By Brian, MW0GKX.

To work out what size hole is required for the tap you are going to use you will need some maths! Don't worry it's fairly easy, well I'm not so sure about imperial but we'll come to that!

Before we can start to number crunch we need to gather some numbers I'll assume that the thread depth will be around 75%, which will give a strong thread and leave a little leeway for unavailable drill sizes. If the drill size is unavailable then use the closest size you can get. I'll deal with the, more common now, Metric first, further down the page is Imperial.

**Be Aware: **Drills tend to make holes slightly oversize than the marked size.

You will need to know:

- Diameter of the thread (
**A**). - Thread Pitch (
**B**).

The math is simple: **A** - **B** = drill size.

Here is a table of examples (note that there are some differences in the table to the math above, but the calculation will work and is easy to remember).

Metric Coarse Sizes (in mm) | Metric Fine Sizes (in mm) | |||
---|---|---|---|---|

Tap size | Drill Size | Tap size | Drill Size | |

4 × .7 | 3.3 | 4 × .35 4 × .5 | 3.6 3.5 | |

4.5 × .75 | 3.7 | 4.5 × .5 | 4 | |

5 × .8 | 4.2 | 5 × .5 | 4.5 | |

6 × 1 | 5 | 6 × .5 6 × .75 | 5.5 5.25 | |

7 × 1 | 6 | 7 × .75 | 6.25 | |

8 × 1.25 | 6.8 | 8 × .5 8 × .75 8 × 1 | 7.5 7.25 7 | |

9 × 1.25 | 7.8 | 9 × 1 | 8 | |

10 × 1.5 | 8.5 | 10 × .75 10 × 1 10 × 1.25 | 9.25 9 8.8 |

You will need to know:

- Diameter of the thread (
**A**). - Thread Pitch (
**B**).

The math is simple (if you're good at fractions!): **A** - **B** = drill size.

For example, the tap drill for 3/8-16 is 5/16. The pitch is 1/16" (1 / threads per inch = inches per thread).

3/8 (6/16) - 1/16 = 5/16.

For sizes that don't work out so nicely, just use the closest drill size.

If, like me, your fractional maths finished when you left school here is a JavaScript calculator to work it out for you (you will need the fractions of an inch to be decimal fractions though!).

Enter appropriate values and press Calculate Drill Size

(enter diameter in decimals eg .500 for 1/2",

use 75 if you're not sure about the Percent of thread)

(enter diameter in decimals eg .500 for 1/2",

use 75 if you're not sure about the Percent of thread)