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Tap Drill Calculator

Find the tap drill size for UNC, UNF and metric threads at any percentage of thread, with the nearest standard drill and the thread it leaves.

Level: Grades 6–7

  • Common Core standard 6.EE.A.2.cEvaluate expressions at specific values of their variables. Include expressions that arise from formulas used in real-world problems. Perform arithmetic operations, including those involving whole-number exponents, in the conventional order when there are no parentheses to specify a particular order (Order of Operations).
  • Common Core standard 7.RP.A.3Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.
Printable worksheet

Tap Drill Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Thread system

75% is the usual chart value; 55–65% is plenty for most jobs and much easier to tap in tough metals.

Result

Nearest standard drill diameter

0.201 in

For 1/4-20 UNC, a #7 drill (0.201 in) gives about 75.4% thread.

Drill size
#7
Thread percentage with this drill
75.4%
Exact drill for 75% thread
0.2013 in
Drill diameter in millimeters
5.105 mm
Basic minor diameter (100% thread)
0.1851 in

See the step-by-step solution ↓

Step-by-step solution

  1. 1/4-20 UNC: the pitch is one inch divided by the threads per inch

    120=0.05 in

  2. Work out how much the threads cut into the diameter for 75% thread: 0.01299 × % × pitch

    0.01299×75×0.05=0.0487125 in

  3. Take that off the outside diameter to get the exact drill size

    0.25−0.0487125=0.2012875 in

  4. Pick the nearest drill you can actually buy: #7

    #7=0.201 in

  5. Run the formula backward to see how much thread that drill really leaves

    0.25−0.2010.01299×0.05=75.44%

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find the right drill bit to use before you tap a threaded hole, plus how much thread it will leave. A tap is the tool that cuts threads inside a hole. Before you use one, you drill a hole a little smaller than the screw. How much smaller decides how deep the tap's teeth bite. Machinists call this the percentage of thread. A 100% thread would cut the full, textbook thread shape. But it takes a huge amount of twisting force (torque) to cut, and it breaks taps. Most charts aim for about 75%, and for many jobs 60% holds nearly as well.

This calculator works out the exact drill size for any percentage of thread. It covers Unified inch threads (UNC and UNF) and ISO metric threads, using the formula tap makers publish. Then it picks the nearest standard drill. For inch threads that's a number, letter or fraction size; for metric, steps of 0.05 to 0.5 mm. It also tells you the thread percentage that drill really gives. That helps you decide whether to go a size up or down.

Where it's used

Why this math matters outside the classroom:

  • Tapping a bracket. A fabricator tapping 1/4-20 holes drills them with a #7 bit, which leaves about 75.4% thread.
  • Metric repairs. A mechanic chasing an M10 × 1.5 thread drills 8.5 mm, since 10 − 0.01299 × 75 × 1.5 is about 8.54 mm.
  • Stainless steel parts. A machinist drops a 1/4-20 hole to 60% thread with a #3 drill so the tap doesn't seize in stainless.
  • Shop class. Students compute 0.5 − 0.01299 × 75 ÷ 13 = 0.4251 in and choose a 27/64 in drill for a 1/2-13 tap.
  • Model engineering. A hobbyist threading #2-56 holes for small screws picks the drill closest to 0.086 − 0.01299 × 75 ÷ 56.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.EE.A.2.c, 7.RP.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose Unified inch or ISO metric threads.
  2. Pick the thread size from the list. Or choose Other and enter the major diameter (the screw's outside width) and the threads per inch or pitch.
  3. Set the percentage of thread you want; 75% is the usual chart value.
  4. Read the nearest standard drill and the thread percentage it leaves. You'll also see the exact size for the percentage you chose.

What the inputs mean

Thread size
UNC is the coarse series and UNF the fine series of Unified inch threads. A 1/4-20 thread is 1/4 in across the outside (its major diameter) and has 20 threads per inch.
Percentage of thread
The percentage of thread compares how tall a thread you actually cut with the full textbook height. Above about 60%, strength rises only a little, but the force needed to turn the tap climbs fast. So tough materials and small taps are often drilled for 55–65%. Thin sheet may need more to get enough grip.

Formula and method

pitch P = 1 ÷ TPI   (inch threads)
drill = D − 0.01299 × %thread × P
%thread = (D − drill) ÷ (0.01299 × P)

D is the major diameter, the outside width of the screw. P is the pitch, the distance from one thread to the next. TPI is threads per inch, and %thread is the percentage of thread you want. A standard 60° thread is about 0.6495 × P deep on each side. So a full thread takes 1.299 × P off the diameter. Taking a percentage of that, and writing 1.299 ÷ 100 as 0.01299, gives the shop formula tap makers use. It works in inches or millimeters, as long as D and P use the same unit.

Real drills come in fixed sizes. So the calculator picks the nearest standard drill. Then it runs the formula backward to show the thread percentage that drill leaves.

Worked example

You need to tap a 1/4-20 UNC thread and want about 75% thread.

  1. Pitch: 1 ÷ 20 = 0.05 in.
  2. Depth to remove: 0.01299 × 75 × 0.05 = 0.0487125 in.
  3. Exact drill: 0.25 − 0.0487125 = 0.2012875 in.
  4. Nearest standard drill: #7, which is 0.201 in.
  5. Thread it leaves: (0.25 − 0.201) ÷ (0.01299 × 0.05) = 75.4%.

Result: Drill with a #7 (0.201 in) bit for a 1/4-20 tap, which gives about 75.4% thread.

Understanding the result

Say the nearest drill gives noticeably more thread than you want. The next size up lowers the percentage and makes tapping easier. In stainless steel and other tough metals, that's often the better trade.

Above about 85% thread, taps jam and break easily, for very little extra strength. The calculator warns you when you're there.

Assumptions and limitations

  • Standard 60° Unified or ISO metric threads, cut with an ordinary cutting tap.
  • The drill cuts exactly its stated size. Real drills often cut slightly oversize, which lowers the thread percentage a little.
  • Forming (roll) taps press threads into shape instead of cutting them. They need a larger hole and follow the maker's own chart, not this formula.
  • Pipe threads (NPT), Acme and other thread forms aren't covered.
  • Some thread classes allow very little size variation. For those, check the allowed hole-size limits (the minor-diameter limits) for that class. You'll find them in the thread standard or the tap maker's chart.

Frequently asked questions

What size drill for a 1/4-20 tap?

A #7 drill (0.201 in) gives about 75% thread, the usual choice. A #3 (0.213 in) gives about 57% and is easier to tap in hard material.

What size drill for an M6 tap?

A 5 mm drill, which leaves about 77% thread on the standard 1 mm pitch. Metric charts often simplify the rule to major diameter minus pitch: 6 − 1 = 5 mm.

Why not drill for 100% thread?

The last part of the thread depth adds very little strength. The screw usually breaks before a 60–75% thread strips. But that last part makes the tap work much harder. Full-depth threads take far more force to tap and are a common cause of broken taps.

Is 'major diameter minus pitch' the same thing?

It's a quick metric rule that works out to about 77% thread. That's because the pitch divided by 0.01299 × pitch is 77. For most coarse metric sizes, it gives the same drill as this calculator.

Sources

How this calculator is checked

Method: Tap makers' percentage-of-thread formula, drill = D − 0.01299 × %thread × P (OSG tap drill chart); thread sizes from ASME B1.1 and ISO 261/262; number and letter drill diameters from the standard wire-gauge and letter series. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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