Tap Drill Size Calculator Online (M3-M30 Chart)

Tap Drill Size Calculator (M3-M30 Chart)

Choosing the right tap drill is one of the most frequently looked-up questions in machining: an oversized hole gives shallow threads that strip easily, while an undersized one overloads the tap - heavier cutting, faster wear, or a broken tap stuck in the part. The online calculator on this page recommends the tap drill diameter from thread size and workpiece material, for both coarse and fine pitches.

The theoretical formula is d0 = d - P for standard thread forms. Adjust for material: use d - P for steel, copper and aluminum; d - 1.1P for cast iron and other brittle materials (less expansion, poorer chip evacuation); for stainless steel add about 0.1 mm to reduce tapping torque and galling. Select the nearest standard drill not larger than the calculated value.

Worn drills produce undersized holes - verify with go/no-go gauges before volume tapping, and deepen blind-hole drills by about 0.5d for the tap lead. Pair this with our torque calculator for assembly. Zhuhai Jiali Hardware manufactures precision screws and custom fasteners - contact us for quotes.