Thread Size Chart: Metric, Pipe & Legacy Thread Dimensions Table (Free PDF)

In the Size column, locate the thread that you want to measure and then move along that row for each of basic diameters of the ISO metric coarse thread. Use these reference numbers to draw a metric thread, identify one, or check its size.

All metric thread tolerances is based off these theoretical sizes, taken from ISO 724. When you measure a male thread with calipers, it should reads as (or just below) the major diameter; if it is well below, it’s likely either worn, rolled, or you’re measuring the next smaller size.

The crest-to-crest distance of one turn: Pitch P

The diameter of a thread that has half the pitch of the thread as its width between groove (the diameter the three-wire measurement and thread micrometer measurements reference): Pitch dia d2 = D2

The nominal outside diameter equal to that of both the nut (D) and the bolt (d): Major dia d = D

The minor diameter D1 is calculated as d minus 1.082532 x P, representing the basic root diameter and the bore of the nut thread.

The design profile external thread have a smaller root diameter. Use d3 instead of D1 for stress calculations and to find the cross-section of bolt cores. The formula is: Minor dia d3 = d minus 1.226869 x P.

It’s a common mistake to read D1 as tap drill size. The standard tap drill is D minus P (e.g., 8.5 mm for M10 x 1.5). It will yield about 75 percent thread engagement, which is enough but not excessive. This size is actualy just barely bigger than D1 (8.376 mm).

Drilling to D1 is 100 percent thread; that doesn’t buy you much in the way of strength and will snap your tap.

A second pitfall involve relying on a caliper readout on an old bolt. Rolled threads runs below nominal. Even crest wear can throw things off. Check the pitch as well before calling out a size.

Some sizes are called second or third choice diameters. Here's a list of them in ISO 261: M1.1, M1.8, M2.2, M3.5, M4.5, M7, M9, M11, M22, M27, M33, M39, M45, M52 and M60.

Those are completely standard and show up on this table. However, stockists only carries first choice sizes (M1, M1.2, M1.6, M2, M2.5, M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30, M36, M42, M48, M56, M64). This means to order from stock, you should of design for those first choice sizes, unless you are fitting into an existing piece of hardware.

This covers dimensions only. Thread pitch chart for conversions from TPI to fine and coarse pitch. The thread engagement chart shows the length of thread engagement into a tapped hole or nut. The UNC UNF thread chart is for unified threads, which have the same 60-degree angle as above but do not have matching diameters different than the two charts.

For example, both a course and fine thread have the same large diameter, but finer thread has different geometry beneath the crest. Because the smaller pitch remove less material, the same nominal size result in a larger pitch diameter and both minor diameters.

The Size x pitch column show where to locate your designation. Because the M12 reference alone mean the coarse 1.75 mm pitch by default, you will see M12 x 1.25 as the designation for a fine thread. It is never written without the pitch.

As with the coarser table, the columns has the same significance: d2, diameter measured, i.e., d less 0.649519 × P; D1, basic bore through nut hole, i.e., d less 1.082532 × P; d3, outer diameter of the root, i.e., d less 1.226869 × P (this is diameter used for calculating cross-section of the core).

Why is the fine bolt of equal size slightly stronger in the shank? This is because d3 is larger. The tap drill to make fine nuts will be correspondingly larger to. The 75 percent rule d minus P = 10.8 mm for M12 x 1.25. This is greater than value of 10.2 mm that would of apply to a coarse M12.

The trap with fine threads is the near miss.

M10 has four pitches (0.75, 1.0, 1.25, and 1.5 mm), which has identical outer diameters. You need a pitch gauge. Why? Force-fit an M10 x 1.25 nut to an M10 x 1.5 stud; it’s gone, stripped in half-turn.

Typical offenders for this problem involve automotive fasteners. Commonly, brake fittings is M12 x 1.25 or M12 x 1.5, while wheel studs are also often M12 x 1.25 or M12 x 1.5. M14 x 1.25 is the classic spark plug thread. None of those accept coarse hardware.

Fine pitch make a difference during fine adjustment or when thin walls and vibration loosen the threads.

Coarse pitch is used anywhere else. Coarse taps bite better into soft metals. They tolerate dirt or plating. They also assemble quicker. Fine strips easily in aluminium. Engagement length is increased if they're used in aluminium.

See chart for engagement length required. Thread pitch chart includes the whole list of available fine pitches per size and choice. The table show the resulting diameters.

Nope. Not a single one. British Standard Pipe sizes is called that because it is the size of the hole in the steel pipe they were cut on. Hence a G 1/2 measure almost 21 mm across.

To recognize a pipe thread, measure its OD. Determine the number of TPI. Locate the row where the major dia is the same as yours. The rest of it wont assist at all.

P stands for pitch. P = 25.4 / TPI

The thread depth in mm are h = 0.640327 x P. On a Whitworth thread with a rounded root and crest of 55 degrees, this is the radial depth.

Pitch Dia = Major Dia minus h. Minor Dia = Major Dia minus 2h.

For internal G threads it's all the same basic diameters. Here they is from the ISO 228-1 specification for basic dimensions to which the tolerance classes A and B is applied.

The trick is that there is a G/R thing going on. G is parallel, doesn't seal onto the thread. Requires an O-ring or bonded washer under the shoulder of the fitting.

R, however, is external taper (EN 10226, ISO 7-1), which seals on the thread itself (with tape or sealant). So it's Rc (internal taper) or Rp (internal parallel) (both is defined in that same ISO standard).

So an R male can begin in a G female, but only a taper-in-taper or taper-in-Rp pair are a pressure joint.

This used to be called BSPP (parallel), BSPT (taper) on old drawings. Both is covered by the diameters in this table, for R threads at the gauge plane.

BSP isn’t interchangeable with NPT. Although they’re nominally half-inch pipe threads (G 1/2), a BSP thread is 55 degrees and parallel while NPT is 60 degrees on a taper. This means that the two won’t mate very well; they’ll just tighten down, weep and ultimatey split a fitting.

If the threaded item was removed from North American equipment and the thread looks obvious tapered, then refer to the table of NPT threads below.

The standard also includes sizes G 5/8, G 7/8 and the intermediate fractions above G 1, which is all listed here. But basically there’s only common fractional sizes, 1/8, 1/4, 3/8, 1/2, 3/4, 1 and up, available as stock fittings.

ISO 228-1 covers all sizes from 1/16 to 6 for thread size. No bigger than that you have a G thread.

Further down on this page is the BSW and BSF table which is also 55 degrees and are fastener threads. They share the form but not the dimension, just like the pipe series and the fastener series.

North American Taper Pipe Thread (NPT): A NPT threaded pipe will have a taper that you can actualy see, meaning the thread tapers continuously at a rate of 1:16 from end to end. So for each foot of length, the diameter change 3/4 inches.

Since the size varies on the thread, a single outside diameter measurement doesn't mean much unless you know exactly where on the thread you measured it. Also check the TPI (threads per inch) and the 60-degree angle. Then cross-reference that with the outside diameter column. This is the number listed for your pipe, which refer to the area of the pipe behind the thread before anything has been touch to it.

The pitch dia E0 at the extreme end of the pipe are the smallest point of the external thread.

The next pitch dia is E1 at the gauge plane. That’s where you can take a L1 ring gauge and just screw it on by hand. And the E1 measure is exactly L1/16 larger then E0.

Hand tight L1 is how far you turn the mating fitting on by hand, usually you turn it another three turns with a wrench to finish connecting it.

Effective thread L2 is the length of the usable thread. Those threads beyond that aren't completed and don't seal.

Thread interference create the seal. As the taper advances, the flanks bite into one another. Any leaks are along the crest-root clearance where there is still a bit of a spiral. These are filled with some kind of sealant or PTFE tape.

If you want to seal a joint without sealant, then you’ll want an NPTF (also called Dryseal) joint. Following its own ASME B1.20.3 standard, the dimensions on this table are all identical between NPT and NPTF, but NPTF tightens the crest and root truncation so the threads crushes together. NPT and NPTF will interchange mechanically; however, a mixture requires sealant to seal. Either will be different than NPS.

NPS is the parallel type which are used on locknuts and mechanical joints.

One error you'll find people making is that they think that BSP threads can be used interchangeably with NPT. 14 TPI is found on both 1/2 BSP (G or R) and 1/2 NPT. Even though they both have 14 TPI, the angle of engagement is slightly different: NPT is 60 degrees while BSP is 55 degrees. And the diameter is off by a couple tenths of a mm which is just large enough for the threads to engage each other but still leak.

If you look at the smaller sizes, 1/4 and 3/8 are easier as these have an 18 TPI which doesn't occur in any BSP size. On the larger sizes, NPT sizes matches true outside-diameter sizes. So 14 OD pipe is actualy 14.000 inches. Above that, the pipe is welded together more often instead of being threaded.

The thread size is all this table shows.

Schedules and wall thickness comes from the type of pipe, not the thread.

For taper threads, tightness comes from turns, not torque. Plumbers meet at hand tight plus two to three wrench turns.

Pre-metric Britain's BSW & BSF are the threads of their fasteners. Today you encounter them every day on old machines, imperial-era plant, motorcycles and even vintage car. The only difference between the two is pitch. Both use a rounded root and crest and a 55 degree form.

The real major diameter in inches is the fractional size. Next locate either core dia or TPI (and then the other) depending on which series you is using. If there is no entry it indicates that size was not standardized by BS 84 for that series.

The nominal outside diameter.

This appears on the big diameter columns in both units. Each core dia are the external thread root diameter. When you ID a worn bolt or want to make an estimate for strength, this is the number to use.

For thread micrometer work, you’ll need the next number, the effective diameter, which equals major diameter minus 0.640327/TPI. We don’t list that to keep the two series side by side.

The traps are the unified near misses. 1/4-20 UNC equals 1/4 BSW 20 TPI but so does 3/8 BSW which equals 3/8-16 UNC. They'll screw into each other but they have 55 degree threads in a 60 degree nut that carry load on the crest corners of the thread. So it'll strip far more lower than its rated strength.

Many British made during and post WW2 purposely used UNC in Whitworth sized parts. Best you should of assume nothing and check all your fasteners for size with a thread gauge.

Anything BSF is never equal to anything UNF. At any given size, the TPI differ; for example, a 1/4 in size has 26 TPI versus 28 TPI.

The other trap is the spanner marking. A wrench stamped 1/2 W fits the head of a 1/2 in Whitworth bolt. The hexagon is much more than 1/2 in across the flats.

Before you buy any spanners, look at a spanner size chart because they do not correspond to the number on this chart for Whitworth ones. Also, below 1/8 in BSW was not used in practice, but instead the BA series covered them. The next table follows.

Decades ago both BSW and BSF becomes outdated. Now BS 84 has been withdrawn. You can only order or roll replacements from specialist stock.

See the BSP table above for pipes with those same 55 degree threads. The other replacement threads is the American ones, which have superseded them on newer machines. See the UNC UNF thread chart.

BA screws are miniature screws used in past British model engineering, instrument and electrical work. They is the sole metric-at-heart legacy series.

The 0 BA size has a 6.0 mm diameter and a 1.0 mm pitch. Each higher number scales the pitch by 0.9. A fixed formula apply to the diameter. So, a larger BA number represent a smaller screw. Look it up in this table or reverse engineer from the major diameter measurement you have.

The BS 93 standard is defined by two values, pitch and major diameter. The inch column is included as a convenience for those of us who measure with imperial tools. TPI is approximate. Because BA pitches are metric, we don’t get to whole TPI numbers.

Major minus 0.6 x pitch. This value is for gauge work and is the pitch diameter. major minus 1.2 x pitch. This is the root of the screw and here’s where you should of check if you want an estimate on how strong a stripped 4 BA stud might be.

It have a unique thread form. It is 47.5 degrees and has deeply rounded roots and crests. This is a shallower form different than either Whitworth or metric.

At the metric boundary is where problems arise. 0 BA screws into M6 x 1.0 far enough to trick you. 47.5 degree flanks that only bear in the corners, making the joint weak. 2 BA (4.7 mm x 0.81) passes for M5 x 0.8 by feel. 5 BA sits close to M3.5. On mixed-era British gear check the thread with a BA pitch gauge; it isn't always as you'd expect. Never tap a metric in an old hole unless you've checked.

The go-to range was even numbered sizes. These are 0, 2, 4, 6, and 8 BA, which covers most of what you're likely to encounter. Odd numbers aren't common. BA sizes above 10 are used in watchmaking and shown here for completion's sake.

Metric replaced the BA system in the 1960s but it's still used today in model engineering as well as for restoring old instrument and radios. Even sizes are still produced for taps and dies.

Stick with the metric coarse table for moddern small screws. The screw gauge chart is not here, it contains a machine screw gauge comparison for wood and self-tapping screws.

This is for those occasions when you have lost your label from a fitting or bolt. A simple way of working out what it is if you know 2 sizes.

Take a reading from the Thread Gauge. If it is imperial, get the number of threads per inch (TPI), then take another reading using the caliper on the outside diameter across the crest. That gives you a row in this table which will give you the most likely pair that can be mixed up. If it's not in there, try searching back through the complete tables above.

Instead of giving you just one number for the measured major diameter, we gives it in a range. That’s because actual male threads is frequently less than that base. For example, cut threads can be a few hundredths less, whereas rolled and plated threads can be up to a couple of tenths smaller.

When you’re down to two candidates with the same diameter and pitch, the flank angle is what decides it. A 60-degree flank angle indicates metric, unified and NPT threads. A 55-degree flank angle indicates British pipe and Whitworth threads. And a 47.5-degree flank angle indicates BA threads.

The formal way is to measure it with an optical comparator. But the practical way, the way most tradespeople do it, is by testing it on a known-good fitting or nut.

Most identification mistakes are made due to two bad habits: Don’t assume the size of a thread is based solely on diameter. A 1 millimeter range encompasses M8, 5/16 inch and 0 BA threads.

And don’t wait until late in the process to determine if your thread is for a fastener or for a pipe. Outside diameters of pipe threads will never equal those of a fastener. The most common ones is 9.7, 13.2, 16.7 and 21 mm. Also, their TPI can be off-the-wall like 27 or 19. Finally, if there’s a taper, it’s always a pipe thread; either R or NPT.

Use the UNC/UNF Thread Chart for all your unified inch threads. The chart gives you a list of all the diameters in inches with the corresponding TPIs for that diameter. From there you can look up the thread pitch chart which details all the available pitches (fine or coarse) per diameter.

You’ll then be able to separate the metric from the imperial threads. Now go back to the series table above. It will tell you the exact working dimensions.