Free Drill Tap Size Calculator

Find the optimal tap drill bit size, compute exact thread engagement percentage, and avoid broken taps in hard metals.

🔩 Thread Size & Workpiece Material

Standard Thread Fastener Presets

Target Thread Engagement (%)

Major Dia D (mm) 0.2362 inches

Pitch (mm or TPI) 25.4 TPI

📊 Recommended Drill Bit & Fit

Recommended Metric Tap Drill

5.0 mm

Exact hole: 5.02 mm (77% thread engagement)

Imperial Drill Bit

#8 or 13/64"

0.197" decimal diameter

Tap Breakage Hazard

Low Risk

Torque is well within tap limit

Standard Rule (75% Thread): Major - Pitch = 5.00 mm

Actual Thread Height: 0.54 mm (0.021")

Holding Strength: > 95% of full bolt shear strength

Recommended Tools & Equipment

Tested hardware and components for high reliability

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Understanding Tap Drill Sizing

Tapping internal threads requires drilling a pre-hole of an exact diameter. If the hole is drilled too small, tapping torque skyrockets by over 300%, jamming the tap and snapping the hardened high-speed steel tool inside the workpiece. If drilled too large, thread height is shallow and bolts strip out under torque.

The Tap Drill Formulas

\text{\bf Metric (75\% Engagement): } D_{\text{drill}} = D_{\text{major}} - P_{\text{pitch}}

\text{\bf Imperial: } D_{\text{drill}} = D_{\text{major}} - \frac{0.01299 \times \%_{\text{thread}}}{\text{TPI}}

\%_{\text{Thread}} = \frac{D_{\text{major}} - D_{\text{hole}}}{1.08253 \times P_{\text{pitch}}} \times 100\%

Why 100% Thread Depth is a Destructive Mistake

Machining handbook testing proves that increasing thread engagement beyond 70% to 75% adds less than 5% to bolt tensile pull-out strength, while doubling tapping cutting resistance. In tough alloys like 304 or 316 Stainless Steel, dropping thread percentage to 60% to 65% virtually eliminates tap breakage while still guaranteeing the bolt breaks before threads strip.

Frequently Asked Questions

What is the quick mental formula for metric tap drills?

For any standard 60-degree metric thread at 75% engagement, simply subtract the thread pitch from the diameter: Drill = Diameter - Pitch. For M6 x 1.0, drill is 5.0 mm. For M8 x 1.25, drill is 6.75 (or 6.8 mm). For M10 x 1.5, drill is 8.5 mm.

What is the drill size for 1/4"-20 UNC?

The standard tap drill for 1/4-20 is a #7 wire drill (0.2010 inches / 5.1 mm), which produces roughly 75% thread engagement. If tapping 304 stainless steel, use a 13/64" drill (0.2031") for easier cutting and zero tap bind.

Why do blind holes require bottoming taps?

Standard taper and plug taps have 3 to 10 chamfered starter threads at the tip that do not cut full-depth threads. In a blind hole where the bolt must thread all the way to the bottom, you must start with a plug tap and finish with a flat-nosed bottoming tap.