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

Calculate drill RPM, feed, and tap-drill size for supported local UNC, UNF, and metric rows

Free drill and tap calculator that combines generic drill speed and feed screening with tap drill size math. Select a supported UNC, UNF, or Metric row, choose a thread percentage from 50% through 83%, and review the theoretical tap drill diameter plus the nearest local number, letter, or fractional drill. The current app includes 20 UNC rows, 20 UNF rows, and 17 metric rows from M1 through M24. It also estimates drilling RPM and feed rate from local material SFM presets and drill type. Treat these values as shop-starting points only: current standards, tap maker data, tool condition, material, coating, hole type, thread class, drilled-hole measurement, and go/no-go gaging can require different values.

Pro Tip: Higher thread percentage usually means a smaller tap drill, more tapping torque, and more tap-breakage risk. Lower thread percentage usually makes tapping easier but changes thread engagement. The right target depends on material, tap style, thread class, lubricant, hole type, drawing requirements, and how the part will be inspected. Use the calculator for a quick calculator, then verify against the drawing, current standards, and tap manufacturer data.

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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

    Use the slider from 50% through 83%. Higher percentages reduce tap drill diameter and usually increase tapping torque. Lower percentages are often easier to tap, but the correct target depends on the job.

  3. Review Tap Drill Size

    The calculator shows the theoretical decimal drill size, the metric equivalent, and the nearest local number, letter, or fractional drill row. It does not approve the final thread class or inspection result.

  4. Enter Workpiece Material

    Select the material and drill type to estimate drilling RPM, SFM, feed per revolution, and feed rate. These are local starting points, not product-specific tool data.

  5. Verify Before Cutting

    Check the current standard, drawing, tap maker data, actual drill size, hole type, lubricant, material condition, and go/no-go gaging before using the result for production work.

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

Assumptions

  • Local UNC and UNF rows cover #0 through 1 inch; local metric rows cover M1 through M24.
  • Thread percentage is calculated from the theoretical shortcut used in the app, not from a reproduced standards table.
  • Nearest drill lookup uses nominal number, letter, and 1/64 inch fractional rows through 1 inch.
  • Material SFM rows are local starting presets and require current tool-maker validation for the exact drill, coating, machine, coolant, and setup.

Limitations

  • Does not recommend or approve tap type, tap speed, lubricant, blind-hole depth, or production process.
  • Thread percentage calculation is theoretical and does not account for drill oversize, runout, tap wear, coating, plating, hole growth, or gaging.
  • Does not calculate actual thread percentage after selecting the nearest local drill row.
  • Pipe threads (NPT, NPTF, BSPT) use tabulated drill sizes, not a percentage-based formula.
  • Left-hand threads, multi-start threads, and Acme/buttress thread forms are not covered.

References

  • ASME B1.1 - Unified Inch Screw Threads (UN and UNR Thread Form)
  • ASME B1.13M - Metric Screw Threads: M Profile
  • ASME B94.11M - Twist Drills
  • ISO 2306 - Drills for use prior to tapping screw threads
  • Machinery's Handbook, Industrial Press
  • Current tap manufacturer data for the specific tool and material

Frequently Asked Questions

The app uses a local thread-percentage shortcut: for inch threads, drill diameter = major diameter - thread percentage / (TPI × 76.980). For metric threads, drill diameter = major diameter - thread percentage × pitch / 76.980. This is a theoretical calculator, not a licensed standards-table reproduction.
At 50% thread, the calculated tap drill is larger and tapping torque is usually lower. At 75%, the calculated drill is smaller and tapping torque usually increases. Strength, stripping risk, and tool life depend on material, thread class, fastener strength, lubricant, tap style, and engagement length, so verify the target for the job.
This app estimates drilling RPM, not a final tapping RPM. Tap speed depends on tap style, holder, material, lubricant, hole type, and machine control. Use the tap manufacturer data for the specific tap and reduce speed where the setup, material, or blind-hole chip control requires it.
Spiral point taps are commonly used for many through holes because they push chips ahead. Spiral flute taps are commonly used for many blind holes because they pull chips back. Forming taps need ductile material, proper hole size, and proper lubricant. The app does not select or approve tap type.
Common causes include a tap drill that is too small for the material and tap, chips packed in the hole, poor lubrication, a blind hole without enough extra depth, wrong tap geometry, misalignment, bad workholding, or too much speed. Use the calculator as one check, then inspect the setup and the tap maker recommendations.
Disclaimer: This calculator provides theoretical tap-drill and generic drill speed/feed screening only. Actual tapping results depend on standards revision, thread class, material condition, tap quality, drilled-hole size, coating, lubrication, machine rigidity, workholding, and inspection method. Verify critical threads with current standards, drawings, tap manufacturer data, calibrated measurement, and go/no-go gauges.

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