions

Can I use a 13/64 in bit for a 1/4-20 tap?

Yes. At 0.2031 in, it is within a few thousandths of a #7 bit and works well in most materials.

Should the pilot hole be as deep as the screw?

In wood, drill the pilot slightly shorter than the screw's length, so the tip still bites. For a tap, drill deeper than the thread you need, so the tap does not bottom out and break.

What happens if I drill a tap hole too small?

The tap has to remove too much metal, which raises the force needed and the chance of a broken tap. Re-drill at the right size or use a bit one size up.

Do I drill a different size for aluminum, steel or plastic?

The chart sizes work for most metals. Softer plastics can also take the same size, but for tough steel some people choose a slightly larger bit for easier tapping. Use cutting oil for all of them.