How do you calculate tap drill size with the standard formula?

For standard 60° cutting taps, use the fundamental machinist formula: Drill Size = Major Diameter - (Pitch × % Thread / 76.98) for metric, or Drill Size = Major Diameter - (% Thread × 0.01299 / TPI) for imperial. For the classic 75% thread rule of thumb: Metric Drill = Major Diameter - Pitch (e.g., M6 × 1.0 requires a 6.0 - 1.0 = 5.0 mm drill). Imperial Drill ≈ Major Diameter - (1 / TPI) (e.g., 1/4-20 UNC requires 0.250 - 0.050 = 0.200 in, matched to a #7 drill at 0.201 in).

What percentage of thread engagement should I use?

A common machining myth is that threads must be 100% engaged. In reality, a 60% to 65% thread provides over 95% of the ultimate pullout strength of a 100% thread because the bolt breaks before the internal thread shears when thread depth exceeds 1.5× bolt diameter. Increasing engagement from 65% to 85% triples tapping torque, exponentially raising the risk of snapping taps—especially in work-hardening stainless steels (304/316) and titanium alloys. Use 70-75% for soft metals (aluminum, brass) and 55-65% for tough alloy steels and stainless.

Why do roll forming taps need much larger drill bits than cutting taps?

Cutting taps slice and evacuate metal chips through flutes, so the pre-drilled hole must equal the thread's minor diameter. Roll forming taps (cold forming or fluteless taps) displace and flow the metal plastically into crests and roots without creating chips. Consequently, the hole must be bored near the pitch diameter. The roll form formula is: Metric Drill = Major Diameter - (0.0068 × % Thread × Pitch), which simplifies at 75% engagement to roughly: Major Diameter - (0.5 × Pitch). For an M8 × 1.25 thread, a cutting tap needs a 6.8 mm drill, whereas a roll form tap mandates a 7.4 mm drill. Using a cutting tap drill for a form tap will shatter the tap instantly!

What drill bit is needed for an M8 tap?

For a standard M8 × 1.25 metric coarse cutting tap at the standard ~75% engagement, use a 6.8 mm drill bit (closest imperial equivalent: 17/64" or letter 'H' at 0.2660 in / 6.76 mm). For an M8 × 1.0 metric fine cutting tap, use a 7.0 mm drill bit. For an M8 × 1.25 roll forming tap, use a 7.4 mm drill bit.

What size drill do I use for a 1/4-20 tap?

For a standard 1/4-20 UNC cutting tap at 75% thread engagement, the industry standard drill is a #7 wire gauge drill (0.2010 in / 5.105 mm). If a #7 drill is unavailable, a 13/64" drill (0.2031 in / 5.16 mm) gives 72% engagement (acceptable in mild steel), and a 5.0 mm metric drill provides 76% engagement.

How deep should a tap drill hole be for a blind hole?

To prevent the tap from bottoming out and shearing off, a blind hole must be drilled deeper than the specified full thread depth. Total Hole Depth = Desired Full Thread Depth + Tap Chamfer Lead (1-2 pitches for bottoming taps, 3-5 pitches for plug taps, 7-10 pitches for taper taps) + Chip Evacuation Clearance (2-4 pitches) + Drill Point Tip Depth (for a standard 118° drill point, add 0.3 × Drill Diameter; for 135°, add 0.2 × Drill Diameter).

Tap Drill Size Calculator & Thread Chart

Calculate exact pre-tapping hole diameters for Metric ISO (M1–M68) and Unified Imperial (UNC, UNF, UNEF #0–2") threads across cutting taps and cold-forming roll taps, with adjustable Percent of Thread (% engagement), material recommendations, and nearest fractional/letter/number/metric drill bit snapping.

Lbl presets

Card title parameters

Lbl thread standard

Lbl nominal size

Lbl pitch

1.000 Unit mm(25.4 Unit tpi)

Lbl tap type

Lbl workpiece material

Lbl target engagement75%

Lbl hole type

Lbl thread depth

Unit mm

Lbl tap chamfer

Card title readout

Lbl recommended drill

5.0 mm(5.00 mm / 0.1969 in)

77.0% Lbl thread engagementLbl delta -0.026 mm

Lbl theoretical bore5.026 mm(0.1979 in)

Lbl cross reference

Col metric5.0 mm77.0% Lbl thread

Col fractional13/64"64.7% Lbl thread

Col number#872.7% Lbl thread

Col letterLetter A4.3% Lbl thread

Lbl permissible range4.96 mm - 5.29 mm

80% (4.96 mm / Max Torque)75%55% (5.29 mm / Stripping Limit)

Lbl depth stops title

Lbl usable thread12.0 mm

Lbl chamfer lead4.00

Lbl stop shoulder19.00

Lbl stop point20.50

Card title reverse

5.0 mm

Desc reverse solver

Lbl in stock drill

Lbl resulting engagement77.0%

75% - 82% — High Torque Warning

Tapping torque rises by 80%. Acceptable in soft aluminum; high risk of breaking small taps in steels.

Lbl torque factor1.08× baseline

Lbl pullout retain~100%

Card title visual

Svg tip thread engage

Card title matrix

Th thread	Th pitch	Th cutting drill	Th cutting65	Th forming drill
M1.6 × 0.35	0.35 mm (72.6 TPI)	1.26 mm	1.30 mm	1.43 mm
M2 × 0.4	0.40 mm (63.5 TPI)	1.61 mm	1.66 mm	1.81 mm
M2.5 × 0.45	0.45 mm (56.4 TPI)	2.06 mm	2.12 mm	2.29 mm
M3 × 0.5	0.50 mm (50.8 TPI)	2.51 mm	2.58 mm	2.76 mm
M3.5 × 0.6	0.60 mm (42.3 TPI)	2.92 mm	2.99 mm	3.21 mm
M4 × 0.7	0.70 mm (36.3 TPI)	3.32 mm	3.41 mm	3.67 mm
M5 × 0.8	0.80 mm (31.8 TPI)	4.22 mm	4.32 mm	4.62 mm
M6 × 1.0	1.00 mm (25.4 TPI)	5.03 mm	5.16 mm	5.52 mm
M7 × 1.0	1.00 mm (25.4 TPI)	6.03 mm	6.16 mm	6.52 mm
M8 × 1.25	1.25 mm (20.3 TPI)	6.78 mm	6.94 mm	7.41 mm
M10 × 1.5	1.50 mm (16.9 TPI)	8.54 mm	8.73 mm	9.29 mm
M12 × 1.75	1.75 mm (14.5 TPI)	10.30 mm	10.52 mm	11.17 mm
M14 × 2.0	2.00 mm (12.7 TPI)	12.05 mm	12.31 mm	13.05 mm
M16 × 2.0	2.00 mm (12.7 TPI)	14.05 mm	14.31 mm	15.05 mm
M18 × 2.5	2.50 mm (10.2 TPI)	15.56 mm	15.89 mm	16.81 mm
M20 × 2.5	2.50 mm (10.2 TPI)	17.56 mm	17.89 mm	18.81 mm
M22 × 2.5	2.50 mm (10.2 TPI)	19.56 mm	19.89 mm	20.81 mm
M24 × 3.0	3.00 mm (8.5 TPI)	21.08 mm	21.47 mm	22.57 mm
M27 × 3.0	3.00 mm (8.5 TPI)	24.08 mm	24.47 mm	25.57 mm
M30 × 3.5	3.50 mm (7.3 TPI)	26.59 mm	27.04 mm	28.33 mm
M36 × 4.0	4.00 mm (6.3 TPI)	32.10 mm	32.62 mm	34.10 mm
M4 × 0.5	0.50 mm (50.8 TPI)	3.51 mm	3.58 mm	3.76 mm
M5 × 0.5	0.50 mm (50.8 TPI)	4.51 mm	4.58 mm	4.76 mm
M6 × 0.75	0.75 mm (33.9 TPI)	5.27 mm	5.37 mm	5.64 mm
M8 × 1.0	1.00 mm (25.4 TPI)	7.03 mm	7.16 mm	7.52 mm
M10 × 1.0	1.00 mm (25.4 TPI)	9.03 mm	9.16 mm	9.52 mm
M10 × 1.25	1.25 mm (20.3 TPI)	8.78 mm	8.94 mm	9.40 mm
M12 × 1.25	1.25 mm (20.3 TPI)	10.78 mm	10.94 mm	11.40 mm
M12 × 1.5	1.50 mm (16.9 TPI)	10.54 mm	10.73 mm	11.29 mm
M14 × 1.5	1.50 mm (16.9 TPI)	12.54 mm	12.73 mm	13.29 mm
M16 × 1.5	1.50 mm (16.9 TPI)	14.54 mm	14.73 mm	15.29 mm
M20 × 1.5	1.50 mm (16.9 TPI)	18.54 mm	18.73 mm	19.29 mm
#0-80 UNC	0.32 mm (80.0 TPI)	1.21 mm	1.26 mm	1.37 mm
#1-64 UNC	0.40 mm (64.0 TPI)	1.47 mm	1.52 mm	1.67 mm
#2-56 UNC	0.45 mm (56.0 TPI)	1.74 mm	1.80 mm	1.97 mm
#3-48 UNC	0.53 mm (48.0 TPI)	2.00 mm	2.07 mm	2.26 mm
#4-40 UNC	0.64 mm (40.0 TPI)	2.23 mm	2.31 mm	2.54 mm
#5-40 UNC	0.64 mm (40.0 TPI)	2.56 mm	2.64 mm	2.87 mm
#6-32 UNC	0.79 mm (32.0 TPI)	2.73 mm	2.83 mm	3.13 mm
#8-32 UNC	0.79 mm (32.0 TPI)	3.39 mm	3.50 mm	3.79 mm
#10-24 UNC	1.06 mm (24.0 TPI)	3.79 mm	3.93 mm	4.32 mm
#12-24 UNC	1.06 mm (24.0 TPI)	4.45 mm	4.59 mm	4.98 mm
1/4"-20 UNC	1.27 mm (20.0 TPI)	5.11 mm	5.28 mm	5.75 mm
5/16"-18 UNC	1.41 mm (18.0 TPI)	6.56 mm	6.75 mm	7.27 mm
3/8"-16 UNC	1.59 mm (16.0 TPI)	7.98 mm	8.18 mm	8.77 mm
7/16"-14 UNC	1.81 mm (14.0 TPI)	9.34 mm	9.58 mm	10.25 mm
1/2"-13 UNC	1.95 mm (13.0 TPI)	10.80 mm	11.05 mm	11.77 mm
9/16"-12 UNC	2.12 mm (12.0 TPI)	12.23 mm	12.50 mm	13.28 mm
5/8"-11 UNC	2.31 mm (11.0 TPI)	13.63 mm	13.93 mm	14.78 mm
3/4"-10 UNC	2.54 mm (10.0 TPI)	16.58 mm	16.91 mm	17.84 mm
7/8"-9 UNC	2.82 mm (9.0 TPI)	19.48 mm	19.84 mm	20.88 mm
1"-8 UNC	3.17 mm (8.0 TPI)	22.31 mm	22.72 mm	23.89 mm
#0-80 UNF	0.32 mm (80.0 TPI)	1.21 mm	1.26 mm	1.37 mm
#1-72 UNF	0.35 mm (72.0 TPI)	1.51 mm	1.56 mm	1.69 mm
#2-64 UNF	0.40 mm (64.0 TPI)	1.80 mm	1.85 mm	2.00 mm
#3-56 UNF	0.45 mm (56.0 TPI)	2.07 mm	2.13 mm	2.30 mm
#4-48 UNF	0.53 mm (48.0 TPI)	2.33 mm	2.40 mm	2.59 mm
#5-44 UNF	0.58 mm (44.0 TPI)	2.61 mm	2.69 mm	2.90 mm
#6-40 UNF	0.64 mm (40.0 TPI)	2.89 mm	2.97 mm	3.20 mm
#8-36 UNF	0.71 mm (36.0 TPI)	3.48 mm	3.57 mm	3.83 mm
#10-32 UNF	0.79 mm (32.0 TPI)	4.05 mm	4.16 mm	4.45 mm
#12-28 UNF	0.91 mm (28.0 TPI)	4.60 mm	4.72 mm	5.05 mm
1/4"-28 UNF	0.91 mm (28.0 TPI)	5.47 mm	5.58 mm	5.92 mm
5/16"-24 UNF	1.06 mm (24.0 TPI)	6.91 mm	7.04 mm	7.43 mm
3/8"-24 UNF	1.06 mm (24.0 TPI)	8.49 mm	8.63 mm	9.02 mm
7/16"-20 UNF	1.27 mm (20.0 TPI)	9.87 mm	10.04 mm	10.51 mm
1/2"-20 UNF	1.27 mm (20.0 TPI)	11.46 mm	11.63 mm	12.10 mm
9/16"-18 UNF	1.41 mm (18.0 TPI)	12.91 mm	13.10 mm	13.62 mm
5/8"-18 UNF	1.41 mm (18.0 TPI)	14.50 mm	14.68 mm	15.20 mm
3/4"-16 UNF	1.59 mm (16.0 TPI)	17.50 mm	17.71 mm	18.29 mm
7/8"-14 UNF	1.81 mm (14.0 TPI)	20.46 mm	20.69 mm	21.36 mm
1"-12 UNF	2.12 mm (12.0 TPI)	23.34 mm	23.61 mm	24.39 mm
1/4"-32 UNEF	0.79 mm (32.0 TPI)	5.58 mm	5.68 mm	5.97 mm
3/8"-32 UNEF	0.79 mm (32.0 TPI)	8.75 mm	8.85 mm	9.15 mm
1/2"-28 UNEF	0.91 mm (28.0 TPI)	11.82 mm	11.93 mm	12.27 mm
1/16"-27 NPT	0.94 mm (27.0 TPI)	7.02 mm	7.14 mm	7.49 mm
1/8"-27 NPT	0.94 mm (27.0 TPI)	9.37 mm	9.49 mm	9.84 mm
1/4"-18 NPT	1.41 mm (18.0 TPI)	12.34 mm	12.52 mm	13.04 mm
3/8"-18 NPT	1.41 mm (18.0 TPI)	15.77 mm	15.95 mm	16.47 mm
1/2"-14 NPT	1.81 mm (14.0 TPI)	19.57 mm	19.80 mm	20.47 mm
3/4"-14 NPT	1.81 mm (14.0 TPI)	24.90 mm	25.14 mm	25.81 mm
1"-11.5 NPT	2.21 mm (11.5 TPI)	31.25 mm	31.54 mm	32.35 mm

Card title setup code

Lbl cnc parameters

Lbl spindle rpm

Lbl surface speed15 m/min (50 SFM)

Lbl feed per rev1.000 mm/rev

Lbl g94 feed rate500 mm/min (19.69 IPM)

Lbl coolant adviceChlorinated / sulfurized heavy cutting oil

(========================================================)
( TOOLS AA CNC TAPPING SETUP SHEET & TOOLING NOTE )
( Standard: METRIC_COARSE | Fastener: M6 )
(========================================================)
THREAD PROFILE:
  Nominal Size:       M6
  Major Diameter:     6.000 mm  (0.2362 in)
  Thread Pitch:       1.000 mm  (25.40 TPI)
  Tap Process Type:   CUTTING TAP (Fluted Shear Cutting)
  Target Engagement:  75%

RECOMMENDED PRE-DRILL HOLE:
  Primary Drill Bit:  5.0 mm
  Drill Diameter:     5.000 mm  (0.1969 in)
  Theoretical Bore:   5.026 mm
  Actual Engagement:  77.0%
  Safe Drill Range:   4.96 mm to 5.29 mm

DEPTH STOPS (BLIND HOLE):
  Full Usable Thread: 12.00 mm
  Chamfer Style:      PLUG TAP (3-5 leads)
  Stop to Shoulder:   19.00 mm
  Stop to Drill Tip:  20.50 mm  (118-deg jobber point)

CNC RIGID TAPPING SPEED & FEED (G84 CYCLE):
  Workpiece Material: Low Carbon / Mild Steel 1018 (70% engagement)
  Cutting Lubricant:  Chlorinated / sulfurized heavy cutting oil
  Spindle Speed:      S500 RPM  (Vc = 15 m/min / 50 SFM)
  Feed Rate (G94):    F500 mm/min
  Feed Rate (G95):    F1.0000 mm/rev  (0.03937 in/rev)

SAMPLE RIGID TAPPING G-CODE SNIPPET:
  G90 G54 G00 X0. Y0.
  G43 H01 Z5.0 M08
  (TOOL 1: PRE-DRILL HOLE)
  G83 Z-20.50 R2.0 Q3.0 F120.
  G80
  (TOOL 2: RIGID TAP)
  M29 S500
  G84 Z-19.00 R5.0 F500
  G80
  G00 Z50. M09
(========================================================)

Primary Metric5.0 mm

Efficiency / Capacity5.026 mm

Net Output500 mm/min

Tap Drill Size Calculator & Thread Chart Guide

Tap Drill Size Calculator & Thread Chart

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Tap Drill Size Calculator & Thread Chart Guide

Key Parameters & Controls

Pro Tips & Best Practices

Common Gotchas & Troubleshooting

⚠️Tap Binds, Challs, or Snaps Within the First 2 to 3 Revolutions

⚠️Internal Threads Fail Go/No-Go Gauge Inspection or Strip Fastener Below Rated Torque

⚠️Chip Packing and Thread Tearing During Spindle Reversal in Blind Holes

Frequently Asked Questions

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