What Size Countersink Bit for a #8 Screw?

The short version for #8 screws

Countersink bits are sold by screw number, so a #8 bit is made for a #8 screw. The bit has two jobs. It drills a pilot hole for the threads, and it cuts a cone at the top so the head can sit flush or slightly below the surface.

For a #8 screw, the pilot hole is usually 7/64 in for softwood and a little larger for hardwood. The cone at the top is commonly about 3/8 in across. Exact dimensions vary a bit from one maker to the next, so treat these as typical numbers and test before you commit to a project piece.

Why the bit angle matters

US wood screws follow an 82 degree head standard, and every set on our best countersink bit page lists 82 degrees. That is the number to look for. A mismatched angle can leave the head sitting high or wedged in the recess.

The same page also covers other features that help with repeat work: a depth stop, replaceable pilot bits, and more flutes for a smoother cut. Those matter most when you are driving many screws of the same size.

Countersink and pilot sizes by screw number

The table below shows approximate sizes for common wood screws. Major diameter is the thread width of the screw. Pilot sizes depend on wood density, so go smaller in softwood and larger in hardwood. Always check the chart that comes with your bit set.

One set in our data lists pilot drill sizes of #6 at 3/32 in, #8 at 7/64 in and #10 at 1/8 in. Those line up with the softwood column below.

What about #6, #9 and #10 screws?

The rule stays the same: pick the countersink that matches the screw number. A #6 screw is smaller, so use a #6 bit and a smaller pilot. A #10 screw needs a #10 bit with a larger pilot and a wider cone.

#9 is the odd one out. Many countersink sets skip it and list #4, #6, #8, #10 and #12 instead. If you have #9 screws, try the #10 bit first and set the depth so the head sits flush. If the head sinks too far, the #8 bit with a slightly deeper pilot may work better. A scrap test settles it in a minute.

Sets that include replaceable pilot bits make this easier. You can swap the pilot for the wood you have without buying a new countersink body. Several sets on our comparison page include a #4 to #12 range with a depth stop, which covers all the sizes in this guide.

Countersinking for deck screws

Many deck screws are in the #8 to #10 range, so a #8 or #10 countersink bit is a good place to start. Check the box for the screw number. Some deck screws have a reamer or nib under the head that cuts its own recess in softer wood, and those may not need a separate countersink at all.

Hardwood decking and some boards near the ends are more likely to split. In those cases, pre-drilling and countersinking helps. Composite decking behaves differently from wood, and makers often publish their own fastening guidance, so follow that if you are using composite boards.

If you are comparing bits for deck work, a depth stop is worth a look. It keeps every head at the same depth across dozens of screws, which gives a more even finish. You can see which sets list one on our countersink bit comparison.

How to countersink a #8 screw step by step

Follow this order for clean results on most wood projects.

• Mark your screw locations and clamp the workpiece so it cannot spin or slide.
• Select the #8 countersink bit and the pilot size for your wood.
• Set the depth stop so the cone ends where the head will sit, or plan to stop by eye on a scrap piece first.
• Drill at a slow, steady speed and let the cutters do the work. Pull the bit out now and then to clear chips.
• Test the fit with a real screw. The head should sit flush or just below the surface.
• Adjust the depth and repeat on the project piece.

Safety notes

Wear safety glasses, since chips and dust can fly out as the bit cuts. Wear hearing protection if you are drilling many holes with a corded drill or in a shop setting, and a dust mask for fine sawdust, especially from treated lumber or composite boards.

Keep your hands clear of the bit and clamp small pieces instead of holding them. Remove the battery or unplug the drill before you change bits or adjust the depth stop. If you are working on a deck or ladder, keep both feet planted and avoid reaching to drill at arm's length.

Frequently asked questions

Can I use a #10 countersink on a #8 screw?

You can, but the cone will be wider than the head needs, so the screw may sink too deep and hold less well. A matching #8 bit gives a snugger fit.

Do I need a separate pilot drill if my countersink already has one?

Many countersink bits drill the pilot and cut the cone in one pass. Check that the pilot diameter suits your wood. Sets with replaceable pilots let you change it.

Why is my screw head still sitting above the surface?

The recess may be too shallow, or the bit angle may not match the screw. Check for an 82 degree bit, then cut a little deeper and test again.

Can I countersink metal with the same bit?

Some sets are rated for wood, plastic and soft metal, while others are wood only. Check the listing for suitable materials before you drill anything but wood.