If you are connecting a stainless steel fitting to a 3/8 inch pipe and the package states that the thread size is 3/8 NPT, but the measurement you obtain shows a thread width of 0.675 inches instead of the expected 0.375 inches, or, if you have already drilled a hole for the 3/8 NPT port but you are not sure in which drill to use — in the case of the confusion we are discussing which is quite common in this field, the meaning of NPT size is “nominal” and the calculation of the tap drill size is not correct and may lead to a leaky connection.
You will find here the size of the 3/8 NPT thread, its dimensions, how to tell it from BSP, and how to achieve a perfect connection.
The quick answer is that 3/8” NPT (National Pipe Taper, ASME B1.20.1) is a type of tapered pipe thread that has very important details about its sizes. Specifically, these sizes are: major (external) diameter of 0.675 inches (17.15 mm), minor (root) diameter of 0.6145 inches (15.61 mm), pitch diameter of 0.6389 – 0.6444 inches (16.2 – 16.4 mm), 18 threads per inch (TPI), pitch of 1.411 mm, taper of 1:16 (3/4 inch per foot) and thread angle of 60 degrees. The recommended drill size is 37/64 inches (0.5781”, 14.68 mm) for ~75% engagement, in addition to normal turning being approximately 4 hand-tight turns + 2 wrench turns.
Table of Contents
- 3/8 NPT Dimensions: The Exact Numbers
- Why "3/8" Isn't 0.375 Inches
- What Size Hole for a 3/8 NPT Tap?
- How to Identify a 3/8 NPT Thread
- 3/8 FIP vs 3/8 NPT: Same Thread, Different Gender
- 3/8 BSP vs 3/8 NPT: Why They're NOT the Same
- Standard NPT Thread Sizes Reference
- How to Make a Proper 3/8 NPT Connection
- Where 3/8 NPT Is Used
- Frequently Asked Questions
- References
- Conclusion
3/8 NPT Dimensions: The Exact Numbers
Here are the official dimensions of the thread 3/8 NPT as outlined in ASME B1.20.1:
| Dimension | Inches | Millimeters |
|---|---|---|
| Nominal size | 3/8" | — |
| Major diameter (thread OD at gauge plane) | 0.6750" | 17.15 mm |
| Minor diameter (root / inside) | 0.6145" | 15.61 mm |
| Pitch diameter | 0.6389"–0.6444" | 16.23–16.37 mm |
| Threads per inch (TPI) | 18 | — |
| Pitch | 0.0556" | 1.411 mm |
| Taper | 1:16 (3/4" per foot) | — |
| Thread angle | 60° | — |
| Tap drill (75% thread) | 37/64" (0.5781") | 14.68 mm |
| Hand-tight engagement | ~4 turns | — |
| Wrench-tight additional | ~2 turns | — |
The diameter of the thread is tapering, that is, it is not the same along its length (1:16) which allows to create a wedge seal when a male and female threads are tightened together.
Why "3/8" Isn't 0.375 Inches
It is one of those questions that raise eyebrows. The "3/8" in "3/8 NPT" denotes a nominal size which used to be based on the old IPS (Iron Pipe Size) inside-diameter system which indicates the approximate inside diameter of the pipe within the historical schedule system and not the outside diameter of the thread itself. However, the actual measurement of the 3/8 NPT thread’s outer diameter equals 0.675 inches (17 mm approximately).
The principle is true for all NPT sizes; for example, 1/4 NPT has the outer diameter of 0.540", 1/2 NPT has the outer diameter of 0.840", while 3/4 NPT has the outer diameter of 1.050", etc. The nominal size works as just labeling while the actual dimensions follow after measuring with calipers: 3/8 and 1/2 NPT fittings measure around 0.675" OD and around 0.840" OD respectively. If you're sizing a whole system of threaded connections, our guide to brass vs stainless steel pipe fittings also covers how material choice interacts with thread sizing.
What Size Hole for a 3/8 NPT Tap?
The suggested drill bit dimension for any 3/8 NPT tap is 37/64 inches (0.5781 inches, 14.68 mm). This results in about 75% thread engagement, which is common practice in the industry. When working with metric measurements, one can use a 14.5-14.7 mm drill (14.68 mm is the precise recommendation; 14.5 mm is the diameter of drill bit to use when tighter threads or softer guidance is needed).
Important NPT tapping notes:
- Since NPT taps have a taper, the hole should be prepared using a taper reamer (1:16) to obtain maximum full threads — for most applications, a straight holed tap is satisfactory but for applications where high pressure is applied, a tap with a starter tapered hole is preferred.
- For soft materials such as aluminum or brass drill slightly oversized and for hard materials such as stainless steel drill at best a little undersized — but do not go significantly beyond recommended because of loss of threads and leak (drilling a 0.375 NPT hole being 19 mm instead of 14.68 mm will lead to loss of approximately 1.5 threads and leakage).
- Basic rule for NPT threading is: “Turn forward one turn and turn back half of the turn” to enable the chips to break and cutting fluid to enter into the gap.
How to Identify a 3/8 NPT Thread
Confirmation of 3/8 NPT fittings can be accomplished using the following steps:
- Measure the diameter of the male and female fitting: The male is 0.675 inches (17 mm) in diameter and has a similar diameter for the female opening.
- Count the number of threads per inch(18) to determine whether the fitting is 3.8 inch NPT. The male fitting has 12 threads while the female fitting has 6 in the same one inch.
- Check the taper angle. The taper of the NPT fitting will have a visibile taper toward the end and it will be at an angle of 16 degrees.
- Check the angle of the threads using a thread gauge. NPT has a 60 degrees angle where as that of BSP is 55 degrees.
- Ensure there are any markings like “NPT” or “3/8 NPT” or the fitting has a standard marking.
If you lack a gauge, then simply check if the NPT male fits into NPT female fitting easily by using your hands. If it gets stuck after one turn or does not start then you are more than likely working with BSP
3/8 FIP vs 3/8 NPT: Same Thread, Different Gender
The term "3/8 FIP" (Female Iron Pipe) refers to 3/8 female NPT, which is just a label used by the plumbing industry to refer to even the female version of the NPT thread standard (sometimes referred to as FPT or FNPT threads). This means that:
- A 3/8 FIP fitting will have 3/8 inch female NPT threads that are internally designed to take in a male NPT pipe or nipple measuring 3/8 inch.
- A 3/8 MIP (male NPT) fitting will screw directly onto the 3/8 FIP fitting because both fittings of this type have threads of the same gender as they refer to the same NPT standard.
In simpler terms, you can say that 3/8 FIP can connect to a 3/8 NPT because both of them actually refer to parts of the same connection. You can rely on using a thread sealant like Teflon tape or pipe dope while working with male threads!
3/8 BSP vs 3/8 NPT: Why They're NOT the Same
It is the greatest compatibility trap among pipe fittings — 3/8 BSP and 3/8 NPT fittings look quite similar but are incompatible.
| Feature | 3/8 NPT | 3/8 BSP |
|---|---|---|
| Threads per inch | 18 TPI | 19 TPI |
| Thread angle | 60° | 55° |
| Thread geometry | Tapered (1:16) | Parallel (G) or tapered (R) |
| OD (male) | 0.675" | ~0.656" |
| Seal method | Thread interference + sealant | Often washer/seal (parallel G), or taper seal (R) |
| Interchangeable with NPT? | — | No |
Due to the difference of TPI and angle, a 3/8 BSP and 3/8 NPT can engage in a couple of threads using brute force — after which they will cross thread, leak, or wear out. There should be no mixing of these standards. Always check to be sure of the standard to be used before making any connection.If you're new to how fittings connect into complete systems, our overview of what a sanitary fitting is explains how the hygienic side of the fitting family is specified when threads aren't appropriate.
Standard NPT Thread Sizes Reference
| Nominal Size | OD (inches) | TPI | Tap Drill (inches) |
|---|---|---|---|
| 1/8 NPT | 0.405 | 27 | 0.332 (Letter Q) |
| 1/4 NPT | 0.540 | 18 | 0.4375 (7/16) |
| 3/8 NPT | 0.675 | 18 | 0.5781 (37/64) |
| 1/2 NPT | 0.840 | 14 | 0.7031 (23/32) |
| 3/4 NPT | 1.050 | 14 | 0.9063 (29/32) |
| 1 NPT | 1.315 | 11.5 | 1.1563 (1-5/32) |
| 1-1/2 NPT | 1.900 | 11.5 | 1.7344 (1-47/64) |
| 2 NPT | 2.375 | 11.5 | 2.2188 (2-7/32) |
The pattern: NPT thread OD is always larger than the nominal size, and larger sizes have coarser (fewer) TPI.
How to Make a Proper 3/8 NPT Connection
- Make both threads clean — remove dirt, burrs, and old sealant.
- Apply the thread sealant to MALE threads: wind PTFE tape for 3–5 turns in tightening direction, alternatively apply pipe dope; if the service is for the high-pressure system use both.
- Start manually for 2–4 turns — tapered NPT can be screwed manually that way; if something goes wrong suddenly stop and check possible cross-threading and BSP mismatch.
- Finish tightening the connection with the wrench, twisting around ~2 more turns than hand tight (this is standard for NPT).
- Don’t apply excessive torque: tapered threads can crack brass or stainless steel, do not overtightened.
Where 3/8 NPT Is Used
- Pneumatic systems - air cylinders, valves, compressers, filters/regulators
- Fuel and oil pipelines
- Hydraulic power units and manifolds (low-pressure side)
- Water supply connections Plumbing devices
- Instrumentalization - pressure gauges and transducers (gauge ports)
- Compressed air accessories and dryers
- Transporter transmission cooler lines.
3/8 NPT's balance between the flow capacity and compact size makes it widely used in numerous mid-sized connections.
Frequently Asked Questions
Is 3/8 BSP and NPT the same?
No, these thread patterns differ and thus cannot be interchanged or used. 3/8 NPT has 18 TPI and a 60-degree angle with a conical shape, whereas 3/8 BSP has 19 TPI and a 55-degree angle with parallel or other types of threads. Their threads may connect temporarily for two rotations, but they do not fit properly, which can result in leaks or breakage. You must check your threads before making a connection!
Is 3/8 FIP the same as 3/8 NPT?
Yep, the concept of 3/8 of an inch FIP (female iron pipe) is basically the same as 3/8 of an inch NPT, which is the male counterpart. The term FIP is just the name used in the plumbing industry when referring to female fittings. Essentially, a 3/8 of an inch FIP component has 3/8 NPT female threads that can connect to a 3/8 NPT male (MIP) component using tape or pipe does for proper sealing. In essence, both the 3/8 of an inch FIP product and its NPT male counterpart are essentially the same product.
What size hole for 3/8 NPT tap?
According to the recommendations, the tap drill size for 3/8 NPT should be 37/64 inches or 0.5781 inches (14.68 mm), which amounts to a 75% connection rate in thread engagement, which is a general standard in the industry. When working in the metric system, one should use a drill of 14.5 mm to 14.7 mm (14.68 mm is direct measurement) as tap drill for 3/8 NPT. As NPT is a tapered connection type, and in order to make the best threads, one should perform a reaming operation using a taper reamer of 1:16 ratio before tapping and applying the ‘advancing one full turn and backing off half a turn’ technique if tapping by hand. The hole should not be excessively oversized since it will lose its thread engagement, which will lead to leaks in the joint from the pressurized side.
References
- ASME B1.20.1 — Pipe Threads, General Purpose (Inch): NPT standard
- PipeSizing — 3/8 NPT thread dimensions and drill sizes
- ThreadSpec — NPT thread chart (OD, TPI, pitch diameter, tap drills)
- Onerovalve — NPT thread size chart and tap drill guide
- Gewinde-Normen — NPT thread data (ANSI B1.20.1)
- NFPA — threaded fitting requirements for gas and fire safety
Conclusion
The term 3/8 NPT refers to a kind of pipe thread measuring 0.675 inches in diameter on the outside, built on the basis of 18 threads per inch with a taper of about 1:16 and the included angle of approximately 60 degrees. Significance of this may be understood in terms of the old designation of the pipe in the American IPS classification. For the original thread fittings identification, it is essential to know part of such important values as OD 0.675,” tap drill 37/64” or 4 hand-tight turns and 2 wrench turns needed for proper joint forming.
The first rule is 3/8 BSP and 3/8 NPT flow specifications are wrong, which means what will work for one type will not pass the other. The second important rule is that all 3/8 FIP fittings correspond to the regular 3/8 NPT fittings. When making your measurements, always use calipers for accurate results checking before applying NPT fittings on one side and sanitary piping on the other, if these belong to food-processing areas.
