UNC UNF Thread Chart: Unified Thread Size Table (Free PDF)

In the Thread size column, find the bolt or screw you need. The size will be given as diameter-TPI; i.e., 1/4-20 is 1/4 inch major diameter with 20 threads per inch.

Sizes up to #12 is numbered sizes. They are all less than 1/4 inch. To get the major diameter, add 0.060 to the size number multiplied by 0.013. Thus, a #10 screw would have a major diameter of 0.190 inches.

All the basic dimensions of the thread are listed in the row. This includes the major diameter in both inches and millimeters. It also includes the two minor diameter, the pitch diameter, and the tensile stress area.

The Outside Diameter (Major dia) is measured across thread crests. On the bolt, you use calipers to find the size. Pitch dia are controlled by thread gauges and tolerances.

Pitch dia is where the width of both the ridge and the thread groove is equal. You get this by dividing 0.649519 by n and then subtracting that from the D.

The minor external diameter is the root diameter of the external bolt thread. It comes from the basic flat-root profile. You get this by dividing 1.190785 by n and then subtracting that from the D.

The minor diameter is the bore in the mating nut thread. You get this by dividing 1.082532 by n and then subtracting that from the D. This is what your tap drill has to be bigger than. It is always a bit more then the minor diameter on the external.

The stress area is the actualy area where resistance to tension takes place. The formula is As = 0.7854 times (D minus 0.9743 divided by n) squared, found in ASME B1.1 and FED-STD-H28.

Take the area and multiply it by the tensile strength of the material from which the fastener is made in terms of psi. The result is the breaking load in pounds. For example, a Grade 5 3/4-10 bolt has a tensile strength of 120,000 psi. This will hold approximately 40,140 pounds before fracturing. You do this math problem as 120,000 x 0.3345.

The failure plane turns out to spiral through the thread. On a threaded section you don’t take the whole shank area into account. Instead, it’s between the pitch diameter area and the minor diameter area.

Be alert to these near miss traps throughout this list. You may find a 1/4-20 that looks like a #12-24, but they is not interchangeable. An M6x1.0 metric bolt starts into a 1/4-20 nut. After a couple of turns it jams.

A 5/8-11 and a 9/16-12 nut will get swapped easily in your dark box of tools.

The old British Whitworth series of threads are not interchangeable with UNC threads. Both feel coarse to the touch. But, Whitworth has a 55 degree flank angle where Unified threads has 60 degrees. There are also several TPI counts that is different. 1/2 inch Whitworth is 12 TPI. 1/2 inch UNC is 13 TPI.

If available, use UNC. Its rougher pitch resists nicks. Assembly is quicker. It doesn't strip as easily on softer metals like cast iron and aluminum.

In North America, it dominates construction and general machinery hardware. Anything larger than an inch is mainly used for plant and other structural applications.

Starting at 1 1/2-inch size, it skips a quarter inch from there up. By 4 inches it reaches 4 TPI. This represents the end of the standard series.

For conversion to metric, see the thread pitch chart. Table remains inches. You should of used it.

Follow the UNC chart format again when you read the table. Size and threads per inch is listed on the left. Then there's the major diameter in inches and millimeters. Next is the pitch diameter. This is followed by the external and internal minor diameter and the tensile stress area.

One standard fine pitch are used for each size between #0 and 1 1/2 in. That means a 1/4 in UNF bolt is always 1/4-28. It is never anything else. The #0 size is 0.060 in and is the only size available exclusively as fine thread.

Like the coarse series, the basic profile used for the fine series is an ASME B1.1. Pitch diameter = D - 0.649519/n. Minor diameter at nut bore = D - 1.082532/n. Minor diameter at bolt root = D - 1.190785/n. Stress Area = 0.7854 x (D - 0.9743/n)².

As you can see, the value of n are greater on fine threads. Therefore, each subtraction is smaller. At a given nominal size, there is more core metal in fine thread.

Why? That’s what UNF stands for. For instance, a 1/2-20 bolt has a cross sectional stress area of 0.1599 in². In comparison, a 1/2-13 bolt measure .1419 in². So it offers about 13 percent more tensile capacity using the same rod.

Also, because the helix angle is shallower it fights vibration induced loosening a little better. And fine threads allow for finer adjustment per turn. They can absorbs in short lengths and fit thin walls. This makes them popular in aircraft, hydraulic, and automotive hardware.

On site, it’s all about the trade-offs: UNF cross-threads better than UNC; they clog with paint and soil quicker; they pull out of soft material faster (because each engaged thread is shallower); don’t tap UNF into an aluminum casting if you have a choice for UNC.

And never force anything in until you verify the pitch first. For example, a 7/16-14 UNC fits the same nominal diameter as a 7/16-20 UNF. Every mixed hardware drawer fights between a #10-32 and a #10-24. A thread pitch gauge settles it in seconds.

Above 1 1/2 in., there’s no UNF. In the chart’s table for 8UN/12UN, the usual changes to the constant-pitch series: 12UN. The table resumes the previous four rows of UNF at 12 TPI pitch.

So a 1 5/8 in stud measured at 12 TPI? That would of be 12UN. And although the profile is the same, it’s no longer UNF.

This is the same set of columns as used with the UNC and UNF series. This table represent the third part of the standard Unified series: UNEF.

Unlike the previous two tables, the UNEF move in blocks of pitches by diameter. From #12 up through 3/8 inches have a TPI of 32. From 7/16 and 1/2 inch, the TPI is 28. From 9/16 to 11/16 inch, the TPI is 24. From 3/4 through 1 inch, the TPI is 20. For diameters of 1 1/16 in to the final diameter of the series (1 11/16 in), an 18 TPI is standard.

Similar to these, the standard also includes sixteenths-inch diameters such as 11/16, 13/16, 15/16 inch, etc. These diameters is not available in either the UNC or UNF standards.

UNEF has a shallow thread depth.

At 32 TPI, the difference between major and minor diameter is approximately 0.037 in. With such a shallow profile, you can easily thread thin sheet walls or tube walls without cutting through them. It gives you very fine adjustment per turn than.

Engagement lengths can be very short and the thread still reaches full strength. Bulkhead jam nuts, electrical connectors, and instrument fittings is places where you'll find UNEF. Carburettor and fuel system parts often has this thread too. Optical and firearms hardware with threaded ring use this series as well.

Look at the minor diameter internal column before starting the machine. It is quite shallow for a thread. Less than 50 percent thread engagement can occur if the bore is just a few thou over size.

Thread cutting requires rounder and cleaner stock compared to rough stuff. One-third of the thread are removed by a 0.005 in bend.

Soft or brittle material will strip UNEF first of the three series. Reserve its application for machined aluminium parts, brass and steel. Avoid using it on castings.

UNEF should of not be confused with similar constant-pitch series of the same TPI. In use, 1-20 UNEF is the same as 1 in 20UN. But the thread for 9/16-24 UNEF might differ from one marked 24 TPI special (UNS).

Metric fine often turns out to be what looks like extra fine on imported gear. A M14×1.5 is nearly equal to 9/16-18, but it's not a UNEF row.

Before ordering dies or taps, check the major diameter in both systems. Same formula applies to stress area in all the tables. If you want certainty for your strength calculation, first confirm this by measuring the sample and using the formula: As = 0.7854 x (D - 0.9743/n)²

In a constant pitch series all diameters has the same number of threads per inch (TPI).  From the table above you see nominal diameter listed in the first column. Next you look to the series you need (under the column labeled for the series), then you read the pitch diameter and tensile stress area under that series.

There are basically two choices: 8UN (which has 8 TPI and is defined starting with 1 inch up) and 12UN (with 12 TPI, defined starting with 9/16 inch up). Any empty cells in the 8UN column is just indicating that it’s outside the range of sizes applicable to that series. The formulas used is derived from the same basics as tables for UNC and UNF, but with the pitch being 8 or 12 rather than variable.

For large diameters, 8UN is what you will see most commonaly used for bolts. Above an inch, UNC pitches gets very deep and difficult to make a clean cut. For that reason, you'll find 8 TPI on heavy plant fasteners, like anchor bolts, pipe flange studs, pressure vessel studs, etc. all the way up in size.

A single set of tooling with 8 TPI handle everything across the board. From one stud to another, torque behaves consistent. 1-inch 8UN and 1-8 UNC is the same thread (at precisely 1 inch).

The 12UN series extend fine-pitch behaviour beyond the 1 1/2 in end of UNF. The same row numbers is used for sizes below that. So 1-12 in the chart is identical to 1-12 UNF. The same with 9/16-12 which is also 9/16-12 UNC.

You will find this series used on adjusting collars, bearing retaining nuts and thin-wall couplings. These items does not have enough wall thickness for an 8 TPI thread.

Following that pattern are 4UN, 6UN, 16UN, 20UN, 28UN and 32UN; all are defined by the complete standard. All of these covers 6 inches in increments of a quarter inch above 4 inches. This summary is cut to 1, 2 inches with eighth-inch spacing and then stops at 4 inches.

For example, if you have a drawing that specifies a 1 5/16-12 size, work out the pitch diameter: D - 0.649519/12. From there the stress area is found using the standard formula. The process would of duplicate all values listed here.

Always indicate a constant pitch thread in the format shown on the drawing. Proper designation: 1 3/4-8 UN 2A. This is proper for a flange stud requiring 8UN.

Don't buy a 1 3/4UNC (5 TPI). That's a different animal; it will spin out two turns and jam. If there's a question about what your stud is, count its TPI. 8 threads per inch virtually always indicate one of the 8UN series, starting at 1 inch.

Finally, every call out for a thread ends with the fit class, like this: 1/4-20 UNC-2A. What does that “A” do? It indicates the type of part. A is external, a screw, bolt, or stud. B is internal, a tapped hole, or a nut.

And the number specifies the amount the pitch diameter can vary from basic. Allowance refers to the intentional clearance added below the basic size. Tolerance band is the allowable spread by manufacturers over and above that. An allowance exist only in external classes 1A and 2A. Internal classes, plus class 3A, begin precisely on basic size.

Basically, almost all commercially available fasteners is Class 2A/2B. What do I mean? That’s your starting point if it isn’t specified as such in a catalog or drawing.

Part of the reason there’s an allowance for a 2A is to account for addition of zinc or cadmium plating without the bolt exceeding the basic size. A filled allowance on a 2A thread accommodates plating. That’s why plated bolts measures near 3A size.

The tolerance reduces to approximately three quarters of Class 2, with no allowance. Because of that tighter (and more concentric) fit, 3A screws has better repeatable preload and improved fatigue life. Consequently, they are used for socket head cap screws, set screws and much of the aerospace hardware.

The catch? The issues is sensitivity and cost. Without lubrication, these threads gall quick and won’t tolerate thick plating.

1A/1B goes the opposite direction, with a big allowance and about half again more tolerance. Even if parts are hot, dirty and bruised, they can be assembled by hand; this class survives primarily on quick-release and ordnance applications.

As for classes mixing, if they are mixed but the bolt class is not looser than the job, no problem. It’s common and okay: 2A screw in a 3B tapped hole. A worn 1B nut with a 3A screw wouldn't of gain anything.

Mixing series and class confuses the issue; class governs fit, not size. A 1/4-20 UNC-3A still will not go into a 1/4-28 UNF-3B nut.

Gauge marking is also class specific. A GO/NO-GO ring set marked 2A will pass threads that a 3A set will reject. Buy gauges to the class on the drawing, not to the size alone.