This reference provides formulas for calculating spindle speed (RPM), feed rate (IPR and IPM), and tap drill size for both unified (UNC/UNF) and metric (M/MF) taps. The formulas cover both cut taps and form taps, with general requirements and special percentage-of-thread calculations for precise thread engagement control.

How do you calculate RPM for UNC and UNF taps?

RPM (revolutions per minute) for UNC and UNF taps is calculated by multiplying the cutting speed in surface feet per minute by 3.82, then dividing by the tap diameter in inches.

RPM = (cutting speed (SFM) × 3.82) / Tap diameter

How do you calculate feed rate for UNC and UNF taps?

Feed rate for UNC and UNF taps depends on the thread pitch, expressed as threads per inch (TPI).

IPR (inch/revolution) = 1 inch ÷ Threads per inch (TPI)

IPM (inch/minute) = RPM ÷ Threads per inch (TPI)

How do you calculate RPM for M and MF taps?

RPM for metric M and MF taps is calculated by multiplying the cutting speed in surface feet per minute by 97.028, then dividing by the tap diameter in millimeters.

RPM = (cutting speed (SFM) × 97.028) / Tap diameter(mm)

How do you calculate feed rate for M and MF taps?

Feed rate for metric M and MF taps is based on the thread pitch in millimeters, converted to inches.

IPR (inch/revolution) = pitch(mm) × 0.03937

IPM (inch/minute) = RPM × pitch(mm) × 0.03937

What is the tap drill size for cut taps (general requirements)?

For both UNC/UNF and M/MF cut taps under general requirements, the tap drill size equals the tap basic major diameter minus the pitch.

Tap Drill Size = Tap basic major diameter – pitch

How do you calculate tap drill size for UNC/UNF cut taps with a specific percentage of thread?

For UNC/UNF cut taps when a specific percentage of thread engagement is required, use this formula:

Drill Size = Basic major diameter – (0.01299 × desired % of thread) / Threads per Inch (TPI)

How do you calculate tap drill size for M/MF cut taps with a specific percentage of thread?

For M/MF cut taps when a specific percentage of thread engagement is required, use this formula:

Drill Size (mm) = Basic major diameter – (desired % of thread × pitch(mm)) / 76.98

What is the tap drill size for form taps (general requirements)?

For both UNC/UNF and M/MF form taps under general requirements, the tap drill size equals the basic major diameter minus half the pitch.

Tap Drill Size = Basic major diameter – pitch / 2

How do you calculate tap drill size for UNC/UNF form taps with a specific percentage of thread?

For UNC/UNF form taps when a specific percentage of thread engagement is required, use this formula:

Drill Size = Basic major diameter – (0.0068 × desired % of thread) / Threads per Inch (TPI)

How do you calculate tap drill size for M/MF form taps with a specific percentage of thread?

For M/MF form taps when a specific percentage of thread engagement is required, use this formula:

Drill Size (mm) = Basic major diameter – (desired % of thread × pitch(mm)) / 147.06

Source and updates

Source: taylortool.com engineering data. Reviewed September 2026.