---
type: "drill"
designation: "18.5 mm"
series: "metric"
series_name: "Metric Drill Sizes"
diameter_in: 0.728346
diameter_mm: 18.5
standard: "ANSI/ASME B94.11M; BS 328"
verified: "August 1, 2026"
url: https://shopmath.org/drills/metric/18-5mm
---

# 18.5 mm Drill Bit Size — 0.7283 in / 18.500 mm

A 18.5 mm drill bit is 0.7283 in (18.500 mm) in diameter. It belongs to the metric series of standard twist drills. It is the recommended tap drill for M20×1.5.

- **Designation**: 18.5 mm
- **Series**: Metric
- **Diameter**: 0.7283 in (18.500 mm)

## 18.5 mm Drill Diameter

| Property | Value |
| --- | --- |
| Designation | 18.5 mm |
| Series | Metric |
| Diameter (in) | 0.7283 |
| Diameter (mm) | 18.500 |

## 18.5 mm Drill Equivalents in the Other Size Series

The nearest standard drill to 18.5 mm 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 | -0.5003 in |
| Letter | [Z](https://shopmath.org/drills/letter/z.md) | 0.4130 | 10.490 | -0.3153 in |
| Fractional | [47/64](https://shopmath.org/drills/fractional/47-64.md) | 0.7344 | 18.653 | +0.0060 in |

## Threads Tapped With a 18.5 mm Drill

| Thread | Pitch | Thread engagement (%) |
| --- | --- | --- |
| [M20×1.5](https://shopmath.org/threads/metric-fine/m20x1.5.md) | 1.500 mm | 77.0 |

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

## Sources and Method for the 18.5 mm 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 18.5 mm Drill Bit

### What size is a 18.5 mm drill bit size?

A 18.5 mm drill bit is 0.7283 in (18.500 mm) in diameter.

### What fraction is a 18.5 mm drill bit?

18.5 mm is 0.7283 in (18.500 mm), which is not itself a fractional size. The closest fractional drill is 47/64 at 0.7344 in (18.653 mm).

### What thread does a 18.5 mm drill bit tap?

A 18.5 mm drill is the recommended tap drill for M20×1.5. That leaves the thread engagement shown in the table above with a cut tap.

## Related pages

- Next size down (any series): [23/32](https://shopmath.org/drills/fractional/23-32.md) — 0.7188 in (18.256 mm)
- Next size up (any series): [47/64](https://shopmath.org/drills/fractional/47-64.md) — 0.7344 in (18.653 mm)
- Previous in the metric series: [18.25 mm](https://shopmath.org/drills/metric/18-25mm.md)
- Next in the metric series: [18.75 mm](https://shopmath.org/drills/metric/18-75mm.md)
- All metric sizes: [Metric drill sizes](https://shopmath.org/drills/metric.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/metric/18-5mm) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*