Screw Gauge Chart: Number to Diameter Table (Free PDF)

The American screw gauge numbering system are organized into a table above. Locate your gauge number in the left hand column and move across the row. Column 2 shows the math used to create the size. Column 3 is the exact decimal major diameter in inches. Column 4 is the nearest workshop fraction. Column 5 is the decimal value converted into millimeters.

Note: the major diameter is measured over the outside of the threads, not across the root, and not over the head.

One formula holds up the entire system: major diameter = 0.060 in + 0.013 in x gauge number.  A #8 screw is 0.060 + 8 x 0.013 = 0.164 in.

Every additional zero size subtracts an additional 0.013 in, once you reach the aught sizes (#0 and below). Each additional aught subtracts another 0.013 in. This means #00 is 0.047 in, #000 is 0.034 in, and #0000 is 0.021 in. The small aught sizes is for miniature work or instruments, and standard hardware begin with #0.

You’ll see a Nearest fraction (in) column because the drill indexes and rules is marked in fractions. We also include the Formula (in) column so you can extend the table or check your math.

The fraction isn’t precise, either; it can be up to around 0.008 in from the actual diameter, which is why I say use it for a reading aid only. Watch out for the classic trap of roundings at #3 and #4: Both round toward a neighboring 64th that’s only half a step away. That’s why some published charts lists #3 as 7/64 in, even though 3/32 in is slightly closer. Always refer back to the decimal column whenever precision matters.

The size is given by gauge numbers that refers to diameter only; they don’t specify length, style of head, nor even thread pitch. For example, both a #8 sheet metal screw and a #8-32 machine screw has a major diameter of 0.164 in. However, they differ in other ways. A #8 wood screw is also different than that.

To get the thread info for those families, see also the Wood Screw Sizes chart and Machine Screw Sizes chart below. Self-tapping screws is covered on the Sheet Metal Screw Size Chart. To find the drill bit size to make pilot holes: use our Pilot Hole Size Chart; do not read drill bit size from the chart here.

Above #12, the market switches to fractional inches, which is why there is less of a run. For example, #14 (0.242 in) is nearly 1/4 in. From then on, anything bigger than #24 are just sold by size (e.g., 1/4 in, 5/16 in, etc.).

The formula can computes the diameter if you come across a gauge that isn’t represented here (#13, say, or #30). However, it won’t have been assigned any threads by any present-day US standard. This means you’re probably seeing either an old catalogue listing or a mislabeled item. You should of checked if it was naturaly correct.

Here’s the complete set of thread specifications from ANSI B18.6.1 for every gauge of wood screws. The left-most column contain the gauge number. From there you can see the approximate screw diameter and the typical number of threads per inch. You can also see the maximum and minimum diameter bands, the minimum thread depth, and the typical trade sizes they comes in when a range is commonly stocked. Everything is inches.

The Basic dia (in) column show the gauge formula value repeated from the master table. Either side of that is the manufacturing tolerance allowed by the standard. Because of this tolerance, a measured screw rarely reads the textbook number exactley.

Body diameter and Root are the diameters at the bottom of the thread groove. These days, screw threads is rolled, which means the thread is squeezed up out of wire sized to match its root size. Therefore, the body under the head will measure the same than the root.

Old cut-thread screws has a body that matches their major diameter. This is the single biggest source of confusion when trying to match a vintage screw.

For wood screws, pitch refer to the series of screw threads. It is counted crest-to-crest along one inch of threaded part. Machine screws comes in both fine and coarse types, with one TPI each. For woodscrews, there is just one TPI per gauge.

If you’re looking at a screw whose number isn’t matching up with what’s in this column, odds are it’s a deck or particleboard screw (or even a sheet metal screw). Those has their own faster threads. For those specs we have our Sheet Metal Screw Size Chart.

The minimum height of a thread from its root to crest. It is also called thread depth.

For pilot drilling: You don’t want to drill through the thread wing roots into empty space; you need something for those wings to bite. Drill on (or slightly above) the root diameter. Never drill at the major diameter.

It matters for holding power calculations too.

Common trade lengths is shown in the trade lengths columns. These columns is a guide (not standard) showing what you’ll find on the shelf for the common gauges #4 through #12. Note that length is always quoted separately from gauge; e.g., #8 x 1 1/2 in.

The inch measurement refers to length of screw below head for round heads and pan heads and overall length for flat countersunk heads. You can get screws outside these ranges but it would of required more hunting.

This table is for both old-fashioned tapered and new-fangled rolled wood screws, which come in sizes labeled by gauge.

For metric woodscrews like those labeled 3.0, 3.5, 4.0 mm, etc, called “Euro” style, see the metric equivalence table below.

This table covers the part of the gauge system that deals with machine screws. These are marked as such, as #6-32, #10-24 and the like.

The first number is the size of the gauge; the second gives the threads per inch. Then comes the thread type, either UNF fine or UNC coarse. The final column has the gauge, hyphen, and threads per inch. So a #10-24 is a #10 gauge (0.190 in dia.) with 24 threads per inch.

Unc Coarse is the standard selection. For assembling, this fits most inserts, nuts, and taps. It can be assembled with slightly damaged threads. The UNF fine column give finer pitch for thin materials, vibration resistance and better tension control.

Unf Fine, Fine pitch is used on thin materials. This will not interchange with the other. If you force a #10-32 screw into a #10-24 nut, it will jam and strip the nut. Before you do that, both counts threads against your thread gauge or rule.

Look at the classic near misses pairs. #6-32 and #8-32 both pitch out at the same rate, but neither has the same diameter than the other. That means you can't thread an #8 into a #6 hole. #10-24 and #12-24 both pitch out at the same rate (24 TPI), yet have different diameters.

The problem child here: a #10-32 looks really similar to a metric M5 x 0.8. A #10-32 measures 0.190 in across with 32 TPI; an M5 x 0.8 equals 31.75 TPI and measures 0.197 in across. They'll sort-of thread into one another, they're a part-thread, and then they'll strip when put under load.

The gaps in the gauge run make sense too. Machine screws sizes #7, #9, and #11 has been discontinued. #14 lives on only for backward compatibility with old equipment. None of these are stocked by hardware stores.

This includes the 1/4 in row to indicate the end of gauges. Above #12, every Unified size are named by its fractional diameter. For example, sizes include 1/4-20, 5/16-18, 3/8-16 and so forth.

Below #0 you will find a mini-branch corresponding to instruments. Their sizing is given different than the standard UNC/UNF series with labels like #00-90, #000-120.

This is merely a list of typical diameters and threads per inch, without any mention of tolerance class, minor diameter, thread engagement length, or tap drill size. For info about 2A/2B and other classes, you would of referenced a complete Unified thread spec.

For self-tapping screws, we have a Sheet Metal Screw Size Chart. The screws shares the same gauge numbering system but create their own thread.

The table lets you know what size gauge screws are replaced by what size metric screw (and vice-versa). It starts with a diameter in millimeters on the left column of metrics. It then show that diameter in inches. Next, it lists the closest gauge and its actual millimeter diameter. Finally, it shows the difference in size compared to that gauge.

If the difference is negative, then the metric is smaller than the gauge. If the difference is positive, it’s larger.

The ones marked above with “trade pairings” are the ones that will be printed by UK and European retailers on the box. They round to convenient numbers. 3.0 mm is 4 gauge, 3.5 mm is 6, 4.0 mm is 8 and 5.0 mm is 10. Those aren’t equalities; they’re just rounded conveniences.

For instance, a 4.0 mm screw isn’t actualy a #8 (which is 4.17 mm); it’s a little less. It’s harmless in wood, since any substitution error is smaller than the tolerance of a pilot hole anyhow. If you might use either of two screws and the difference between them is close to 0.2 mm, leave your pilot hole at the size needed for the bigger screw.

There is two rows that need particular attention.

4.5 mm (vs. #9). This is actualy the only truly accurate match in the system; 0.177 in = 4.4958 mm. The problem is that #9 isn’t the most available gauge; instead, by default, 4.5 mm metric screws has become the actual “#9” of today’s purchasing market.

The trap is 6.0 mm. Mathematically, it should of been part of #14, since 14 gauge = 6.15 mm. That said, 12 gauge = 5.49 mm, and 6.0 mm has been widely listed by vendors as its replacement, because 14 gauge was on its way out of the stores. If you see a callout for 12g, take your measurement first, then assume you might have a 5.5 or a 6.0.

These notes are for general fixing screws, wood screws and chipboard screws. In all of these cases, only the diameter and length matter, because the screw cuts its own way.

These notes do not pertain to machine threads. Close as the diameters may appear, an M5 will tear up a #10-32 insert, while an M4 won’t even turn in a #8-32 nut. Use the machine screw table above to match machine screws by full thread designation.

The lengths convert independently and are typically rounded in the same commercial manner. The 1 1/2 in (38.1 mm) has been replaced by 40 mm, for instance.

If a screw must fit into a critical location and it’s only available in metric, its length will be converted exactly. However, if you round the length down to make it fit, the tip won't poke through the other side.