---
title: "Tap Drill Size Chart: Inch & Metric Threads | OpenSpindle"
description: "Tap drill size chart for common inch (UNC/UNF) and metric coarse threads at ~75% thread engagement, plus clearance-hole guidance for through holes."
canonical: https://openspindle.com/reference/tap-drill-thread-chart
---

# Tap Drill & Thread Chart

Tap drill sizes for common inch and metric threads at about 75% thread engagement, plus how to size a clearance hole for the mating screw.

To cut an internal thread, you drill a hole slightly larger than the thread's minor diameter, then tap it. The standard target is about 75% thread engagement, which holds nearly all the strength of a full thread while cutting far less material and taking less tapping torque. The inch sizes below are the standard 75% tap drills; the metric coarse rule is simply tap drill = nominal diameter minus pitch. To pass a screw through a part instead of threading it, use a clearance hole sized larger than the screw.

## Inch tap drills (UNC / UNF, ~75% thread)

These are the standard 75%-thread tap drills for common inch machine-screw and fractional threads. Coarse (UNC) and fine (UNF) share several sizes; both are listed where common.

| Thread | Tap drill | Decimal (in) |
| --- | --- | --- |
| #4-40 UNC | #43 | 0.0890 |
| #6-32 UNC | #36 | 0.1065 |
| #8-32 UNC | #29 | 0.1360 |
| #10-24 UNC | #25 | 0.1495 |
| #10-32 UNF | #21 | 0.1590 |
| 1/4-20 UNC | #7 | 0.2010 |
| 1/4-28 UNF | #3 | 0.2130 |
| 5/16-18 UNC | F | 0.2570 |
| 3/8-16 UNC | 5/16 | 0.3125 |
| 1/2-13 UNC | 27/64 | 0.4219 |

*Standard 75%-thread tap drills. Softer materials (aluminum, brass, plastics) tap well at 75%; in hard steels many shops drop to ~65% thread with a slightly larger drill to cut tapping torque and reduce broken taps.*

## Metric coarse tap drills (tap drill = nominal minus pitch)

For metric coarse threads the tap drill in millimeters equals the nominal diameter minus the pitch. For example, M6 x 1.0 uses a 6 - 1.0 = 5.0 mm drill.

| Thread | Pitch (mm) | Tap drill (mm) |
| --- | --- | --- |
| M3 | 0.5 | 2.5 |
| M4 | 0.7 | 3.3 |
| M5 | 0.8 | 4.2 |
| M6 | 1.0 | 5.0 |
| M8 | 1.25 | 6.8 |
| M10 | 1.5 | 8.5 |

*Coarse-pitch threads. Fine-pitch metric threads use a smaller pitch and therefore a larger tap drill (e.g. M8 x 1.0 uses a 7.0 mm drill).*

## Clearance holes (for passing a screw through)

When a screw passes through a part rather than threading into it, drill a clearance hole larger than the screw's major diameter. ISO 273 defines three fits for metric fasteners: close, normal (medium), and loose. Normal fit is the usual default.

| Screw | Clearance drill (normal fit) |
| --- | --- |
| M3 | 3.4 mm |
| M4 | 4.5 mm |
| M5 | 5.5 mm |
| M6 | 6.6 mm |
| M8 | 9.0 mm |
| M10 | 11.0 mm |

*ISO 273 normal (medium) fit. Use close fit for precise location and loose fit for more play. For inch screws, a clearance hole is typically the screw major diameter plus about 0.010 to 0.030 in depending on fit.*

Inch tap drills follow the standard 75%-thread chart; metric coarse follows tap drill = nominal minus pitch; clearance holes follow ISO 273. Confirm thread class, material, and fit for your part before cutting.

## On OpenSpindle

- [CNC machining capability](https://openspindle.com/capabilities/cnc-machining.md)
- [CNC machining tolerance chart](https://openspindle.com/reference/cnc-machining-tolerance-chart.md)

## Frequently asked questions

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

Use a #7 drill (0.201 in) for a standard 75%-thread 1/4-20 UNC tapped hole. In harder materials some shops step up slightly (toward ~65% thread) to reduce tapping torque and the risk of breaking the tap.

### How do I calculate a metric tap drill size?

For metric coarse threads, tap drill diameter (mm) equals the nominal diameter minus the pitch. M6 x 1.0 uses 6 - 1.0 = 5.0 mm; M8 x 1.25 uses about 6.8 mm. Fine-pitch threads have a smaller pitch, so they need a larger tap drill.

### What is 75% thread engagement?

It is the depth of thread the tap cuts relative to a theoretical full (100%) thread. A 75% thread holds nearly all the strength of a full thread while cutting much less material, so it is the standard target on most tap drill charts and lowers tapping torque and tap breakage.

### What is the difference between a tap drill and a clearance hole?

A tap drill is sized just above the thread's minor diameter so the hole can be tapped with internal threads. A clearance hole is larger than the screw's major diameter so the screw passes through freely. Use the tap drill where threads are cut and the clearance drill where a screw only passes through.