Sheet Metal Screw Size Chart: Gauge and Pilot Hole Table (Free PDF)

The column at left contain a list of screw gauges to match your actual shank diameter in both inches and millimeters. Scroll down that column until you see your measurement.

The last two columns contains the number of threads per inch. Choose one of the last two columns depending on if the thread stamp on the box say it's Type B or AB, or the obsolete Type A. This is the most common spaced thread found on almost all sheet metal screw currently available.

Choose the last column if the box say Type A; this is a obsolete coarse thread. So a #8 sheet metal screw has a shank diameter of 0.164 in (4.17 mm). That same #8 is typically stocked 8-18, which mean Type AB.

American Screw Gauge: This is the Gauge No. It doesn’t tell you anything about the length; each step up (minus the base) add 0.013 in to the total. The nominal outside major diameter is the basic diameter.

The nearest fractional equivalent is the closest fractional equivalent that’s commonly used. This can be helpful when referencing hole charts and drill bit size, but it is approximate and not the actual size. The two TPI columns represents the standard thread pitch for each thread family per ASME B18.6.4.

Mixing machine screw and sheet metal pitches is the most common trap. For instance, a #10-16 sheet metal screw isn’t a #10-24, or a #10-32 machine screw. Even though their gauge diameters matches, they are NEVER interchangeable.

Also keep in mind that although the industry use 1/4-14 for their 1/4 inch fasteners, the actual size of 1/4-14 is essentially identical to 14 x 14. Type A is #14 (0.242 in) and Type AB/B is 1/4 in (0.250 in). Most catalogs just shows them as the same screw. They are 1/4-14 and 14 x 14.

Legacy Type A diameters includes sizes #16, #18, #20 and #24. They stick around mostly due to replacement work and old drawings.

Odd gauges are standardized as #3, #5 and #7, but they is not stocked as widely as the even ones. Standardize on #4, #6, #8, #10, #12 and 1/4 in. Use these if you’re starting from scratch rather than patching old hole.

Use #8 stock for general body panel, appliance and ductwork work. Light trim uses #6 gauge. Where extra pull-out or clamp strength is required, use #10 or #12 gauge.

This is a table of tapping screws for threads formed by the screw. This does not apply to machine, wood, or drywall screws. They has different thread standards even if the label shows the same gauge number.

The metric table below has metric ST sizes.

In the first column, look up your screw size and you’ll see the thread pitch and then actual manufactured thread diameters. You’ll see two numbers. The Major dia max are the biggest allowable diameter over the crest of the threads, measured in both inches and millimeters to compare directly to metric ST screws. Then there is the Minor dia max/min, which are the maximum and minimum core or root diameter that’s allowed.

The minor diameter is important for pilot holes because the hole should of no smaller than the root diameter; otherwise, it can jams and twist right off the screw.

Type AB is a spaced thread screw with a sharp gimlet point that will locates and begin in a drilled or punched hole.

Type B is the same as above but has a blunt point with a short taper. It is suitable where you want full thread engagement all the way to the tip or where a sharp point might wander, such as in plastics, non-ferrous casting, and thick sheet.

Type BP is the same as above, but it have a cone point for aligning misaligned holes or punching fabric.

Types BF and BT is thread cutting versions of the above for brittle plastics.

Also note that because the spaced thread is rolled, a #8 is sized up to .166 in whereas the base gauge is only .164. Don’t write off a screw as to big based off this alone.

Likewise a 1/4 in size has a maximum major diameter of only 0.246 in, or just shy of 1/4 in, meaning a tapping screw 1/4-14 will pass a ring gauge a true 1/4 in bolt would not.

The millimetre column show why metric crossover works so well. The inch threads and ISO 1478 ST threads are dimensionally identical (same series).

The following sizes apply: #8-18 Type AB is ST4.2, #10-16 is ST4.8, and 1/4-14 is ST6.3. Metric and inch pan head sheet metal screws of like size runs in like holes.

Please note that this table do not include Type A screws (the old coarse thread) or Type U hammer drive.

Also excluded are the types listed in machine screws like Types D, F, G, and T since these types follows machine screw pitch. If you find something tapped for 6-32 or 10-24 and it has a tapping point, it's a Type F screw; otherwise, it is a sheet metal screw found on this table.

For those of you who still have to match screws on older products, or even read old drawings that call out the now obsolete Type A gimlet point thread, it’s all right there in a handy table. Find your size, then simply read the pitch and the range of allowable diameters (both major and minor) just like you see them in the Type AB table, above.

The standard dropped Type A decades ago; so if you’re calling out new hardware, stop here and go with Type AB.

From size #6 up, Type A runs visibly coarser different than Type AB. At size #6 it’s 18 versus 20 TPI; #8, 15 versus 18; #10, 12 versus 16. This difference mean an old screw will strip a hole tapped by its moddern replacement (and vice-versa).

Happily, the pitches does match Type AB in sizes #2 to #5. Because the pitches line up here in smaller gauges, small screws tend to interchange far more forgivingly then mid sizes.

In the mid sizes, Type A is also a bit thicker than Type AB. For instance, an #8 Type A might be measured at 0.168 in over the crest compared to 0.166 in on a Type AB. The minor diameter are bigger so the old published pilot holes is a bit larger as well.

In practice, the fix for replacing a stripped Type A screw is often to drive another of that gauge but Type AB into the same hole. If the hole is chewed out, go one gauge up.

There are several large legacy diameters for these: Type A #16 (0.268 in), #18 (0.294 in), #20 (0.320 in) and #24 (0.372 in).

These primarily appear on older agricultural equipment, as well as railcar and HVAC ducting. The closest equivalent today is a 5/16-12 or 3/8-12 Type AB or B. You’ll require slightly different holes, so measure your existing hole first before ordering.

This table is not used for wood screws. Wood screws has the same gauge numbers as old Type A screws, but they have different thread profiles.

However, the threads on wood screws follow a different length standard and are cut to a slightly different profile (ASME B18.6.1). The pilot holes that is published are for wood not for sheet metal.

Drill the diameter of the sheet the thread bites into. Anywhere within the minimum-maximum diameter of hole works. Look at the chart for a standard drill. It is listed as a letter or a number. If neither fits, use the closest fraction above or below the middle of the range. In the case of an #8 sheet metal screw, the No. 29 drill (0.136 in) is right there in the middle, or just under 9/64 in in workshop lingo.

It's not just for show. You want to work toward the minimum on thin, soft sheets like aluminium or 22-26 gauge steel, where a few of the threads will be carrying all the load. You also want to work toward the maximum on thick, hard material to ensure the drive torque do not exceed what is needed to turn the screw off its thread.

Many installers find that with very thin duct work they can drill a size smaller than the listed bit, and rely on the gimlet point to do the rest.

The pilot hole go only in the bottom sheet. The clearance hole in the top sheet must be just a little larger than the major diameter of the screw.

Without a clearance hole, the screw threads into both sheets, engages the flanks of those threads to hold the sheets apart and does not clamp the sheets together. Loose sheet metal joint are most commonly caused by this alone.

A pierced or punched hole is not equivalent to a drilled hole. For a punched hole, because it does a bit of tapering and hardening of the metal at the hole, make the punched hole about 0.004 to 0.012 in larger than a drilled hole. Also, always drive screws into the punched hole in the same direction the hole was punched.

With an extruded or pierced hole, there is added thread engagement due to a raised collar. This additional engagement accounts for the larger pierced hole sizes given in old handbooks for equal sized screws.

All these are based off ordinary ductile sheet such as aluminium, low carbon steel etc, driven with case hardened steel screws.

Should you use stainless screws in stainless sheet? Try at the top. However, galling is a factor that changes things. Work hardening also matter, so manufacturers advise you to trial fit. For castings or plastics, substitute this table with information from your screw manufacturer about the specific material concern.

Identify the type of screw (sheet metal) and look across to read the size. For two of the most common head types, like the Hex Washer Head, the A/F columns identify the wrench size, then the hex head height, followed by washer diameter.

This information include a nominal fraction wrench size and an exact decimal limit value. Hex head height mean the distance from the sheet to the top of the head. Washer diameter equal the outside diameter of the integral washer flange that distributes bearing load. For Pan Heads, these columns list the head height and outside diameters for both Phillips and Slotted type.

Run all of the most common sheet metal screws from #6 up to 1/4 inch. You’ll need 1/4, 5/16 and 3/8 inch nut drivers for this.

For #6, #7 and #8 use a magnetic 1/4 inch driver. For #10 and #12 go with the 5/16 inch, and for 1/4 inch and #14 take out the 3/8 inch driver. There’s also a tiny 3/16 inch driver for the #4 size that looks like a 1/4 inch at first but isn't.

For any slotted holes, cover strips or flanges, use the washer diameter column. If the integral washer does not sit squarely on solid material all the way around, the joint will loosen and dish out.

The washer also establishes the minimum edge distance. Where possible, maintain a hole center of at least one washer diameter away from sheet edge.

Clearance at folds in the edges and behind the panels matter, because of head height. A #10 hex washer head projects .120 in or so from the surface (plus whatever washer height).

By comparison, a pan head (the same size) has a flatter, broader dome shape. That's the distance between them that determines whether your joint catches as you slide a duct over a framing member.

The ASME B18.6.3 standard applies to tapping screw heads and has been merged with the head standard since 2013; it include maximum values. The machine screw heads is sized slightly smaller, so do not reference the size of a counter sink from a machine screw chart.

The ASME B18.6.3 standard also covers tapping screw heads, but flat, oval, and truss heads is sized according to a different standard: ASME B18.6.4. They include flat, oval, and truss heads that feature larger diameter heads than their machine screw counterparts.

These are metric sheet metal screw numbers.

The letters “ST” appear at the beginning of the number to indicate that it’s a Metric sheet metal screw with the following digits representing the nominal outside diameter in millimeters. Thus, ST4.2 mean the screw is 4.2 mm across its diameter on top of the threads.

The limits for minor diameter (d2) and major diameter (d1) along with the size is listed. Use the minor diameter or core diameter (d2) to determine how small your pilot hole should of be. The pitch is how far apart the thread crests are from each other. The dedicated metric pilot hole table below list all the sizes with their finished recommendations.

To help you blend metric and inch stock, use the Former No. column. Each ST size are a replacement for an old inch gauge, which can be found on list of standard sizes (ISO 1478). The ST2.9 replaces a #4 screw, the ST3.5 replaces a #6, and the ST4.2 replaces a #8. The ST4.8 replaces a #10, the ST5.5 replaces a #12, and the ST6.3 replaces a ¼ in screw.

It is the same thread, just different paperwork. A hole drilled to accept a #8-18 Type AB screw accept a pan head ST4.2 screw just fine, and vice-versa.

The flank angle is 60 degrees, and the pitch is really coarse compared to a metric machine thread of equal diameter. For example, a metric M4 machine screw have a pitch of only 0.7 mm; while ST4.2 is 1.4 mm. Never attempt to sub in an M4 for an ST4.2. They look close enough if you only look at the diameters, but they is no match for the threads.

Then there’s the point style that comes from the product standard. There are two basic styles, a cone or pointed one called Type C (inch = Type AB) and a flat blunt one called Type F (inch = Type B). They has a rounded point as well called Type R on some sizes.

The common DIN head standards all ride on this thread, including: DIN 7981 / ISO 7049 for pan head, DIN 7982 / ISO 7050 for countersunk, DIN 7983 / ISO 7051 for raised countersunk and DIN 7976 / ISO 1479 for hex head.

The standard also lists ST1.5 and ST1.9 but they is seldom carried. The most commonly available sizes are ST2.2 through to ST6.3 covering nearly all commercially produced hardware. For structural sheet and heavy duct work use ST8 and ST9.5.

All sizes listed here contain thread information only. These do not apply to chipboard screws or self-drilling screws to DIN 7504. They also do not apply to thread rolling screws that produces metric machine threads like the Taptite type.

The first column (ST) shows your screw size, and second column tell you its inch equivalent. From there, you can find your minimum/maximum range of sizes from the table on the sheet and make a pilot hole inside that zone. Metric twist drills comes in 0.1 mm steps, so you can find a stock drill for every value in that range. The last column translates them into inches if you work with fractional or number bits.

To determine what size drill bit to use, select from this range based off the strength and thickness of your sheet: Select the small end of the range for thin and/or soft sheet materials (e.g., 1 mm mild steel or aluminium, Rm up to roughly 300 N/mm2). Select the large end for thick or high strength sheet (2.5 to 3 mm and up, or Rm approaching 500 N/mm2). This ensures that the screw do not twist off due to excessive drive torque. DIN 7975 specifies these figures in very fine increments, and the ranges given above includes these specs.

The inch pilot table rules apply. Only drill a pilot hole in the nut member; drill the clearance hole in the top sheet. The sheet must also be thicker than about a single turn of the screw's thread. Otherwise, the screw would of not develop its full clamp load beneath the screw head.

Drive the screw from the punched side (and keep the punched hole about 0.1-0.3 mm larger than the pilot hole). Use an extruded hole or change the type of fastener if the sheet is to thin.

For aluminium sheet, the table values for the equivalent sheet thickness are roughly 0.1-0.2mm lower (according to published trade tables), which maintains the choice within the range.

When stainless steel screws is used with stainless steel sheet, the generic values given above cannot be applied directly. Try your joint by trial, beginning towards the upper end of the scale. That is what the supplier guides compiled according to DIN 7975 advise for stainless applications.

These holes is intended only for thread forming ST screws.

Pilot holes are unnecessary in the rated thickness range of self-drilling screws (DIN 7504). There is also separate hole tables for thread rolling screws for machine threads (family DIN 7500). Using this table with any of these screws will cause broken or loose joint.