---
type: "drill"
designation: "1"
series: "fractional"
series_name: "Fractional Drill Sizes"
diameter_in: 1
diameter_mm: 25.4
fraction_label: "1"
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/fractional/1
---

# 1 Drill Bit Size — 1.0000 in / 25.400 mm

A 1 drill bit is 1.0000 in (25.400 mm) in diameter. It belongs to the fractional series of standard twist drills. It is the recommended tap drill for 1-11.5 NPT, 1-11.5 NPTF, 1 1/4-11.5 NPT, 1 1/4-11.5 NPTF, 1 1/2-11.5 NPT, 1 1/2-11.5 NPTF, 2-11.5 NPT, 2-11.5 NPTF, 2 1/2-8 NPT, 3-8 NPT, 3 1/2-8 NPT, 4-8 NPT, 5-8 NPT, 6-8 NPT, 8-8 NPT, 10-8 NPT, 12-8 NPT, 14-8 NPT, 16-8 NPT, 18-8 NPT, 20-8 NPT, 24-8 NPT, 1 1/16"-18 UNEF, 1 1/8"-18 UNEF, 1 1/8"-12 UNF, 1 3/16"-18 UNEF, 1 1/4"-18 UNEF, 1 1/4"-12 UNF, 1 1/4"-7 UNC, 1 5/16"-18 UNEF, 1 3/8"-18 UNEF, 1 3/8"-12 UNF, 1 3/8"-6 UNC, 1 7/16"-18 UNEF, 1 1/2"-18 UNEF, 1 1/2"-12 UNF, 1 1/2"-6 UNC, 1 9/16"-18 UNEF, 1 5/8"-18 UNEF, 1 11/16"-18 UNEF, 1 3/4"-5 UNC, 2"-4 UNC, 2 1/4"-4 UNC, 2 1/2"-4 UNC, 2 3/4"-4 UNC, 3"-4 UNC, 3 1/4"-4 UNC, 3 1/2"-4 UNC, 3 3/4"-4 UNC, 4"-4 UNC.

- **Designation**: 1
- **Series**: Fractional
- **Diameter**: 1.0000 in (25.400 mm)
- **Fraction**: 1

## 1 Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 1 |
| Series | Fractional |
| Diameter (in) | 1.0000 |
| Diameter (mm) | 25.400 |
| Fractional call-out | 1 |

## 1 Drill Equivalents in the Other Size Series

The nearest standard drill to 1 in each of the other three series. Nothing here is an exact match unless the difference column reads 0.0000 in.

| Series | Nearest drill | Diameter (in) | Diameter (mm) | Difference |
| --- | --- | --- | --- | --- |
| Number | [#1](https://shopmath.org/drills/number/1.md) | 0.2280 | 5.791 | -0.7720 in |
| Letter | [Z](https://shopmath.org/drills/letter/z.md) | 0.4130 | 10.490 | -0.5870 in |
| Metric | [25 mm](https://shopmath.org/drills/metric/25mm.md) | 0.9843 | 25.000 | -0.0157 in |

## Threads Tapped With a 1 Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [1-11.5 NPT](https://shopmath.org/threads/npt/1.md) | 11.5 TPI | — |
| [1-11.5 NPTF](https://shopmath.org/threads/nptf/1.md) | 11.5 TPI | — |
| [1 1/4-11.5 NPT](https://shopmath.org/threads/npt/1-1-4.md) | 11.5 TPI | — |
| [1 1/4-11.5 NPTF](https://shopmath.org/threads/nptf/1-1-4.md) | 11.5 TPI | — |
| [1 1/2-11.5 NPT](https://shopmath.org/threads/npt/1-1-2.md) | 11.5 TPI | — |
| [1 1/2-11.5 NPTF](https://shopmath.org/threads/nptf/1-1-2.md) | 11.5 TPI | — |
| [2-11.5 NPT](https://shopmath.org/threads/npt/2.md) | 11.5 TPI | — |
| [2-11.5 NPTF](https://shopmath.org/threads/nptf/2.md) | 11.5 TPI | — |
| [2 1/2-8 NPT](https://shopmath.org/threads/npt/2-1-2.md) | 8 TPI | — |
| [3-8 NPT](https://shopmath.org/threads/npt/3.md) | 8 TPI | — |
| [3 1/2-8 NPT](https://shopmath.org/threads/npt/3-1-2.md) | 8 TPI | — |
| [4-8 NPT](https://shopmath.org/threads/npt/4.md) | 8 TPI | — |
| [5-8 NPT](https://shopmath.org/threads/npt/5.md) | 8 TPI | — |
| [6-8 NPT](https://shopmath.org/threads/npt/6.md) | 8 TPI | — |
| [8-8 NPT](https://shopmath.org/threads/npt/8.md) | 8 TPI | — |
| [10-8 NPT](https://shopmath.org/threads/npt/10.md) | 8 TPI | — |
| [12-8 NPT](https://shopmath.org/threads/npt/12.md) | 8 TPI | — |
| [14-8 NPT](https://shopmath.org/threads/npt/14.md) | 8 TPI | — |
| [16-8 NPT](https://shopmath.org/threads/npt/16.md) | 8 TPI | — |
| [18-8 NPT](https://shopmath.org/threads/npt/18.md) | 8 TPI | — |
| [20-8 NPT](https://shopmath.org/threads/npt/20.md) | 8 TPI | — |
| [24-8 NPT](https://shopmath.org/threads/npt/24.md) | 8 TPI | — |
| [1 1/16"-18 UNEF](https://shopmath.org/threads/unef/1-1-16-18.md) | 18 TPI | 86.6 |
| [1 1/8"-18 UNEF](https://shopmath.org/threads/unef/1-1-8-18.md) | 18 TPI | 173.2 |
| [1 1/8"-12 UNF](https://shopmath.org/threads/unf/1-1-8-12.md) | 12 TPI | 115.5 |
| [1 3/16"-18 UNEF](https://shopmath.org/threads/unef/1-3-16-18.md) | 18 TPI | 259.8 |
| [1 1/4"-18 UNEF](https://shopmath.org/threads/unef/1-1-4-18.md) | 18 TPI | 346.4 |
| [1 1/4"-12 UNF](https://shopmath.org/threads/unf/1-1-4-12.md) | 12 TPI | 230.9 |
| [1 1/4"-7 UNC](https://shopmath.org/threads/unc/1-1-4-7.md) | 7 TPI | 134.7 |
| [1 5/16"-18 UNEF](https://shopmath.org/threads/unef/1-5-16-18.md) | 18 TPI | 433.0 |
| [1 3/8"-18 UNEF](https://shopmath.org/threads/unef/1-3-8-18.md) | 18 TPI | 519.6 |
| [1 3/8"-12 UNF](https://shopmath.org/threads/unf/1-3-8-12.md) | 12 TPI | 346.4 |
| [1 3/8"-6 UNC](https://shopmath.org/threads/unc/1-3-8-6.md) | 6 TPI | 173.2 |
| [1 7/16"-18 UNEF](https://shopmath.org/threads/unef/1-7-16-18.md) | 18 TPI | 606.2 |
| [1 1/2"-18 UNEF](https://shopmath.org/threads/unef/1-1-2-18.md) | 18 TPI | 692.8 |
| [1 1/2"-12 UNF](https://shopmath.org/threads/unf/1-1-2-12.md) | 12 TPI | 461.9 |
| [1 1/2"-6 UNC](https://shopmath.org/threads/unc/1-1-2-6.md) | 6 TPI | 230.9 |
| [1 9/16"-18 UNEF](https://shopmath.org/threads/unef/1-9-16-18.md) | 18 TPI | 779.4 |
| [1 5/8"-18 UNEF](https://shopmath.org/threads/unef/1-5-8-18.md) | 18 TPI | 866.0 |
| [1 11/16"-18 UNEF](https://shopmath.org/threads/unef/1-11-16-18.md) | 18 TPI | 952.6 |
| [1 3/4"-5 UNC](https://shopmath.org/threads/unc/1-3-4-5.md) | 5 TPI | 288.7 |
| [2"-4 UNC](https://shopmath.org/threads/unc/2-4.md) | 4 TPI | 307.9 |
| [2 1/4"-4 UNC](https://shopmath.org/threads/unc/2-1-4-4.md) | 4 TPI | 384.9 |
| [2 1/2"-4 UNC](https://shopmath.org/threads/unc/2-1-2-4.md) | 4 TPI | 461.9 |
| [2 3/4"-4 UNC](https://shopmath.org/threads/unc/2-3-4-4.md) | 4 TPI | 538.9 |
| [3"-4 UNC](https://shopmath.org/threads/unc/3-4.md) | 4 TPI | 615.8 |
| [3 1/4"-4 UNC](https://shopmath.org/threads/unc/3-1-4-4.md) | 4 TPI | 692.8 |
| [3 1/2"-4 UNC](https://shopmath.org/threads/unc/3-1-2-4.md) | 4 TPI | 769.8 |
| [3 3/4"-4 UNC](https://shopmath.org/threads/unc/3-3-4-4.md) | 4 TPI | 846.8 |
| [4"-4 UNC](https://shopmath.org/threads/unc/4-4.md) | 4 TPI | 923.8 |

_Engagement is what the real drill leaves with a cut tap, not the requested target._

## Sources and Method for the 1 Drill Size

### Drill index

- **Source**: Number/letter: bd_warehouse drill_sizes.csv (Apache-2.0) cross-checked against Wikipedia Drill bit sizes (CC BY-SA 4.0). Fractional: computed 1/64 in steps per ANSI B94.11M. Metric: computed BS 328 increment pattern.
- **Method**: computed
- **Verified against**: Wikipedia Drill bit sizes (CC BY-SA 4.0); 31/31 checks passed

### Assumptions

- Drill diameters are nominal. A worn or run-out drill cuts oversize; measure the hole, not the shank, when the fit matters.
- Dataset 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

## Common Questions About the 1 Drill Bit

### What size is a 1 drill bit size?

A 1 drill bit is 1.0000 in (25.400 mm) in diameter. The nearest fractional call-out is 1.

### What is the metric equivalent of a 1 drill bit?

The closest metric drill to 1 (1.0000 in (25.400 mm)) is 25 mm at 0.9843 in (25.000 mm) — a difference of 0.0157 in.

### What thread does a 1 drill bit tap?

A 1 drill is the recommended tap drill for 1-11.5 NPT, 1-11.5 NPTF, 1 1/4-11.5 NPT, 1 1/4-11.5 NPTF, 1 1/2-11.5 NPT, 1 1/2-11.5 NPTF, 2-11.5 NPT, 2-11.5 NPTF, 2 1/2-8 NPT, 3-8 NPT, 3 1/2-8 NPT, 4-8 NPT, 5-8 NPT, 6-8 NPT, 8-8 NPT, 10-8 NPT, 12-8 NPT, 14-8 NPT, 16-8 NPT, 18-8 NPT, 20-8 NPT, 24-8 NPT, 1 1/16"-18 UNEF, 1 1/8"-18 UNEF, 1 1/8"-12 UNF, 1 3/16"-18 UNEF, 1 1/4"-18 UNEF, 1 1/4"-12 UNF, 1 1/4"-7 UNC, 1 5/16"-18 UNEF, 1 3/8"-18 UNEF, 1 3/8"-12 UNF, 1 3/8"-6 UNC, 1 7/16"-18 UNEF, 1 1/2"-18 UNEF, 1 1/2"-12 UNF, 1 1/2"-6 UNC, 1 9/16"-18 UNEF, 1 5/8"-18 UNEF, 1 11/16"-18 UNEF, 1 3/4"-5 UNC, 2"-4 UNC, 2 1/4"-4 UNC, 2 1/2"-4 UNC, 2 3/4"-4 UNC, 3"-4 UNC, 3 1/4"-4 UNC, 3 1/2"-4 UNC, 3 3/4"-4 UNC, 4"-4 UNC. Each leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [63/64](https://shopmath.org/drills/fractional/63-64.md) — 0.9844 in (25.003 mm)
- Previous in the fractional series: [63/64](https://shopmath.org/drills/fractional/63-64.md)
- All fractional sizes: [Fractional drill sizes](https://shopmath.org/drills/fractional.md)
- All drill sizes: [Drill bit size reference](https://shopmath.org/drills.md)
- Complete index by diameter: [Drill size chart](https://shopmath.org/charts/drill-sizes.md)
- Tap drills by thread: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

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