---
type: "calculator"
name: "Tap Drill Size Calculator"
standard: "NBS H28 / ASME B1.1 and ISO 68-1 profile geometry"
verified: "August 1, 2026"
url: https://shopmath.org/calculators/tap-drill
---

# Tap Drill Size Calculator

The tap drill for 1/4"-20 UNC at 75% thread engagement is #7 — 0.2010 in (5.105 mm), which actually leaves 75.4% thread. Any thread in the database works the same way, at any engagement from 50% to 100%.

## Inputs

| Input | Range | Notes |
| --- | --- | --- |
| Thread | any inch or ISO metric thread in the database | Taper pipe threads are excluded: NPT is cut with a taper tap into a straight hole at one published drill size, so percent thread is not a quantity it has. |
| Thread engagement | 50–100% | The proportion of the full theoretical thread height the tap will cut. |
| Drill index | the thread's own system, or all four indexes | Inch drills for inch threads and metric for metric is the assumption every published chart makes silently; this control makes it explicit. |

## Worked example — 1/4"-20 UNC at 75% thread

- **Thread**: [1/4"-20 UNC](https://shopmath.org/threads/unc/1-4-20.md)
- **Basic major diameter**: 0.2500 in (6.350 mm)
- **Pitch**: 20 TPI
- **Target engagement**: 75%
- **Theoretical hole**: 0.2013 in (5.113 mm)
- **Nearest standard drill**: #7 — 0.2010 in (5.105 mm)
- **Engagement that drill leaves**: 75.4%

```text
hole Ø  =  0.2500 − (75 ÷ 100) × (1.29904 ÷ 20)
        =  0.2013 in
```

Asking for a lighter 60% thread moves the recommendation to #3 (0.2130 in), which leaves 57.0%. The gap between what you ask for and what the nearest real drill leaves is the number published charts omit.

## 1/4"-20 UNC across the engagement range

| Engagement | Target Ø (in) | Target Ø (mm) | Nearest drill | Actual % |
| --- | --- | --- | --- | --- |
| 50% | 0.2175 | 5.524 | [7/32](https://shopmath.org/drills/fractional/7-32.md) | 48.1 |
| 55% | 0.2143 | 5.443 | [#3](https://shopmath.org/drills/number/3.md) | 57.0 |
| 60% | 0.2110 | 5.359 | [#3](https://shopmath.org/drills/number/3.md) | 57.0 |
| 65% | 0.2078 | 5.278 | [#4](https://shopmath.org/drills/number/4.md) | 63.1 |
| 70% | 0.2045 | 5.194 | [#6](https://shopmath.org/drills/number/6.md) | 70.8 |
| 75% | 0.2013 | 5.113 | [#7](https://shopmath.org/drills/number/7.md) | 75.4 |
| 80% | 0.1980 | 5.029 | [#8](https://shopmath.org/drills/number/8.md) | 78.5 |
| 85% | 0.1948 | 4.948 | [#9](https://shopmath.org/drills/number/9.md) | 83.1 |

Every other thread in the database has the same table on its own page — see [the tap drill chart](https://shopmath.org/charts/tap-drill.md) for all of them at the shop default.

## Sources and Method for the Tap Drill Size Calculator

### Inch thread dimensions

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)
- **Method**: computed
- **Verified against**: threads-unified: 1064 checks passed across 3 harness(es)

### Metric thread dimensions

- **Source**: ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled
- **Method**: computed+compiled
- **Verified against**: threads-metric: 215 checks passed across 3 harness(es)

### Standard drill index

- **Source**: bd_warehouse drill_sizes.csv (Apache-2.0) + Wikipedia Drill bit sizes (CC BY-SA 4.0); fractional and metric computed from ANSI B94.11M / BS 328 increment patterns
- **Method**: computed+compiled
- **Verified against**: drills: 31 checks passed across 1 harness(es)

### Formulas

**Tap drill hole diameter — inch threads**

```text
hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)

  D = basic major diameter (in)
  n = threads per inch
```

**Tap drill hole diameter — metric threads**

```text
hole Ø  =  D − (%thread ÷ 100) × 1.29904 × P

  D = basic major diameter (mm)
  P = pitch (mm)
```

_1.29904 = 2 × 0.64952, twice the basic thread height per side on the 60° V profile shared by the Unified and ISO systems._

**Engagement the chosen drill actually produces**

```text
%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100        [inch]
%thread  =  (D − drill Ø) ÷ (1.29904 × P) × 100      [metric]
```

_The recommendation rounds to a real drill; this reports what that drill leaves._

### Assumptions

- Every figure is for a CUT tap. Form (roll) taps need a larger hole and are not in this dataset.
- The drill index defaults to the thread's own system — inch drills for inch threads, metric for metric. That is the assumption behind every published chart, and the drill-index control makes it explicit and changeable.
- The recommendation is the nearest standard drill in either direction, not the next size up. Selecting nearest-to-75% reproduces 36 of the 39 rows of the classic published UNC/UNF chart; every tie-break variant tested made the agreement worse.
- Drill diameters are nominal. A worn drill, spindle run-out, or a soft material closing in behind the drill all move the finished hole toward a heavier thread than the table says.
- The engagement bands described beside the slider come from practitioner evidence in our audience research, not from a standard. They describe what the number means; they are not a recommendation for a specific job.
- Inch thread dimensions scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.
- Metric thread dimensions scope: EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).
- Standard drill index scope: Number sizes #61-#80 are stored but tagged demandTier 'extended' — query-space-map.md Appendix A found the tail thinning in the 60s. Sizes below #80 and metric sizes above 25.0 mm excluded.

**Data last verified**: August 1, 2026

## Related pages

- Lookup version: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)
- The reverse question: [Thread engagement calculator](https://shopmath.org/calculators/thread-engagement.md)
- Threads: [Screw thread reference](https://shopmath.org/threads.md)
- All calculators: [Machining calculators](https://shopmath.org/calculators.md)

---

*Source: [ShopMath](https://shopmath.org/calculators/tap-drill) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*