drilling pilot holes in wood and metal

How Do You Know What Size Pilot Hole to Drill?

We get asked this question a lot and for many people, especially first time or beginners to drilling, pilot holes can be just a bit confusing. By the time you finish this article, then you will know all you need to know about drilling pilot holes.

The only reason you drill a pilot hole is to prepare the material for a larger drill bit or screw. The simplest method is to take the screw that you plan on using, and measure the width of it across the threads. Then take off a couple of millimetres to allow for the thread.

You want the pilot hole to be large enough to allow the screw to easily go in. At the same time it needs to be small enough to allow the threads of the screw to actually catch the wood. Finding that perfect size is usually what causes people the concern.

Worry not though, this is actually a pretty easy thing to do as you will see below.

Pilot holes can be used for one of two reasons:

  1. To drill through material as a guide for making a bigger hole with a bigger drill bit
  2. To make a prepared hole for a screw or nail
how to drill pilot holes in materials

Why Do You Make a Pilot Hole in Wood or Timber?

The main reason that pilot holes are drilled into timber is so that a screw or nail can then be easily screwed or hammered in without the wood splitting or being damaged. This is really important when doing neat wood work and where the wood or lumber will be seen.

If for example you are laying deck boards, then many people will simply use their drill/driver to screw the boards directly on to the deck. In the middle of a deck board that is the best way to do that.

However if you are at the end of a board, and you try forcing a screw through the last inch or so, then more often than not the board will crack. Even if it doesn't crack, the board is weakened and may crack later.

Also, in rough work such as making a stud wall it really doesn't matter a great deal as no-one gets to see the finished job. Nevertheless when drilling into the end of a board, it is always a very good idea to drill a pilot hole, to avoid any risk of damaging the wood.

When you drill a pilot hole, and then drive in the screw it is highly unlikely to split or damage the wood.

Why Do You Make a Pilot Hole in Metal?

Sometimes you may also need to drill a pivot hole into metal. When used for metal, the pivot hole really acts as a guide for the larger drill bit. The pivot hole then becomes the guide for the tip of the larger drill bit. Having this smaller hole does make drilling the larger hole a great deal easier.

The smaller hole is much easier to control when drilling, and is a more accurate way of lining up a bigger drill bit. The drilled pilot hole acts as a guide for the tip of the larger drill bit.

How Do You Know Which Drill Bit Size to Use?

This is often the hardest thing to figure out. We have provided all the information just below, and hopefully that will make this task a great deal easier.

Pilot Hole Drill Bit Size Chart (Metric)

Some people like to stick with the older UK method of screw sizes. These are often known as imperial measurements and are expressed in the fractions of an inch measurement.

Most people though have now adopted what is called the metric sizes. There will continue to be this difference in how people ask for and buy screws. Some people will talk about a 2" screw whereas others will talk about a 50 mm screw.

The chart below shows the metric measurements as that is what the majority of people now use.

Understanding UK Screw Sizes

As you will know screws come in different thicknesses and in different lengths. There are also different types of screws, but for the purpose of pivot holes, we will stick to your normal wood screw.

  • All metric screw sizes are measured by diameter and length in mm

Sounds confusing already? Worry not it is a lot easier than you may think.

Screw Size Metric

Pilot Hole Size

Available Screw Lengths

2.5 mm diameter

1 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm and 40 mm

3.0 mm diameter

1.5 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm and 40 mm

3.5 mm diameter

2.0 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm and 40 mm

4.0 mm diameter

2.5 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 60 mm and 70 mm

5 mm diameter

3 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 60 mm, 70 mm, 75 mm, 80 mm, 90 mm and 100 mm

5.5 mm diameter

3.5 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 60 mm, 70 mm, 75 mm, 80 mm, 90 mm, 100 mm, 110 mm, 130 mm and 150 mm

6.0 mm diameter

4.0 mm

12 mm, 16 mm, 20 mm, 25 mm,30 mm, 35 mm, 40 mm, 45 mm, 50 mm, 60 mm, 70 mm, 75 mm, 80 mm, 90 mm, 100 mm, 110 mm, 130 mm and 150 mm

So when drilling a pilot hole the best advice is to use a drill bit 2 mm smaller than the screw size. The depth of the whole should then be the length of the screw. If you just stick to that rule, then you will never go wrong.

pilot hole drilling facts

How to Drill a Pilot Hole

Ideally you should always test any pilot holes you may want to drill on an old piece of material. That way if you make a mistake it really does not matter a great deal.

Most times I cut off a short piece of wood, clamp it to a vice or a workbench, and then make a couple of pilot holes using my drill, and then drive in a couple of screws.

Once I am completely satisfied with the results, then I do feel really confident when it comes to working on the main project. Each time I have to drill a different size hole, I then first test that size on an old piece of wood.

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cordy

I am a retired construction worker and I also happen to love power tools. I have used hundreds over the years and offer my opinions on them here.

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