Which drill goes under your tap?

What a tap drill size is

A tap does not cut the whole depth of the thread out of solid metal. It cuts the grooves in a hole you drilled first, so the hole's diameter sits between the bottom and the top of the thread. A smaller hole leaves more metal for the tap to cut into, which makes a fuller thread and a harder job for the tap; a larger hole makes tapping easy and the thread shallower. The share of the full thread depth you leave is the percent of thread (or thread engagement). This calculator gives the drill for the percentage you want, finds the nearest standard sizes among the fractional, number, letter and metric drills, and shows what percentage each one really gives. In the second mode you enter the drill you have and read the engagement.

The formula

The full depth of a 60-degree thread is 0.6495 × pitch, so a hole for 100 percent of the thread is smaller than the major diameter by twice that, 1.299 × pitch. For a percentage p: drill = D − (p ÷ 100) × 1.299 × pitch, where D is the major (outside) diameter and the pitch is 1 ÷ TPI for inch threads and the thread pitch in millimeters for metric ones. For inch threads the formula is often written D − 0.01299 × p ÷ TPI. To go the other way: percent = (D − drill) ÷ (1.299 × pitch) × 100.

Worked examples

1/4-20 at 75 percent: 0.2500 − 0.75 × 1.299 ÷ 20 = 0.2013 inch, so the drill is a #7 (0.201 inch), which leaves 75.4 percent. M6 × 1 at 75 percent: 6 − 0.75 × 1.299 × 1 = 5.03 mm; the 5.0 mm drill leaves 77.0 percent. M12 × 1.75: the formula gives 10.30 mm, so 10.3 mm (74.8 percent), while the rule drill = diameter − pitch gives 10.25 mm (77.0 percent) and the usual chart value of 10.2 mm leaves 79.2 percent. 3/8-16 in steel at 50 percent: 0.3344 inch, a Q drill. The table gives 75 percent drills for common sizes.

Limits and common mistakes

Charts do not agree to the last drill. The University of Florida chart, for example, lists a #22 drill for a #10-24 thread at 75 percent, while the formula gives 0.1494 inch and a #25 drill; the nearest drill is a judgement call, which is why the table above shows what each one leaves rather than just one name. The formula also assumes a cutting tap in a straight, round hole: a drill that runs oversize, a hole in a soft or sticky material that springs back, or a plated surface will change the real thread. A thread-forming tap needs a bigger hole and its own chart, and the clearance hole for the bolt is a different, larger drill. For blind holes drill deeper than the thread, because the chamfer of the tap does not cut full threads to the bottom. For a part that carries a load, use the thread class and the tap maker's recommendation on the drawing; this tool gives the standard shop starting point.

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

A #7 drill (0.201 inch). The formula D − 0.01299 × 75 ÷ 20 gives 0.2013 inch for a 75 percent thread, and #7 is the nearest standard drill; it leaves a 75.4 percent thread. A 7/32 inch drill (0.219 inch) is the usual choice for a 50 percent thread in steel.

What is the tap drill formula?

Drill diameter = major diameter − (percent thread ÷ 100) × 1.299 × pitch, with the pitch equal to 1 ÷ TPI for inch threads. The constant comes from the depth of a full 60-degree thread, 0.6495 × pitch, taken on both sides of the hole: 2 × 0.6495 = 1.299. For inch threads the same thing is written D − 0.01299 × percent ÷ TPI. The Army's TM 9-243 hand-tools manual notes that the back of the center gage carries a table of the double depth of thread for each pitch, which is exactly this number, for choosing tap drills.

Why do most charts use 75 percent?

A 75 percent thread is the traditional compromise: the hole is large enough for the tap to cut without huge torque and still leaves most of the strength of a full thread. A chart from the University of Florida's machine design lab lists 75 percent drills for aluminum, brass and plastics and 50 percent drills for steel, stainless steel and iron, where a hard material makes the tap work harder. For a 3/8-16 thread that is 5/16 inch at 75 percent and a Q drill (0.3320 inch, 53.0 percent) at 50 percent.

What is the tap drill for M6, M8, M10 and M12?

For the coarse metric threads the common drills are 5.0 mm for M6×1, 6.8 mm for M8×1.25, 8.5 mm for M10×1.5 and 10.2 mm for M12×1.75. The formula gives 5.03 mm for M6 at 75 percent, so 5.0 mm leaves 77.0 percent. For M12 it gives 10.30 mm, which rounds to 10.3 mm (74.8 percent); the shop rule drill = diameter − pitch gives 10.25 mm (77.0 percent), and the usual chart value is 10.2 mm (79.2 percent). All three are acceptable.

Can I use the same drill for a thread-forming (roll) tap?

No. A forming tap pushes the metal aside instead of cutting a chip, and it needs a larger hole than a cutting tap. This calculator is for cutting taps only; use the drill size printed in the tap maker's chart for a forming tap, because too small a hole breaks the tap.

Which drill do I use for the clearance hole of the bolt?

The tap drill is for the threaded part only. The clearance hole for the bolt passing through the other part is larger than the major diameter; the University of Florida chart lists close-fit and free-fit clearance drills for each screw size next to the tap drills. Do not use the tap drill for it.

Sources: tap drill formula D − 0.01299 × % ÷ TPI the usual shop formula given by machining references (60-degree thread, depth 0.6495 × pitch), checked here against the University of Florida chart; US Army TM 9-243, “Use and Care of Hand Tools and Measuring Tools” (1983), chapter 16 (center gage, double depth of thread); University of Florida, Department of Mechanical and Aerospace Engineering design laboratory, “Tap & Drill Chart” (75 percent and 50 percent drills for the standard inch sizes, used to check this calculator); drill sizes #1 to #80 and A to Z as listed in Wikipedia, “Drill bit sizes”; thread series ASME B1.1 (UNC, UNF) and ISO 261 / ISO 262 (metric). Related: bolt torque calculator, machining speeds calculator, decimal to fraction calculator and length converter. How we check our calculators.