How to Know What Size Drill Bit to Use for Screws

To choose the right drill bit for a screw, match the bit to the screw’s inner shaft, not the outer threads. Use a slightly smaller pilot bit for softwood, a slightly larger one for hardwood, and a tap drill chart for machine screws in metal.

Picking a drill bit sounds easy until you are standing in front of a wall of bits and screw boxes. I get it. The sizes look random. The charts look technical. And one wrong choice can split wood, strip a screw, or leave a loose hole.

I’m Tyler Bennett, and I’ve used this simple process for years in DIY projects, repairs, and shop work. In this guide, I’ll show you how to choose the right bit fast, read the charts, and avoid the mistakes that cause most problems.

Quick Answer: What Size Drill Bit Should You Use for Screws?

The basic rule of thumb

For most screws, use a pilot bit that matches the screw’s minor diameter. That is the solid core of the screw, without the threads. If you are drilling a clearance hole, use a bit close to the screw’s outer diameter so the screw can pass through freely. If you are threading metal for a machine screw, use a tap drill chart.

• Pilot hole = match the screw’s inner shaft
• Clearance hole = match the screw’s outer diameter
• Tap hole = use a proper tap drill size from a chart

Fast answer by situation

• Wood screws: Use a pilot hole close to the screw’s core diameter.
• Softwood: Use a slightly smaller pilot hole so the threads bite well.
• Hardwood: Use a slightly larger pilot hole to prevent splitting and snapped screws.
• Sheet metal screws: Match the hole to the screw style and gauge chart.
• Machine screws: Use a tap drill chart if you are cutting threads.
• Self-tapping screws: Many still need a pre-drilled hole. Check the package or chart.

One-sentence takeaway

If you’re unsure, match the drill bit to the screw’s solid inner shaft, not the outer threads.

Understanding Screw and Drill Bit Sizes

What screw size actually means

In the USA, many screws use a gauge number like #4, #6, #8, #10, and #12. As the number goes up, the screw gets thicker. That gauge tells you the diameter range, but it does not tell you the exact pilot hole by itself. The material matters too.

For example:

• #4 = small trim and light-duty screws
• #6 = general repair and hardware screws
• #8 = very common for wood projects
• #10 = stronger fastening, hinges, framing hardware
• #12 = heavier-duty wood and structural work

Major diameter vs minor diameter

This is the part that clears up most confusion.

• Major diameter = the full width across the outer threads
• Minor diameter = the width of the inner shaft, also called the root or core

When I choose a pilot hole, I look at the minor diameter. The threads still need wood or metal to bite into. If you drill a hole as big as the major diameter, the screw may not hold well.

Pilot hole vs clearance hole vs tap hole

• Pilot hole: A starter hole that guides the screw and reduces splitting.
• Clearance hole: A larger hole that lets the screw slide through the top piece without threading into it.
• Tap hole: A hole sized for a tap so you can cut threads for a machine screw in metal or plastic.

If you are joining two wood pieces, a pilot hole in the lower piece and a clearance hole in the top piece often gives the tightest joint.

Drill bit sizing systems in the USA

In U.S. shops, you will see drill bits marked in several systems:

• Fractional sizes like 1/8 inch, 5/32 inch, and 3/16 inch
• Numbered bits like #43, #36, and #29
• Letter bits like F, H, and Q
• Metric bits like 3 mm, 4 mm, and 5 mm

If you want a good reference for bit size systems, Engineering ToolBox has a useful drill bit size chart. For unit basics, NIST explains measurement standards clearly.

Why Material Changes the Drill Bit Size

Softwood

Softwood gives a little. Pine, cedar, and fir usually let screw threads bite in easily. That means you can use a slightly smaller pilot hole and still drive the screw cleanly.

Good reasons to pre-drill softwood:

• Better screw placement
• Less wandering
• Cleaner entry
• Less chance of splitting near edges

Hardwood

Hardwood is less forgiving. Oak, maple, hickory, and walnut can split if the pilot hole is too small. They can also snap a brittle screw. In hardwood, I usually go one bit size larger than I would in softwood for the same screw.

If you are driving long screws into hardwood, a pilot hole is not optional. It is the safe move.

Metal

Metal needs more precision. A pilot hole that works in wood does not mean much here. For sheet metal screws, the correct size depends on the screw design and metal thickness. For machine screws, you need the right tap drill size if you plan to cut threads.

Also remember:

• Use cutting oil when drilling steel
• Use slower drill speeds for larger bits
• Clamp the work so the bit does not grab

Plastic and composite materials

Plastic can crack if the hole is too tight. Composite boards can mushroom or split around the screw. In both cases, a clean pilot hole helps. I often test on scrap first because some plastics expand with heat and some composite boards act more like hardwood than softwood.

How to Measure a Screw So You Pick the Right Drill Bit

Method 1: Match the bit to the screw’s root diameter

This is my favorite method because it is simple and accurate enough for most jobs.

1. Hold the screw next to a few drill bits.
2. Look at the solid center shaft of the screw.
3. Choose a bit that matches that shaft, but does not cover the threads.

If the bit hides the threads, it is too big for a pilot hole. If the bit looks much smaller than the center shaft, it is probably too small.

Method 2: Hold the drill bit in front of the screw

Put the bit directly in front of the screw and line them up by eye.

• If you can still see the threads sticking out on both sides, the bit is in the right range.
• If the bit covers the whole screw, it is too large.
• If the bit looks tiny compared to the shaft, it is too small.

This is the fast shop method. I use it all the time for basic woodwork.

Method 3: Use calipers or a drill gauge

If you want a more exact answer, use a caliper to measure:

• Major diameter for clearance holes
• Minor diameter for pilot holes

A drill gauge also helps if your bits are worn or the markings are hard to read. This is the best method when you are working with metal, machine screws, or expensive materials.

What to do if you’re between two bit sizes

If you are stuck between sizes, use this rule:

• Softwood: Choose the smaller bit
• Hardwood: Choose the larger bit
• Metal tapping: Follow the chart exactly
• Plastic: Test the larger bit on scrap if cracking is a risk

When in doubt, drill a test hole in scrap first. That one habit saves a lot of repairs.

Drill Bit Size Chart for Common Screw Sizes

Table: Common pilot hole sizes for wood screws

These sizes are practical starting points for standard wood screws. Always test in scrap if the wood is brittle, very hard, or close to an edge.

Table: Common clearance hole sizes

Use a clearance hole when you want the screw to pass through the top piece without grabbing it. This helps pull the two pieces together tightly.

Notes for using the chart

• These are common starting sizes, not a perfect answer for every screw brand.
• Tapered wood screws may want a tapered pilot bit for the cleanest fit.
• Near the edge of a board, I often pre-drill even in softwood.
• For hardwood, wax on the screw threads can make driving easier.
• If the screw binds too hard, go up one bit size.
• If the screw feels loose, go down one bit size on your test piece.

Tap Drill Chart for Machine Screws

Table: Common tap drill sizes

Use this chart when you are drilling a hole before tapping threads for common machine screws.

When to use a tap drill chart

Use a tap drill chart when:

• You are threading a hole for a machine screw
• You are working in metal or rigid plastic
• You need the screw to thread into the material itself
• You want repeatable, accurate thread engagement

If you want a second reference, Engineering ToolBox also lists common tap drill sizes.

UNC vs UNF threads

UNC means Unified National Coarse. UNF means Unified National Fine.

• UNC has fewer threads per inch and is common in general fastening.
• UNF has more threads per inch and is often used where finer adjustment or more holding strength in thin material is needed.

You cannot guess between the two. A #10-24 and a #10-32 do not use the same tap drill.

Step-by-Step: How to Choose the Right Drill Bit for Any Screw

Step 1: Identify the screw type

Start with the screw itself. Ask:

• Is it a wood screw?
• Is it a sheet metal screw?
• Is it a machine screw?
• Is it self-tapping?

This matters because a wood screw chart and a tap drill chart are not the same thing.

Step 2: Identify the material

Look at what you are drilling into:

• Softwood = smaller pilot hole
• Hardwood = slightly larger pilot hole
• Metal = more exact drill size
• Plastic or composite = test first to avoid cracking

Step 3: Decide whether you need a pilot, clearance, or tap hole

This is where many people go wrong.

• Use a pilot hole when the screw will cut into the material.
• Use a clearance hole in the top part when you want the screw to pull two pieces together.
• Use a tap hole when a tap will cut threads for a machine screw.

Step 4: Match the bit size to the screw diameter

Now choose the bit:

• For a pilot hole, match the minor diameter.
• For a clearance hole, match the major diameter.
• For metal threading, use the tap chart.

If the screw is long, hard, or going into dense material, I lean toward a cleaner, slightly larger pilot hole. It makes driving easier and lowers the chance of breaking the screw.

Step 5: Test on scrap material first

This is the step that saves the project.

1. Drill a test hole in scrap.
2. Drive the screw.
3. Check the fit.

• If the screw fights hard, the hole is too small.
• If the screw spins or feels weak, the hole is too large.
• If it drives firm and holds tight, you have the right size.

Common Mistakes When Choosing a Drill Bit for Screws

Using a bit that is too large

If the bit is too large, the screw threads do not have enough material to bite. The screw may feel loose or strip out. This happens a lot when people match the bit to the outer threads instead of the inner shaft.

Using a bit that is too small

If the bit is too small, the screw takes too much force to drive. That can lead to:

• Split wood
• Broken screws
• Stripped screw heads
• Poor alignment

Skipping pilot holes in hardwood

I see this all the time. A screw may go into pine without a pilot hole, but hardwood is different. Skipping the pilot in oak or maple is asking for trouble.

Confusing wood screw charts with tap drill charts

A wood screw pilot hole chart is for a screw that cuts into the material. A tap drill chart is for a hole that will be threaded with a tap. They are not interchangeable.

Not accounting for screw material and brittleness

Some screws are harder and more brittle than others. Stainless and hardened screws can snap if the pilot hole is too tight, especially in hardwood. Cheap screws can twist off even faster. If the screw feels weak, increase the pilot slightly and use lubrication or wax when appropriate.

Pro Tips for Cleaner Holes and Better Screw Holding

Use brad point bits for wood

Brad point bits start accurately and leave cleaner holes in wood than standard twist bits. I use them any time appearance matters, like cabinets, trim, or visible furniture parts.

Countersink when needed

If the screw head needs to sit flush, add a countersink. This gives the head a clean seat and helps prevent surface tear-out.

If you do a lot of woodworking, a combo countersink bit is handy because it drills the pilot and countersink together.

Drill to the correct depth

A pilot hole should usually be at least as deep as the screw’s threaded length. If the hole is too shallow, the screw can bottom out and split the material. A piece of tape on the bit works as a simple depth mark.

Lubricate when drilling metal

Cutting fluid reduces heat and helps the bit cut cleaner. It also makes tapping easier and improves thread quality.

Keep a screw-and-bit reference card

Once you find the sizes you use most, write them down. I keep a small card in my drill case with my common pilot, clearance, and tap sizes. It saves time every single week.

FAQ

What size drill bit do I use for a screw?

For a pilot hole, use a bit close to the screw’s inner shaft or minor diameter. For a clearance hole, use a bit close to the screw’s outer diameter.

Should the pilot hole be smaller than the screw?

Yes. The pilot hole should be smaller than the full thread width so the threads can still grip the material.

What size pilot hole should I use for a #8 screw?

A common starting point is 1/8 inch in softwood and 9/64 inch in hardwood.

Do wood screws always need pilot holes?

No, but they often help. I strongly recommend pilot holes in hardwood, near board edges, and when using long or thick screws.

Can I use the same drill bit size as the screw?

Usually no for a pilot hole. If you match the full thread diameter, the screw may not hold. That size is closer to a clearance hole.

How deep should a pilot hole be?

It should usually match the length of the threaded part of the screw, or be close to it, so the screw drives smoothly without forcing the material apart.

What is the difference between a pilot hole and a tap hole?

A pilot hole guides a screw into material like wood. A tap hole is drilled before using a tap to cut threads for a machine screw.

Conclusion

The easiest way to pick the right drill bit is to match the bit to the screw’s inner shaft, then adjust for the material. Softwood usually wants a smaller pilot. Hardwood usually wants a slightly larger one. For machine screws in metal, always use a tap chart.

If you keep one chart nearby and test on scrap first, you will get cleaner holes, tighter joints, and far fewer broken screws.