Thread types: The practical engineer's guide
Learn how to correctly design threads for manufacturing in order to save time and costs.
Read articleLearn how to correctly design threads for manufacturing in order to save time and costs.
At university we learn how to design parts to optimize design efficiency - less material, higher performance. In the real world the details, such as threads, can cause you to waste time and lose money.
There is little practical literature targeting threads in the context of design for manufacturing. Most articles you will find talk about the actual machining of the parts and are catered towards machinists. This article seeks to be a practical guide for engineers designing physical products.
Threads can be either internal or external and typically fit together. They are called out (or specified) in a technical drawing by defining the following parameters.
There are two types of holes where you can define threads: blind holes and thru holes.
There are various different thread types available, the most common are metric and unified.
The following threads are the simplest type of threads and are understood by both local and global suppliers. The dimensions should be placed on the view that describes the feature most clearly.
As a standard, you can define metric threads by simply putting the type and size (e.g. M10). The size, as mentioned above, is defined by the nominal diameter (major diameter) of the thread. The pitch and depth that follow will follow the common metric common standards (see table below).
All metric threads are assumed to be “thru” on thru holes unless indicated otherwise. Depths for blind holes must always be indicated.
Class fit - metric threads
Class fit defines the tolerance of the threads, and is also sometimes referred to as "allowance" or "thread fit". Internal thread tolerances are always written with capital letters, and external threads are lowercase. The standard thread tolerances are 6H for internal threads and 6g for internal threads, which is what your supplier will default to if you don’t call any different tolerances in your drawing. If you’re interested, below is a deep dive into why this is.
The class fits are defined typically from ISO 965-1, based on the thread engagement length. The thread engagement length is important for you because it denotes the length of your assembly, which will be useful for your load bearing calculations, as friction changes with this thread engagement.
The default thread engagement length considered is N, which denotes a typical screw/screw hole connection. Therefore, the class fits default to “medium” which is 6H for (internal threads) and 6g (for external threads), as highlighted below.
The tolerance position determines the type of tolerance to be used. The main ones are g and h.
Why are these call outs so different?
With metric threads, you can get away with a very simple call out, like in the second example, where you only indicate the thread series (M) and size (10). In this case, the rest will default to standard. Remember that for blind holes, you also must indicate the depth of the thread, as there is no standard to fall back to (see the table below).
Thread series | Thread size (nominal diameter) | Pitch | Class fit | Depth | Modification |
---|---|---|---|---|---|
M | 10 | 1.5 | 6H | THRU | LH - left hand thread |
M | 10 | Not indicated - default to standard pitch for this thread size (table) | Not defined - default to 6H | Not indicated so default to THRU for thru holes. For blind holes, this indication would be incomplete | Not indicated - default to RH (right hand) |
Drill size = Hole size required
Size | Drill size | Drill size, decimal size equivalent | Thread type |
---|---|---|---|
0-80 | 3/64" | 0.0469" | UNF |
1-64 | 53 ga. | 0.0595" | UNC |
1-72 | 53 ga. | 0.0595" | UNF |
2-56 | 51 ga. | 0.067" | UNC |
2-64 | 50 ga. | 0.07" | UNF |
3-48 | 47 ga. | 0.0785" | UNC |
3-56 | 46 ga. | 0.081" | UNF |
4-40 | 43 ga. | 0.089" | UNC |
4-48 | 42 ga. | 0.0935" | UNF |
5-40 | 38 ga. | 0.1015" | UNC |
5-44 | 37 ga. | 0.104" | UNF |
6-32 | 36 ga. | 0.1065" | UNC |
6-40 | 33 ga. | 0.113" | UNF |
8-32 | 29 ga. | 0.136" | UNC |
8-36 | 29 ga. | 0.136" | UNF |
10-24 | 25 ga. | 0.1495" | UNC |
10-32 | 21 ga. | 0.159" | UNF |
12-24 | 16 ga. | 0.177" | UNC |
12-28 | 15 ga. | 0.18" | UNF |
1/4"-20 | 7 ga. | 0.201" | UNC |
1/4"-28 | 3 ga. | 0.213" | UNF |
1/4"-32 | 5.6 mm | 0.2204" | UNEF |
5/16"-18 | F | 0.257" | UNC |
5/16"-24 | I | 0.272" | UNF |
5/16"-32 | 9/32" | 0.2813" | UNEF |
3/8"-16 | 5/16" | 0.3125" | UNC |
3/8"-24 | Q | 0.332" | UNF |
3/8"-32 | 11/32" | 0.3438" | UNEF |
7/16"-14 | U | 0.368" | UNC |
7/16"-20 | 25/64" | 0.3906" | UNF |
7/16"-28 | 13/32" | 0.4063" | UNEF |
1/2"-13 | 27/64" | 0.4219" | UNC |
1/2"-20 | 29/64" | 0.4531" | UNF |
1/2"-28 | 15/32" | 0.4688" | UNEF |
9/16"-12 | 31/64" | 0.4844" | UNC |
9/16"-18 | 33/64" | 0.5156" | UNF |
9/16"-24 | 33/64" | 0.5156" | UNEF |
5/8"-11 | 17/32" | 0.5313" | UNC |
5/8"-18 | 37/64" | 0.5781" | UNF |
5/8"-24 | 37/64" | 0.5781" | UNEF |
11/16"-24 | 16.5 mm | 0.6496" | UNEF |
3/4"-10 | 21/32" | 0.6563" | UNC |
3/4"-16 | 11/16" | 0.6875" | UNF |
3/4"-20 | 45/64" | 0.7031" | UNEF |
7/8"-9 | 49/64" | 0.7656" | UNC |
7/8"-14 | 13/16" | 0.8125" | UNF |
7/8"-20 | 53/64" | 0.8281" | UNEF |
15/16"-20 | 57/64" | 0.8906" | UNEF |
1"-8 | 7/8" | 0.875" | UNC |
1"-12 | 59/64" | 0.9219" | UNF |
1"-20 | 61/64" | 0.9531" | UNEF |
1 1/8"-7 | 63/64" | 0.9844" | UNC |
1 1/8"-12 | 1 1/32" | 1.0313" | UNF |
1 1/4"-7 | 1 3/32" | 1.0938" | UNC |
1 1/4"-12 | 1 11/64" | 1.1719" | UNF |
1 1/4"-18 | 1 3/16" | 1.1875" | UNEF |
1 3/8"-6 | 1 7/32" | 1.2188" | UNC |
1 3/8"-12 | 1 19/64" | 1.2969" | UNF |
1 1/2"-6 | 1 11/32" | 1.3438" | UNC |
1 1/2"-12 | 1 27/64" | 1.4219" | UNF |
Size | Pitch, mm | Drill size | Thread type Coarse/Fine |
---|---|---|---|
M1.6 | 0.35 | 1.25 mm | C |
M1.8 | 0.35 | 1.45 mm | C |
M2 | 0.4 | 1.6 mm | C |
M2.2 | 0.45 | 1.75 mm | C |
M2.5 | 0.45 | 2.05 mm | C |
M3 | 0.5 | 2.5 mm | C |
M3.5 | 0.6 | 2.9 mm | C |
M4 | 0.7 | 3.3 mm | C |
M4 | 0.75 | 3.2 mm | F |
M4.5 | 0.75 | 3.7 mm | C |
M5 | 0.5 | 4.5 mm | F |
M5 | 0.8 | 4.2 mm | C |
M6 | 0.5 | 5.5 mm | F |
M6 | 0.75 | 5.2 mm | F |
M6 | 1 | 5 mm | C |
M6.3 | 1 | 5.3 mm | F |
M7 | 1 | 6 mm | C |
M8 | 0.5 | 7.5 mm | F |
M8 | 0.75 | 7.2 mm | F |
M8 | 1 | 6.9 mm | F |
M8 | 1.25 | 6.7 mm | C |
M9 | 1 | 8 mm | F |
M9 | 1.25 | 7.7 mm | C |
M10 | 1 | 8.9 mm | F |
M10 | 1.25 | 8.7 mm | F |
M10 | 1.5 | 8.5 mm | C |
M11 | 1 | 10 mm | F |
M11 | 1.5 | 9.5 mm | C |
M12 | 1 | 11 mm | F |
M12 | 1.25 | 10.8 mm | F |
M12 | 1.5 | 10.5 mm | F |
M12 | 1.75 | 10.2 mm | C |
M14 | 1 | 13 mm | F |
M14 | 1.25 | 12.8 mm | F |
M14 | 1.5 | 12.5 mm | F |
M14 | 2 | 12 mm | C |
M16 | 1 | 15 mm | F |
M16 | 1.5 | 14.5 mm | F |
M16 | 2 | 14 mm | C |
M18 | 1 | 17 mm | F |
M18 | 1.5 | 16.5 mm | F |
M18 | 2.5 | 15.5 mm | C |
M20 | 1 | 19 mm | F |
M20 | 1.5 | 47/64" | F |
M20 | 2.5 | 17.5 mm | C |
M22 | 1.5 | 20 mm | F |
M22 | 2.5 | 25/32" | F |
M24 | 1.5 | 57/64" | F |
M24 | 2 | 22 mm | F |
M24 | 3 | 21 mm | C |
M26 | 1.5 | 24.5 mm | F |
M27 | 1.5 | 1" | F |
M27 | 2 | 25 mm | F |
M27 | 3 | 24 mm | C |
M30 | 1.5 | 1 1/8" | F |
M30 | 2 | 28 mm | F |
M30 | 3.5 | 1 1/32" | C |
M33 | 2 | 31 mm | F |
M33 | 3.5 | 1 11/64" | C |
M35 | 1.5 | 33.5 mm | F |
M36 | 1.5 | 34.5 mm | F |
M36 | 2 | 34 mm | F |
M36 | 4 | 32 mm | C |
Size | Drill size | Drill size, decimal size equivalent | Thread type |
---|---|---|---|
00-90 | 62 ga. | 0.038" | UNS |
6-48 | 2.9 mm | 0.1142" | UNS |
8-24 | 31 ga. | 0.12" | UNS |
8-40 | 28 ga. | 0.1405" | UNS |
10-28 | 23 ga. | 0.154" | UNS |
10-36 | 20 ga. | 0.161" | UNS |
10-40 | 20 ga. | 0.161" | UNS |
10-48 | 18 ga. | 0.1695" | UNS |
10-56 | 18 ga. | 0.1695" | UNS |
12-36 | 12 ga. | 0.189" | UNS |
3/16"-24 | 9/64" | 0.147" | UNS |
3/16"-100 | 16 ga. | 0.177" | UNS |
7/32"-32 | 13 ga. | 0.185" | UNS |
1/4"-24 | 4 ga. | 0.209" | UNS |
1/4"-27 | 3 ga. | 0.213" | UNS |
1/4"-36 | 2 ga. | 0.221" | UNS |
1/4"-40 | 2 ga. | 0.221" | UNS |
1/4"-48 | 1 ga. | 0.228" | UNS |
1/4"-56 | 1 ga. | 0.228" | UNS |
1/4"-80 | B | 0.238" | UNS |
9/32"-32 | 1/4" | 0.25" | UNS |
5/16"-20 | G | 0.261" | UNS |
5/16"-27 | 6.9 mm | 0.2716" | UNS |
5/16"-28 | J | 0.277" | UNS |
5/16"-40 | 7.2 mm | 0.2834" | UNS |
11/32"-32 | 5/16" | 0.3125" | UNS |
3/8"-18 | 8.1 mm | 0.3188" | UNS |
3/8"-20 | 8.2 mm | 0.3228" | UNS |
3/8"-27 | R | 0.339" | UNS |
3/8"-28 | R | 0.339" | UNS |
3/8"-40 | 8.9 mm | 0.3503" | UNS |
7/16"-16 | 9.5 mm | 0.374" | UNS |
7/16"-18 | 9.6 mm | 0.3779" | UNS |
7/16"-24 | 9.9 mm | 0.3897" | UNS |
7/16"-27 | 10.2 mm | 0.4015" | UNS |
7/16"-32 | 13/32" | 0.4063" | UNS |
7/16"-40 | 10.5 mm | 0.4134" | UNS |
15/32"-32 | 7/16" | 0.4375" | UNS |
1/2"-12 | 10.5 mm | 0.4134" | UNS |
1/2"-16 | 7/16" | 0.4375" | UNS |
1/2"-18 | 11.2 mm | 0.4409" | UNS |
1/2"-24 | 11.5 mm | 0.4528" | UNS |
1/2"-27 | 11.5 mm | 0.4528" | UNS |
1/2"-32 | 15/32" | 0.4688" | UNS |
1/2"-40 | 12 mm | 0.4724" | UNS |
9/16"-16 | 1/2" | 0.5" | UNS |
9/16"-20 | 33/64" | 0.5156" | UNS |
9/16"-27 | 33/64" | 0.5156" | UNS |
9/16"-32 | 17/32" | 0.5313" | UNS |
5/8"-12 | 35/64" | 0.5469" | UNS |
5/8"-16 | 9/16" | 0.5625" | UNS |
5/8"-20 | 37/64" | 0.5781" | UNS |
5/8"-27 | 37/64" | 0.5781" | UNS |
5/8"-28 | 37/64" | 0.5781" | UNS |
5/8"-32 | 19/32" | 0.5938" | UNS |
11/16"-16 | 5/8" | 0.625" | UNS |
11/16"-18 | 16 mm | 0.6299" | UNS |
11/16"-20 | 41/64" | 0.6406" | UNS |
11/16"-32 | 21/32" | 0.6563" | UNS |
3/4"-12 | 16.5 mm | 0.6496" | UNS |
3/4"-18 | 45/64" | 0.7031" | UNS |
3/4"-24 | 45/64" | 0.7031" | UNS |
3/4"-27 | 23/32" | 0.7188" | UNS |
3/4"-32 | 23/32" | 0.7188" | UNS |
13/16"-12 | 47/64" | 0.7344" | UNS |
13/16"-16 | 3/4" | 0.75" | UNS |
13/16"-18 | 19 mm | 0.748" | UNS |
7/8"-12 | 51/64" | 0.7969" | UNS |
7/8"-16 | 13/16" | 0.8125" | UNS |
7/8"-18 | 53/64" | 0.8281" | UNS |
7/8"-27 | 21 mm | 0.8268" | UNS |
7/8"-32 | 27/32" | 0.8438" | UNS |
15/16"-16 | 7/8" | 0.875" | UNS |
1"-14 | 23.5 mm | 0.9252" | UNS |
1"-16 | 15/16" | 0.9375" | UNS |
1"-18 | 61/64" | 0.9531" | UNS |
1"-24 | 61/64" | 0.9531" | UNS |
1"-27 | 24.5 mm | 0.9646" | UNS |
1"-32 | 31/32" | 0.9688" | UNS |
Header | Header | Header | Header |
---|---|---|---|
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Cell | Cell | Cell | Cell |
Pipe size | Thread type | TPI | Drill size | Drill size, decimal size equivalent |
---|---|---|---|---|
1/16 | NPT | 27 | C | 0.242" |
1/8 | NPT | 27 | Q | 0.332" |
1/4 | NPT | 18 | 7/16" | 0.4375" |
3/8 | NPT | 18 | 9/16" | 0.5625" |
1/2 | NPT | 14 | 45/64" | 0.7031" |
3/4 | NPT | 14 | 29/32" | 0.9063" |
1 | NPT | 11 1/2 | 1 9/64" | 1.1406" |
1 1/4 | NPT | 11 1/2 | 1 31/64" | 1.4844" |
1 1/2 | NPT | 11 1/2 | 1 23/32" | 1.7188" |
2 | NPT | 11 1/2 | 2 3/16" | 2.1875" |
1/8 | NPS | 27 | T | 0.358" |
1/4 | NPS | 18 | 15/32" | 0.4688" |
3/8 | NPS | 18 | 19/32" | 0.5938" |
1/2 | NPS | 14 | 19 mm | 0.748" |
3/4 | NPS | 14 | 61/64" | 0.9531" |
1 | NPS | 11 1/2 | 1 13/64" | 1.2031" |
Pipe size | Thread type | TPI | Drill size | Drill size, decimal size equivalent |
---|---|---|---|---|
1/8 | BSPP | 28 | 8.8 mm | 0.3464" |
1/4 | BSPP | 19 | 11.8 mm | 0.4645" |
3/8 | BSPP | 19 | 15 mm | 0.5905" |
1/2 | BSPP | 14 | 19 mm | 0.748" |
3/4 | BSPP | 14 | 24 mm | 0.9449" |
1 | BSPP | 11 | 30 mm | 1.1811" |
1 1/4 | BSPP | 11 | 39.5 mm | 1.5551" |
1 1/2 | BSPP | 11 | 45 mm | 1.7716" |
2 | BSPP | 11 | 57 mm | 2.244" |
1/8 | BSPT | 28 | 8.4 mm | 0.3307" |
1/4 | BSPT | 19 | 11.2 mm | 0.4409" |
3/8 | BSPT | 19 | 14.5 mm | 0.5709" |
1/2 | BSPT | 14 | 18 mm | 0.7087" |
3/4 | BSPT | 14 | 22 mm | 0.8661" |
1 | BSPT | 11 | 30 mm | 1.1811" |
1 1/4 | BSPT | 11 | 38.5 mm | 1.5157" |
1 1/2 | BSPT | 11 | 44.5 mm | 1.7519" |
2 | BSPT | 11 | 56 mm | 2.2047" |
Pipe size | Thread type | Drill size | Drill size, decimal size equivalent |
---|---|---|---|
PG-7 | PG | 11.2 mm | 0.4409" |
PG-9 | PG | 35/64" | 0.5468" |
PG-11 | PG | 17.25 mm | 0.6791" |
PG-13.5 | PG | 3/4" | 0.75" |
PG-16 | PG | 21.25 mm | 0.8366" |
PG-21 | PG | 1 1/16" | 1.0625" |
PG-29 | PG | 1 13/32" | 1.4063" |
PG-36 | PG | 1 51/64" | 1.7969" |
Drill size = Hole size required
Class fit - Unified threads (UN)
Remember that class fit defines the tolerance range of thread dimensions, this is always coupled up with an indication of whether the thread is external (B) or internal (A). There are 6 types of class fit for unified threads:
Examples:
These examples are very different - how do they relate to the formula?
Unified threads have various different ways of being called out, you are better off picking one way and sticking to it, but if ever you need to look at another engineers drawings here’s a list of common ways to call them out broken out by section.
Nominal diameter (major diameter/thread size) | TPI | Thread series & type | Class fit & external/internal designation (optional) | Modification (optional) |
---|---|---|---|---|
.250 | 20 | UNC | 2A | MOD |
1/4" | 20 | UNC | Not defined - default to 2A | No modification |
#4 | 20 | UNC | Not defined - default to 2A | No modification |
.375 | 24 | UNEF | 2B | No modification |
10 | 32 | UNF | 2A | No modification |
0.190 | 32 | UNF | 2A | No modification |
If there’s one thing that will without fail save you time and money when it comes to manufacturing is to keep things common.
You don’t learn this when you’re getting your degree, and it's only through trial and error that seasoned engineers realise the importance of keeping components as common as possible - even when designing custom products. If there’s an off the shelf solution, don’t over complicate it. This also applies to threads, always seek to choose common series of threads as much as possible and common sizes.
By common I mean globally understood. We know that there are threads such as NPT, BSP, etc. that are also common locally but overseas suppliers tend to have issues with these unless they're specialized. I suggest to always make design choices that maximize the amount of manufacturers eligible to supply them. Why? Because having a diversified supply mitigates risks and gives you more buying power.
One of the most common mistakes we’ve seen when we make parts containing threads (we’ve manufactured more than 6 million parts at the time of writing) is that the thread sizes do not seem to match the hole sizes they are called out in.
This is a problem because it leads to confusion on whether it’s the right thread size and the diameter of the hole is wrong or vice-versa. As manufacturers, we cannot make assumptions, and this confusion leads to a delay in manufacturing time (1-2 days at least).
Another reason for delay is incomplete thread specifications.
Blind holes are holes that do not go through the full wall thickness of the part. If you call out a thread on a blind hole, you must always indicate the depth of the thread.
![]() | About the author Agata is a Product Manager at Hubs responsible for the manufacturing service offering. Having worked with multiple customers and suppliers, Agata has an excellent understanding of the common hiccups in custom manufacturing. She holds an Aerospace Engineering Bachelors and a Masters in Innovation Management. |