# M18x1.5 tap drill size

The tap drill for an M18 × 1.5 metric fine thread is 16.5 mm (0.6496 in). It leaves about 77 % thread, the normal range for steel.

## Drill options for M18 × 1.5

| Drill | Inch | Thread % |
|---|---|---|
| 16.3 mm | 0.6417 | 87 % |
| 16.4 mm | 0.6457 | 82 % |
| 16.5 mm | 0.6496 | 77 % |
| 16.6 mm | 0.6535 | 72 % |
| 16.7 mm | 0.6575 | 67 % |

## M18x1.5 thread data

- Thread: M18 × 1.5 (fine)
- Pitch: 1.5 mm
- Tap drill: 16.5 mm (0.6496 in)
- Thread engagement: 77 %
- Clearance hole (ISO 273): 19 close, 20 normal, 21 loose (mm)

## Method

Thread % = 76.98 × (D − drill) / P = 76.98 × 1.5 / 1.5 = 77 %. Aim for 65–75 % in steel, 75–85 % in aluminium, brass and plastics.

## FAQ

**What size drill for an M18 × 1.5 tap?**
Use a 16.5 mm drill. It leaves about 77 % thread engagement.

**What is 16.5 mm in inches?**
16.5 mm is 0.6496 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/m18x1-5/
Updated: 2026-10-05
Publisher: MachinistCharts (https://machinistcharts.com)