Tap Drill Size Calculator

How to use

1. 1Choose Standard thread to pick a named size, or Custom thread to enter your own. For standard, select Inch (UNC / UNF) or Metric (ISO) and then the thread size.
2. 2For a custom thread, set the system and enter the major diameter with TPI for inch (a decimal like 0.5 or a fraction like 1/2 is accepted) or the major diameter and pitch in millimetres for metric.
3. 3Set the target thread engagement with the slider. 75 percent is the standard default; lower it for easier tapping in hard materials, raise it for a fuller thread.
4. 4Read the recommended drill bit, shown in both inches and millimetres, with the actual thread engagement that bit produces, the theoretical hole size, and the minor diameter.
5. 5Check the other close drills if the recommended size is not in your index, and use Copy summary or Copy drill size to save the result.
6. 6Use the quick reference chart for common sizes at 75 percent thread, and click any row to load that thread into the calculator.

About this tool

Tap Drill Size Calculator finds the correct drill bit to use before you cut an internal thread with a tap, the single most common lookup in machining, metalworking, and DIY fastener work. Drill the hole too small and the tap binds and snaps; drill it too large and the thread is shallow and strips under load, so the right drill is the difference between a clean thread and a ruined part or a broken tap stuck in the hole. The tool covers both thread systems. Inch threads include the Unified Coarse (UNC) and Unified Fine (UNF) series from #0 up to 1 inch, the sizes used across hardware, automotive, and machine building, and metric threads cover the ISO coarse series plus the common fine pitches from M1.6 to M20. You can pick a standard size from the list or switch to Custom thread and enter any major diameter and TPI (inch) or any major diameter and pitch in millimetres (metric), which is useful for odd or proprietary threads. The math is the established Machinery's Handbook relation for 60-degree threads: the theoretical hole diameter is the major diameter minus (percent of full thread divided by 76.98) times the pitch, where the pitch is one over TPI for inch threads. That theoretical diameter is then snapped to the nearest real drill bit drawn from the full standard sets, the fractional series in 64ths, the number (wire gauge) series from #1 to #80, the letter series A to Z, and the common metric series, and the tool reports the actual thread engagement that snapped bit produces along with the drill size in both inches and millimetres. A thread engagement slider lets you trade strength against ease of tapping. The shop standard is 75 percent, which keeps almost the same joint strength as a fuller thread while cutting tapping torque and tap breakage substantially; dropping toward 65 percent in hard or gummy materials reduces breakage further, and pushing toward 100 percent gives the maximum thread but is rarely worth the extra force. For each result the tool also shows the minor diameter (the 100 percent thread hole) and up to three other close drills from any series, so you can choose what is actually in your index. The recommended drills match standard published tap drill charts exactly: 1/4-20 returns a #7, 3/8-16 returns a 5/16 inch, 1/2-13 returns a 27/64 inch, M6 x 1.0 returns a 5.0 mm, M8 x 1.25 returns a 6.8 mm, and M10 x 1.5 returns an 8.5 mm. A built-in quick reference chart lists the common inch and metric sizes with their 75 percent drills, and any row loads into the calculator with one click. A few honest limits: this calculator is for cut taps that remove material; form (roll) taps need a different, larger hole and you should follow the form tap maker chart for those. Drill diameters are nominal and real holes drift slightly oversize, so treat the result as a starting point and confirm on scrap for critical or production work. Everything is computed locally in your browser; nothing you enter is uploaded or stored.

Free to use. Works in your browser. No signup, no login.