Tap Drill Size Chart — Metric, UNC & UNF

Quick tap-drill lookup

Search for a thread such as M8 × 1.25, 1/4-20 UNC, or #10-32 UNF. The chart keeps cutting-tap and forming-tap recommendations in separate columns because the processes remove and displace material differently.

Cutting-tap values use the public manufacturer chart and the supporting technical references listed below. Forming-tap values use the public Fastenal metalworking chart where a corresponding row is listed. The structured dataset identifies converted fields separately from directly sourced values.

How to read a tap-drill chart

1. Identify the thread system and designation, including pitch or threads per inch. M8 × 1.25 is not the same as M8 × 1.00, and 1/4-20 is not 1/4-28.
2. Choose a cutting tap or forming tap. Do not select one drill column and use it for both processes.
3. Choose a drill designation that your tooling system supports. Number, letter, fractional, and metric labels are identifiers; do not convert a drill number into a guessed diameter.
4. Confirm the tap manufacturer’s current recommendation for the material, hole depth, coating, lubrication, and required thread engagement.
5. Deburr and verify the drilled diameter before tapping. A damaged drill, runout, coating, or material springback can change the result.

Why tap-drill sizes differ

A tap drill leaves room for the tap’s thread form. The best hole size depends on the thread percentage that the application needs, the material’s strength and ductility, the tap geometry, the hole depth, and the process. A simple major-diameter-minus-pitch rule can be a starting estimate for some cutting taps, but it is not a validated recommendation for every thread or material.

Forming taps need particular care. They displace material instead of cutting chips, so they commonly need a larger pre-hole than a cutting tap. The workpiece must also be suitable for cold forming. Follow the tap maker’s chart rather than copying the cutting-tap value.

Metric, UNC, and UNF notation

Related Mechanitec references

• How to Use Hole Wizard in SOLIDWORKS — CAD workflow for creating standard tapped holes.
• How to Make a Threaded Hole With Hole Wizard — model the selected thread in SOLIDWORKS.
• Mikuni Pilot Jet Drill Size Chart — a separate small-hole drill reference with a different primary intent.
• How to Use SOLIDWORKS Toolbox — related fastener-library workflow.

Source note

The draft uses the public standards pages and manufacturer tap-drill references listed in the accompanying provenance file. Verify production-critical values against the current tap manufacturer’s chart and the applicable thread standard.

Frequently asked questions

What drill size is used for an M8 × 1.25 tap?

Guhring’s public popular-size chart lists 6.75 mm for an approximately 75% cutting-tap recommendation. The Fastenal chart lists 7.40 mm for a forming tap. Confirm the recommendation for your tap model and material before drilling production parts.

What drill size is used for a 1/4-20 tap?

The public Fastenal chart lists a number 7 drill, 0.2010 in, for a cutting tap and a number 1 drill, 0.2280 in, for a forming tap.

Can I use the major diameter minus pitch formula?

Use it only as a rough calculation, not as a universal production recommendation. Thread percentage, tap type, material, and the tap manufacturer’s geometry can change the required hole size.

Are cutting-tap and forming-tap drill sizes interchangeable?

No. Forming taps usually require a different, often larger, pre-hole and are limited by the workpiece material and process.