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Drill & Tap Calculator - Tap Drill Sizes, Drill Speeds & Feed for UNC, UNF & Metric Threads

Look up tap drill sizes for any thread and calculate drill RPM and feed rate by material

Free drill and tap calculator that combines tap drill size lookup with drill speed and feed calculations in one tool. Select your thread size - UNC, UNF, or Metric - and get the correct tap drill diameter for 50%, 65%, 75%, or any custom thread percentage. Then enter your workpiece material and drill type to get the recommended RPM and feed rate for the tap drill. Most machinists know the common tap drills by heart: 7/16-14 UNC uses a 3/8 drill, 1/2-13 UNC uses a 27/64 drill. But what about 5/8-18 UNF, M12×1.25, or the tap drill for 75% thread in stainless? This calculator covers every standard UNC and UNF size from #0-80 through 1-1/2-6, plus Metric from M1.6×0.35 through M30×3.5, at any thread percentage you specify. It shows the exact decimal drill size, the nearest fractional or number drill, and the resulting thread percentage with that drill. For tapping, it provides recommended cutting speeds and tap type recommendations (spiral point for through holes, spiral flute for blind holes, forming taps for ductile materials) based on your workpiece material.

Pro Tip: The textbook says use 75% thread for maximum strength, but in practice 60-65% thread is the sweet spot for most applications. Going from 65% to 75% thread increases the minor diameter reduction by a large amount but only adds about 5% more thread engagement strength - while dramatically increasing tapping torque and the risk of tap breakage, especially in stainless and tough alloys. Use 65% for general work, 75% only when you need maximum pull-out strength in soft materials like aluminum or when the code specifically requires it.

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Drill & Tap Calculator
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How It Works

  1. Select Thread Standard

    Choose UNC (Unified National Coarse), UNF (Unified National Fine), or Metric. Then select the specific thread size from the dropdown. UNC is the default for general-purpose fasteners in the US; UNF for precision and vibration-resistant applications; Metric for international work.

  2. Choose Thread Percentage

    Select 50%, 65%, 75%, or enter a custom value. Thread percentage determines the tap drill size. Higher percentages give more thread engagement but require more tapping torque and increase tap breakage risk. 65-75% is standard for most applications.

  3. Review Tap Drill Size

    The calculator shows the exact decimal drill size for your selected thread percentage, the nearest standard fractional, number, or letter drill, and the actual thread percentage achieved with that standard drill. It flags cases where the nearest standard drill gives less than 50% or more than 83% thread.

  4. Enter Workpiece Material

    Select the material you are drilling and tapping: mild steel, alloy steel, stainless, aluminum, cast iron, brass, or plastic. The calculator provides drilling RPM, feed per revolution, and tapping speed for the tap drill size in that material.

  5. Get Tapping Recommendations

    Review the recommended tap type (spiral point, spiral flute, or forming), cutting speed in SFM, and lubricant recommendation. For blind holes, the calculator shows the minimum drill depth needed for full thread engagement plus chip room at the bottom.

Built For

  • Machinists looking up tap drill sizes for uncommon thread sizes or metric conversions
  • CNC programmers determining drill and tap cycle parameters for production runs
  • Maintenance mechanics selecting the right drill for field tapping repairs
  • Engineering students learning the relationship between tap drill size and thread percentage
  • Tool crib managers verifying drill stock against upcoming job requirements

Frequently Asked Questions

Tap drill diameter = Major diameter - (1 / TPI) for 100% thread. For a specific thread percentage: Tap drill = Major diameter - (Thread percentage / 100 × pitch). For quick 75% thread: subtract the pitch from the major diameter. For example, 1/2-13 UNC: 0.500 - (1/13) = 0.500 - 0.0769 = 0.423 inches for 100% thread. At 75%: 0.500 - 0.75 × 0.0769 = 0.442, which rounds to a 7/16 (0.4375) drill giving about 77% thread.
At 50% thread, the tap drill is larger, tapping is easier, and the threads are shallower. Pull-out strength is about 90% of a 75% thread. At 75%, the tap drill is smaller, tapping requires more torque, and tap breakage risk increases significantly in hard materials. The extra 25% thread depth only adds about 10% more pull-out strength. For most applications, 60-65% is the practical optimum that balances strength, ease of tapping, and tool life.
Tapping speed depends on material: mild steel at 30-50 SFM, stainless at 10-20 SFM, aluminum at 60-90 SFM, cast iron at 40-60 SFM. Convert SFM to RPM using RPM = (SFM × 3.82) / tap major diameter. For a 1/2-13 tap in mild steel at 40 SFM: (40 × 3.82) / 0.5 = 306 RPM. On a manual machine, go slower rather than faster - a broken tap in a hole is a nightmare to remove.
Spiral point (gun) taps push chips forward through the hole. Use them for through holes where chips can exit the far side. Spiral flute taps pull chips back up out of the hole. Use them for blind holes where chips have no exit path. Roll-forming taps displace material rather than cutting and produce no chips at all - ideal for ductile materials like aluminum, copper, and low-carbon steel in blind holes.
The most common reasons: (1) Tap drill is too small, giving too high a thread percentage and excessive torque. Use a larger drill for 60-65% thread instead of 75%. (2) Blind hole not deep enough - the tap bottoms out before reaching full depth. Drill 3-4 pitch lengths deeper than the needed thread depth. (3) Chips packing in the flutes, especially in blind holes with the wrong tap type. (4) No cutting fluid or wrong fluid for the material. (5) Spindle speed too high, especially in stainless and alloy steels.
Disclaimer: This calculator provides tap drill sizes based on standard Unified and Metric thread specifications. Actual tapping results depend on material condition, tap quality, lubrication, and machine rigidity. Thread percentages are theoretical - verify critical threads with go/no-go gauges. Always follow tap manufacturer recommendations for speeds, feeds, and lubrication.

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