Pilot Hole Size Chart: Wood & Metal Screw Table (Free PDF)

Find the screw size in the left-hand column and select the pilot drill from the appropriate hardwood or softwood column. If you use an ordinary twist bit to drill, select columns headed straight bit. If you have tapered countersink bits, select columns headed tapered bit (the bits matches the cone shape of a conventional cut-thread screw).

Drill a pilot hole in the receiving piece. Drill a clearance hole through the top piece. Countersink the hole if the screw is oval- or flat-headed.

Look at the shank diameter column. This lists the screw’s nominal body diameter. Note that there is a specific formula used by US wood screw gauges. The diameter equals 0.060 + 0.013 x gauge number. So a #8 screw is 0.164 in (or roughly 4.2 mm) and a #10 screw is 0.190 in (roughly 4.8 mm).

Look also at the pilot sizes, which are typically close to the screw's root diameter when using a straight bit. You will see the root diameter of the screw is near this number. Use this as a guideline when choosing what bit to use. You want the core of the screw to have a clear path, but you want it so the wood has something to bite into once the screw thread starts.

The shank clearance drill is the smallest standard bit that lets the unthreaded shank pass freely through the top board. What does that mean? This way the head of the screw can pull the two pieces together. Otherwise the threads will try to jack the pieces apart.

Softwood has low density and includes pine, spruce, cedar, fir, and others. Hardwood consists of dense species like oak, maple, beech, walnut, and most tropical timbers.

Hardwood pilots are one size larger than softwood pilots. Why? Because it’s hard to squish dense fibers. Don’t force a softwood-sized pilot into oak or you’ll split the board or snap a #6 brass screw.

Note that you can often just skip the pilot when using #6 or smaller screws in clear softwood away from edges. Always use hardwood column for brass, stainless or other soft-metal screws. Pre-form threads with a same-size steel screw if in very hard stock.

There are two traps. First, straight and tapered bits of the same size are not interchangeable. Using a straight twist bit instead of a tapered-sized bit will result in a hole whose tip is too large for good holding.

The chart refers to traditional tapered wood screw geometry. Moddern coarse-thread production screws feature deeper threads and a thinner core. A screw typically requires a pilot hole one size smaller than indicated on this chart. Many can be driven without any pilot whatsoever into softwood.

If uncertain, hold bit against screw and hold both up to the light. Bit should conceal the core while exposing the threads.

The countersink diameter is the size of the drill needed to seat a standard flat head flush with the surface. Various sources differ by a size on certain gauges. I followed Bolt Depot’s chart. Other sources say go down one size.

Measure the real-world diameter of the head of your screw if you can see it on the piece you’re working on. Countersink to that dimension.

Drill the pilot hole as deep into the piece as the screw body goes. Use a tape flag on the bit to avoid leaving a through-hole on a show face.

Engineering rule from the American Wood Council says 70 percent of root diameter in softwoods; 90 percent in hardwoods. That’s the max strength spec for structural applications. The sizes listed above are what we practice in woodwork. And they guard against splitting too.

Bolts and machine screws are an exception. Machine screws and bolts cut into tapped metal threads. Instead, they use a tap drill chart. That means it is not relevant for this table.

Also excluded from the table are drywall screws, masonry screws, lag screws (table 2) and self-tapping screws into sheet metal (table 3). Each has its own size of holes.

Find the nominal diameter of the lag screw in the first column. Locate the wood part that the screw threads grip. Read pilot hole dimension.

Drill side member with shank clearance hole. Size equals nominal diameter. Depth equals depth of any unthreaded shank.

Next drill the pilot hole to full depth of thread penetration. It is smaller than the clearance hole. Lag screw requires this hole more than a wood screw. Without it, the coarse threads will grip the top board and prevent the joint from staying together.

The three pilot drill sizes reflect the National Design Specification wood groups. These is divided by the specific gravity of the wood when dried in an oven.

The softwood (Group III & IV) has a specific gravity below 0.5 and consists of spruce, pine, fir, cedar and redwood. The medium (Group II) has a specific gravity from 0.5, 0.6 and is made up of Douglas fir-larch, yellow pine (southern), etc. The hardwood (Group I) has a specific gravity above 0.6 and consists of oak, hard maple, hickory and other dense tropical woods.

According to the NDS 2005, the pilot size ranges from 40 to 70 percent of the shank diameter for soft wood, 60 to 75 percent for medium wood and 65 to 85 percent for hard wood. The sizes given here is the commonly accepted sizes that fastener suppliers publish within these bands.

The hard and soft wood drill sizes are not published separately for some diameters. For instance, this holds true for the following: 7/16 in, 9/16 in, and all sizes above 3/4 in.

For diameters where soft and hard wood drills are not published, the value shown is the general-purpose pilot used by Portland Bolt and other manufacturers. That diameter is right smack dab in the middle, what we called the medium band. It make up around 62 to 75 percent of the shank.

In general framing lumber, you could step down to the next size smaller drill if known to be soft. Or you can increase to one size larger in dense hardwood material. Alternatively, you can simply calculate the NDS percentage straight from the decimal column.

If your connection is being designed to a load table, then use the exact percentage stated either by the NDS or your engineer. Don’t use a rounded shop size for structural connections.

Drill the pilot as deep as the full screw goes, because if a lag bottoms out in a short pilot, it will twist off at the head. Always run a washer under the hex head so it do not crush the surface fibers, and drive with a socket or impact driver, never a screwdriver bit in the hex corners. Soap or wax on threads cuts torque needed for driving about in half; no loss in holding power.

Too-large pilot is just as bad as too-small. Threads shear out past the top band of NDS, and the withdrawal capacity is lost.

The table covers the old ASME B18.2.1 geometry for traditional hex-head lag screws. It's not applicable to moddern engineered structural screws such as GRK RSS, TimberLOK, SPAX PowerLags etc. Those are self drilling, hardened screws typically used without a pilot hole in softwoods.

Lag shields and concrete screw anchors in masonry are not covered. It's not a tap drill chart for machine threads.

The screw gauge tells you which drill size to use; you then drill the listed size hole and insert and drive in the screw. As you do this, a self-tapping screw create or cuts its own thread in the wall of that hole.

There’s no separate option as with a wood screw between hard- and soft-wood varieties. It’s just the thickness and hardness of the metal and hole diameter to consider.

From the drill bit column: the exact matching decimal hole size are given by the wire-gauge letter or number drill bit. For metric drills, the mm approx column indicate the closest 0.1 mm.

The OD is nominal.

Nominal OD mean outside diameter of the thread crest. Wood screws has the same number system for gauges.

The pilot hole is located at a size that’s always between the outside diameter of the screw and the root diameter. That will be roughly 80 to 85 percent of the outside diameter.

So there’s plenty of steel left for the thread to bite into, yet enough removed so the screw does not seize or snap. Fastener suppliers publishes these sizes for Type 25, Type B and Type AB screws. Any of the three will fit the same hole size.

For example, different sheet gauges requires a different optimal hole diameter. ANSI has tables to match hole size with gauge of steel for sharp point screws.

For example a #8 takes a 0.125 in hole (1/8 in bit) in sheet up to 0.036 in thick but 0.128 in at 0.048 in; a #10 goes from 0.136 in (#29) in thin sheet to 0.149 in (#25) at 0.048 in. Thicker sheets/stainless: go one size larger. For piercing extruded/punched holes: the hole runs larger then a drilled one because the punched lip adds thread depth.

For duct-gauge thin material (26 to 24 ga), you back off one size; the few engaged threads bite hard. For heavy sheet (16 ga and up), go up one size. Manage the torque and do not shear.

Please note that the above is for self-tapping screws used in unthreaded holes. Bolts or machine screw screwed into predrilled threaded holes require a tap drill chart. In that case the sizes will be different.

For example, coincidentally, a 10-32 tap drill is also a #21; but a 1/4-20 tap drill is not 7/32 in., but a #7. There are even published tables of holes for thread-forming machine-thread screws. These will be different still.

And then there are self-drilling Tek points which don't require any pilot hole whatsoever (up to their rated drilling capacity). This make them dominant in applications like framing and metal roofing.

So here’s a summary level chart on screws. See the dedicated Tabelic sheet metal screw size chart. It shows the gauge-by-gauge dimensions of sheet metal screws, along with head style, length, and pitch of threads.

This one is used if the screw is already in your hand. It is just asking which bit to use.

The provided table is only for wood screws measured by thread diameter. They’re labeled by outer diameter of the thread in millimeters. A 4.0 x 40 is a screw that’s 4.0 mm around its thread and 40 mm long. So that size are listed in the first column.

From there you drill the same way you did from the gauge chart: exactly like it says. Round off to the next bit you have. If it forces you to pick one or the other, round DOWN on soft, clear stock; and round UP if it’s close to an edge or if you’re working in hardwood.

The nearest gauge column ties this table to the imperial chart. Each pair is used by retailers and on packaging: #4 = 3.0 mm; #8 = 4.0 mm. A #14 would be 0.242 inches or 6.1 mm. It’s also available as 6.0 mm and 6.5 mm screws, which is both the same. You just get a larger clearance hole with the 6.5 mm row.

The pilot columns are metric equivalents to the straight-bit sizes above. The clearance hole is the next metric bit above the screw diameter. The hole is about 0.5 mm larger then the diameter, which is enough so the shanks and threads clears easily.

Metric pilots follow two different approachs. For traditional (e.g., rolled-thread) screws like the ones shown in this chart, the pilot is sized at the root. However, according to UK/European trade publications, newer hardened production screws is about 0.5 mm less. These have sharper threads and a thinner core.

If your screws were packaged in a branded box indicating a pilot size, then you go by what’s on the box. If not (they’re plain or slot rolled-thread screws), then refer to this table.

Nowadays, many metric screws has cutting points that drill easily into softwood without a pilot at all. Only around the end of boards do manufacturers recommend pre-drilling and also in hardwood. This is because beech and oak split easy; here again you need to pre-drill.

The same upsizing guidance applies for brass and stainless metric screws. For dense timber, pre-thread with a steel screw from the hardwood column.

Note that this table applies to wood screws, by the diameter of the screw thread. This does not apply to metric machine screws (e.g., M3 or M4), which require a tap drill equal to the thread diameter minus the pitch when tapping a metal hole.

This table is not for metric sheet-metal ST screws (self-tapping) such as DIN 7970; they require their own hole tables.