# M18 tap drill size

The tap drill for an M18 × 2.5 metric coarse thread is 15.5 mm (0.6102 in). It leaves about 77 % thread, the normal range for steel.

## Drill options for M18 × 2.5

| Drill | Inch | Thread % |
|---|---|---|
| 15.3 mm | 0.6024 | 83 % |
| 15.4 mm | 0.6063 | 80 % |
| 15.5 mm | 0.6102 | 77 % |
| 15.6 mm | 0.6142 | 74 % |
| 15.7 mm | 0.6181 | 71 % |

## M18 thread data

- Thread: M18 × 2.5 (coarse)
- Pitch: 2.5 mm
- Tap drill: 15.5 mm (0.6102 in)
- Thread engagement: 77 %
- Clearance hole (ISO 273): 19 close, 20 normal, 21 loose (mm)

## Method

Thread % = 76.98 × (D − drill) / P = 76.98 × 2.5 / 2.5 = 77 %. Aim for 65–75 % in steel, 75–85 % in aluminium, brass and plastics.

## FAQ

**What size drill for an M18 × 2.5 tap?**
Use a 15.5 mm drill. It leaves about 77 % thread engagement.

**What is 15.5 mm in inches?**
15.5 mm is 0.6102 inch.

**What size clearance hole for M18?**
ISO 273 gives 19 mm for a close fit, 20 mm normal and 21 mm loose.

**How tight should an M18 bolt be?**
About 312 N·m for class 8.8 with dry threads.

Source page: https://machinistcharts.com/tap-drill/m18/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)