Tap Drill Size Calculator
Drill Too Small and You'll Snap the Tap
Cutting internal threads means removing a hole slightly smaller than the tap's major diameter and letting the tap form the rest — drill too large and the threads come out shallow and weak, drill too small and the tap binds and breaks off in the hole. The right drill size sits at a specific point between the two, targeting roughly 75% thread engagement, which is strong enough for most applications without excessive cutting force.
The Formulas
Metric: Tap Drill Size = Major Diameter − Pitch
Worked Examples
| Thread | Major Diameter | TPI / Pitch | Calculated Drill Size |
|---|---|---|---|
| 1/4-20 (imperial) | 0.25 in | 20 TPI | 0.2000 in |
| 3/8-16 (imperial) | 0.375 in | 16 TPI | 0.3125 in |
| M6 (metric) | 6 mm | 1.0 mm pitch | 5.000 mm |
| M10 (metric) | 10 mm | 1.5 mm pitch | 8.500 mm |
These calculated values land on or near widely published standard tap drill sizes for these thread specifications, which is a useful sanity check — always round to the closest available standard drill bit at or just above the calculated value.
Where This Calculation Matters
- Machine shop setup — selecting the correct drill bit before tapping a hole, especially for less common or non-standard thread sizes not on a printed chart.
- Custom thread specifications — calculating drill size for non-standard pitches where a reference chart doesn't have a direct entry.
- Verifying a chart value — double-checking a tap drill chart figure against the underlying formula when the two don't seem to agree.
How to Use This Calculator
- Choose the mode: Imperial (TPI) or Metric (Pitch).
- For imperial, enter Major Diameter (in) and Threads per Inch (TPI).
- For metric, enter Major Diameter (mm) and Thread Pitch (mm).
- Select Calculate to get the tap drill size, then round to the nearest standard drill bit available.
Related Calculations
Once the hole is tapped and threaded fasteners go in, check tightening torque with the tools in the Bolt Torque Calculator.