About the Handiwork Tap Drill Size Calculator
The Tap Drill Size Calculator finds the correct drill bit diameter to use before cutting an internal thread, for metric (ISO), UNC, and UNF threads. It computes the exact diameter from the thread’s major diameter and pitch (or threads per inch) at any chosen percentage of thread engagement, rather than only offering a fixed chart entry, and rounds to the nearest practical drill bit size.
How to use the Handiwork Tap Drill Size Calculator
- Choose Metric, UNC (coarse), or UNF (fine).
- Select the thread designation, like M8 or 3/8"-16.
- Adjust thread engagement if needed — 75% is the standard default — and read the nearest practical drill size plus the exact calculated diameter.
How the tap drill size is calculated
Cutting an internal thread removes material equal to the thread depth from the drilled hole. For a 60° thread profile, that depth for full (100%) engagement is 1.082532 × pitch for metric threads, or 1.299038 ÷ TPI for Unified (UNC/UNF) threads. This calculator scales that depth by your chosen engagement percentage and subtracts it from the major diameter: drill diameter = major diameter − (engagement% ÷ 100) × depth constant × pitch (or ÷ TPI for inch threads).
Why 75% is the standard default
A 100%-engagement hole would cut a thread with the maximum possible strength but takes far more torque to tap and dramatically increases the risk of breaking the tap, especially in harder metals or blind holes. Testing across the fastener industry has shown that 75% engagement provides close to full thread strength (the difference in shear strength between 75% and 100% engagement is small) while cutting noticeably easier and being far more forgiving of a slightly off-size hole. For softer materials like aluminum or plastic, some machinists go as low as 50-65% to reduce tapping torque further.
Exact diameter vs. nearest drill bit
The exact calculated diameter is rarely a size that drill bits actually come in, so this calculator also shows the nearest practical size — the closest 0.1 mm increment for metric, or the closest 1/64" fraction for UNC/UNF, both common jobber twist drill increments. A machinist’s numbered/lettered drill index (like a #7 drill for 1/4"-20 UNC) can get closer still; use the exact decimal value shown here to pick the closest bit from a full index if you have one.
Blind holes and tap breakage
In a blind hole (one that does not go all the way through), chips have nowhere to go, which is the most common cause of a broken tap. Drilling slightly deeper than the required thread depth to leave a clearance pocket below the last full thread, backing the tap out frequently to clear chips, and using cutting fluid all reduce that risk — none of which this calculator can account for, since it only computes the hole diameter.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
Assumptions and limitations
- Assumes a standard 60° thread profile (ISO metric or Unified/UN); it does not cover pipe threads (NPT/BSP), Acme, buttress, or other non-60° thread forms.
- The nearest practical drill size is rounded to a common increment (0.1 mm metric, 1/64" imperial) — a full numbered/lettered drill index can get closer for critical work.
- Does not account for the material being tapped; softer materials (aluminum, plastic, wood) often tap well at a lower engagement percentage than the 75% default, while brittle materials may need a size closer to 100%.
- Assumes the hole is drilled straight and to the correct depth — it does not calculate minimum drilled depth for blind holes.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
What drill size do I use for an M8x1.25 tap?
At the standard 75% thread engagement, an M8x1.25 calculates to 6.985 mm exactly, which this calculator rounds to a practical 7.0 mm drill. Many published charts list 6.8 mm for M8, which cuts a slightly deeper thread (closer to 90% engagement) — both are commonly used in practice.
What drill bit do I use for a 1/4-20 UNC tap?
At 75% engagement, 1/4"-20 UNC calculates to 0.2013" exactly. The nearest common jobber drill in 64ths is 13/64" (0.203"); a numbered-drill index gets closer still with a #7 (0.201").
Should I use a smaller drill for more thread strength?
A smaller drill does cut a deeper, stronger thread, but the strength gain past about 75% engagement is small while the tapping torque and risk of breaking the tap both increase sharply. Going below 50% is not modeled here and is generally too weak for a structural fastener.
Why is the calculated size different from the chart I usually use?
Published tap drill charts vary because they assume different thread engagement percentages, round to different drill bit increments, or reference numbered/lettered drills instead of fractional or metric decimal sizes. Set the engagement percentage here to match what your chart targets (usually 75%) to compare directly.
Does this work for tapping plastic or wood?
The 60° thread-depth formula still applies, but you may want to reduce the engagement percentage (50–65%) for softer materials like plastic, aluminum, or wood, since they tap more easily and are more prone to stripping at full engagement.