Pipe Thread Size Charts in MM/Inches (Dimensions, Types, Standards, Calculation)

2025.4.15

Among the many types of threads, those used to connect and join piping systems – such as water supply lines and pipe fittings – are known as pipe threads. However, the types and naming conventions of pipe threads can be somewhat complex, often leading to confusion. Using pipe threads from different standards can result in leaks, malfunctions, or even damage. To ensure safe and proper connections, it’s important to thoroughly understand and verify the types, standards, and names of the pipe threads being used. 

So here we will cover the basics of pipe threads, the main standards used globally, detailed size charts in both millimeters and inches, and how to measure and calculate thread dimensions accurately.

Pipe Thread Size Charts in MM/Inches (Dimensions, Types, Standards, Calculation)

What is a Pipe Thread?

A pipe thread is a screw-type connection used to join pipes and fittings. These threads can form a mechanical connection or provide a seal that prevents leaks of air, water, oil, gas, and other fluids.

Pipe Thread Types

Pipe threads fall into two main categories:

Tapered Pipe Threads

Tapered threads are commonly used in piping connections where leak prevention is critical. The thread diameter decreases along the axis (tapered), and as the threads are tightened, surface contact increases, improving the seal and airtightness.

Applications: High-pressure systems like plumbing, gas, and hydraulic systems.

Common Examples:

Parallel Pipe Threads

Parallel threads are used for mechanical joints, especially in high-pressure applications. The diameter remains constant, and the threads run parallel to the axis. To prevent leakage, they are typically used with O-rings or gaskets.

Applications: Low-pressure systems, machinery, and industrial systems.

Common Examples:



Pipe Thread Size Chart (Standards & Dimensions)

Pipe threads are governed by several regional and international standards:

What Are Pipe Thread Standards?

Pipe thread standards are formalized systems used to define the shape, size, and function of threads used on pipes and fittings. They ensure that threaded parts fit together properly and can reliably create a mechanical connection and/or pressure-tight seal.

Each standard varies in terms of:

These standards ensure compatibility across fittings, valves, and piping systems. However, each standard has its own dimensions and tolerances, meaning threads from different standards may look similar but are not interchangeable. 

So, below we will list some major internationally used pipe thread standards and the commonly used pipe thread size tables in these standards. Each specification includes both inch and metric (mm) dimensions, along with thread characteristics like thread angle (typically 55° or 60°), taper (tapered or parallel), and sealing method (interference fit or O-ring/gasket).

1. NPT (National Pipe Thread Standard)

NPT Pipe Thread Size Chart (Inch & MM dimensions)

Nominal Pipe SizeThreads per Inch (TPI)Pitch (in)Pitch (mm)Pitch Dia at External Thread Start E0 (in)E0 (mm)Hand Tight Engagement Length L1 (in)L1 (mm)Max Thread Engagement L2 (in)L2 (mm)E1 (in)E1 (mm)E2 (in)E2 (mm)
1⁄16″270.037040.9410.271186.8870.1604.0640.26116.6330.281187.1420.287507.302
1⁄8″270.037040.9410.363519.2330.16154.1020.26396.7050.373609.4900.380009.652
1⁄4″180.055561.4110.4773912.1270.22785.7860.401810.2070.4916312.4900.5020012.753
3⁄8″180.055561.4110.6120115.5440.2406.0960.407810.3630.6270115.9280.6375016.192
1⁄2″140.071431.8140.7584319.2600.3208.1280.533713.5580.7784319.7770.7917720.119
3⁄4″140.071431.8140.9676824.5780.3398.6100.545713.8670.9888725.1181.019725.891
1″11.50.086962.2091.2136330.8260.40010.1600.680217.2771.2386331.4641.265032.131
1 1⁄4″11.50.086962.2091.5571339.5480.42010.6680.706817.9571.5833840.2191.601340.675
1 1⁄2″11.50.086962.2091.7960945.6170.42010.6680.723518.3701.8223446.2801.841346.768
2″11.50.086962.2092.2690257.6630.43611.0740.756519.2172.2962758.3182.316358.844
2 1⁄2″80.125003.1752.7195369.0730.68217.3231.125028.5752.7621670.1702.790670.858
3″80.125003.1753.3406284.8560.76619.4561.200030.4803.3885086.0433.415686.782
3 1⁄2″80.125003.1753.8375097.5100.82120.8541.250031.7503.8888198.7663.915699.484
4″80.125003.1754.38741111.4480.83721.2521.312533.3384.43272112.5894.4561113.178
5″80.125003.1755.39073136.9210.93723.8001.406335.7215.44929138.4155.4746139.054
6″80.125003.1756.44609163.7320.95824.3331.562539.6886.50957165.3426.5406166.127
8″80.125003.1758.43359214.2101.06327.0001.718843.6648.50003215.9008.5406216.927
10″80.125003.17510.54531267.8471.21030.7341.906348.40610.62094269.77110.6540270.596
12″80.125003.17512.53281318.3331.36034.5442.125053.97512.61781320.49412.6540321.392
14″80.125003.17513.77500350.8851.56239.6752.250057.15013.87262352.37513.9156353.476
16″80.125003.17515.76250400.3681.81246.0252.500063.50015.87575403.22915.9156404.256
18″80.125003.17517.75000450.0502.00050.8002.750069.85017.87575454.04217.9156455.069
20″80.125003.17519.73750501.9382.12553.9752.875073.02519.87031504.71219.9056505.601
24″80.125003.17523.71250602.3002.37560.3253.250082.55023.86094605.07023.9156607.464

2. NPTF (National Pipe Taper Fuel)

NPTF Pipe Thread Size Chart (Inch & MM Dimensions)

Nominal Pipe SizeThreads per Inch (TPI)Pitch (in)Pitch (mm)Major Diameter (in)Major Diameter (mm)Minor Diameter (in)Minor Diameter (mm)Pitch Diameter (in)Pitch Diameter (mm)
1⁄16″270.03700.9410.31257.9380.26396.7050.28827.318
1⁄8″270.03700.9410.405010.2870.35248.9540.37879.620
1⁄4″180.05561.4110.540013.7160.476112.0950.508112.906
3⁄8″180.05561.4110.675017.1450.608115.4470.641916.308
1⁄2″140.07141.8140.840021.3360.742218.8570.790920.090
3⁄4″140.07141.8141.050026.6700.951424.1671.000725.410
1″11.50.086962.2091.315033.4011.200130.4821.257431.939
1 1⁄4″11.50.086962.2091.660042.1641.537339.0551.598640.622
1 1⁄2″11.50.086962.2091.900048.2601.772445.0271.835246.626
2″11.50.086962.2092.375060.3252.231356.6832.303158.504
2 1⁄2″80.12503.1752.875073.0252.695568.4562.785270.741
3″80.12503.1753.500088.9003.316084.2263.408086.558
3 1⁄2″80.12503.1754.0000101.6003.814596.8983.907399.249
4″80.12503.1754.5000114.3004.3100109.4744.4061111.887

3. BSP (British Standard Pipe)

BSP Pipe Thread Size Chart (Inch & MM Dimensions)

BSP Thread SizeThreads per Inch (TPI)Pitch (mm)Major Diameter (in)Major Diameter (mm)Minor Diameter (in)Minor Diameter (mm)Pitch Diameter (in)Pitch Diameter (mm)
1/16″ BSP280.9070.26206.6550.23505.9690.24856.312
1/8″ BSP280.9070.38309.7280.33708.5660.36009.147
1/4″ BSP191.3370.518013.1570.451011.4450.484012.301
3/8″ BSP191.3370.656016.6620.589014.9500.622015.806
1/2″ BSP141.8140.825020.9550.733018.6310.779019.793
5/8″ BSP141.8140.902022.9110.811020.5870.857021.749
3/4″ BSP141.8141.041026.4410.949024.1170.995025.279
1″ BSP112.3091.309033.2491.191030.2911.250031.770
1 1/8″ BSP112.3091.491037.8971.376034.9391.433036.418
1 1/4″ BSP112.3091.650041.9101.533038.9521.591040.431
1 1/2″ BSP112.3091.882047.8031.765044.8451.823046.324
2″ BSP112.3092.347059.6142.230056.6562.288058.135
2 1/4″ BSP112.3092.588065.7102.471062.7522.529064.231
2 1/2″ BSP112.3092.960075.1842.843072.2262.901073.705
3″ BSP112.3093.460087.8843.343084.9263.401086.405
3 1/2″ BSP112.3093.927099.7703.810096.8123.868098.291
4″ BSP112.3094.3920111.5514.2750108.5934.3330110.072

4. ISO Pipe Threads

G (ISO 228) Pipe Thread Size Chart (MM & Inch Dimensions)

G Thread SizeThreads per Inch (TPI)Pitch (mm)Major Diameter (mm)Major Diameter (in)Pitch Diameter (mm)Pitch Diameter (in)Minor Diameter (mm)Minor Diameter (in)
G 1/8″280.9079.7280.38309.1470.36028.5660.3373
G 1/4″191.33713.1570.518312.3010.484611.4450.4509
G 3/8″191.33716.6620.656115.8060.622414.9500.5886
G 1/2″141.81420.9550.825319.7930.779618.6310.7339
G 5/8″141.81422.9110.902521.7490.856920.5870.8112
G 3/4″141.81426.4411.041025.2790.995324.1170.9496
G 1″112.30933.2491.309231.7501.250030.2911.1909
G 1 1/8″112.30937.8971.491936.4181.433034.9391.3740
G 1 1/4″112.30941.9101.650840.4311.591838.9521.5327
G 1 1/2″112.30947.8031.881246.3241.822144.8451.7630
G 2″112.30959.6142.347458.1352.288356.6562.2292
G 2 1/4″112.30965.7102.588064.2312.528362.7522.4693
G 2 1/2″112.30975.1842.959673.7052.900672.2262.8415
G 3″112.30987.8843.460486.4053.401384.9263.3422
G 3 1/2″112.30999.7703.927298.2913.868196.8123.8091
G 4″112.309111.5514.3925110.0724.3335108.5934.2744

5. Metric Pipe Threads (ISO 261 / ISO 724)

Metric Pipe Thread Size Chart (Inch & MM Dimensions)

Metric Thread SizeThread Pitch (mm)Major Diameter (mm)Major Diameter (in)Pitch Diameter (mm)Pitch Diameter (in)Minor Diameter (mm)Minor Diameter (in)
M5 × 0.80.85.000.19694.350.17133.870.1524
M6 × 1.01.06.000.23625.350.21064.770.1878
M8 × 1.01.08.000.31507.350.28946.770.2665
M10 × 1.01.010.000.39379.350.36818.770.3453
M10 × 1.251.2510.000.39378.750.34457.990.3146
M12 × 1.51.512.000.472410.350.40759.190.3620
M14 × 1.51.514.000.551212.350.486011.190.4406
M16 × 1.51.516.000.629914.350.564613.190.5192
M18 × 1.51.518.000.708716.350.643315.190.5979
M20 × 1.51.520.000.787418.350.722017.190.6766
M22 × 1.51.522.000.866120.350.800719.190.7553
M24 × 2.02.024.000.944921.930.863620.160.7937
M27 × 2.02.027.001.063024.930.981523.160.9116
M30 × 2.02.030.001.181127.931.099326.161.0295
M33 × 2.02.033.001.299230.931.217029.161.1472
M36 × 3.03.036.001.417332.831.292530.731.2091
M39 × 3.03.039.001.535435.831.410233.731.3278
M42 × 3.03.042.001.653538.831.527936.731.4465
M45 × 3.03.045.001.771741.831.645739.731.5652

6. API Pipe Threads (American Petroleum Institute)

API Pipe Thread Size Chart (MM & Inch Dimensions) – EUE (External Upset End) / NUE (Non-Upset End) / LTC (Long Thread Casing)

Size (in)Threads per Inch (TPI)Taper (in/ft)Outer diameter of pipe (in)Outer diameter of pipe (mm)Inner diameter of pipe (in)Inner diameter of pipe (mm)Thread Length (mm)Make-up Length (mm)
1.050″11.50.751.31533.400.86521.9744.544.5
1.315″11.50.751.66042.161.12228.5044.544.5
1.660″11.50.751.90048.261.37034.8047.647.6
1.900″11.50.752.37560.331.61040.8950.850.8
2.063″11.50.752.37560.331.75044.4550.850.8
2.375″80.752.87573.031.99550.6758.758.7
2.875″80.753.50088.902.44162.0063.563.5
3.500″80.754.000101.62.99276.0063.563.5
4.000″80.754.500114.33.47688.3063.563.5
4.500″80.755.000127.03.958100.5563.563.5
5.000″80.755.500139.74.408111.9666.766.7
5.500″80.756.050153.74.892124.2866.766.7

7. JIS Threads (Japanese Industrial Standard)

JIS Pipe Thread Size Chart (MM & Inch Dimensions)

JIS threads are closely related to BSP threads (British Standard Pipe), often used in Japanese hydraulic, pneumatic, and plumbing systems.

JIS B0202 Parallel Pipe Thread Dimensions (Similar to BSPP / ISO 228)

JIS Thread SizeTPIPitch (mm)Major Diameter (mm)Major Diameter (in)Minor Diameter (mm)Minor Diameter (in)
PT 1/16280.9077.7230.3046.5610.258
PT 1/8280.9079.7280.3838.5660.337
PT 1/4191.33713.1570.51811.4450.451
PT 3/8191.33716.6620.65614.9500.589
PT 1/2141.81420.9550.82518.6310.733
PT 5/8141.81422.9110.90220.5870.811
PT 3/4141.81426.4411.04124.1170.949
PT 1112.30933.2491.30930.2911.191
PT 1 1/4112.30941.9101.65038.9521.533
PT 1 1/2112.30947.8031.88244.8451.765
PT 2112.30959.6142.34756.6562.230
PT 2 1/2112.30975.1842.96072.2262.843
PT 3112.30987.8843.46084.9263.343
PT 4112.309113.0304.449110.0724.333

JIS B0203 Tapered Pipe Thread Dimensions (Similar to BSPT / ISO 7-1)

JIS Tapered ThreadTPITaper (1:X)Major Dia (mm)Major Dia (in)Thread Length (mm)Common Use
R 1/8281:169.7280.383~10Pneumatics
R 1/4191:1613.1570.518~13Plumbing
R 3/8191:1616.6620.656~14Hydraulic
R 1/2141:1620.9550.825~19General Fluid
R 3/4141:1626.4411.041~20Steam
R 1111:1633.2491.309~22Industrial
R 1 1/4111:1641.9101.650~23Water Lines
R 1 1/2111:1647.8031.882~24HVAC
R 2111:1659.6142.347~25Large Fluid Pipes

8. SAE Straight Pipe Thread (Society of Automotive Engineers)

SAE Straight Thread Size Chart (MM & Inch Dimensions) – ORB / O-Ring Boss

SAE Dash SizeThread Size (UN/UNF)Threads per Inch (TPI)Major Diameter (in)Major Diameter (mm)Pitch (mm)Common Name
-25/16-24 UNF240.31257.941.0585/16″ ORB
-33/8-24 UNF240.37509.531.0583/8″ ORB
-47/16-20 UNF200.437511.111.2707/16″ ORB
-51/2-20 UNF200.500012.701.2701/2″ ORB
-69/16-18 UNF180.562514.291.4119/16″ ORB
-83/4-16 UNF160.750019.051.5883/4″ ORB
-107/8-14 UNF140.875022.221.8147/8″ ORB
-121 1/16-12 UN121.062526.992.1171-1/16″ ORB
-141 3/16-12 UN121.187530.162.1171-3/16″ ORB
-161 5/16-12 UN121.312533.342.1171-5/16″ ORB
-201 5/8-12 UN121.625041.282.1171-5/8″ ORB
-241 7/8-12 UN121.875047.632.1171-7/8″ ORB
-322 1/2-12 UN122.500063.502.1172-1/2″ ORB

9. DIN 2999 (German Standard)

DIN Pipe Thread Size Chart (MM & Inch Dimensions)

DIN 2999 / DIN ISO 7-1 – Tapered Pipe Threads (Similar to BSPT)

DIN SizeNominal SizeThreads per Inch (TPI)Pitch (mm)Major Diameter (mm)Major Diameter (in)Thread Type
R 1/81/8″280.9079.7280.383Tapered (R)
R 1/41/4″191.33713.1570.518Tapered (R)
R 3/83/8″191.33716.6620.656Tapered (R)
R 1/21/2″141.81420.9550.825Tapered (R)
R 3/43/4″141.81426.4411.041Tapered (R)
R 11″112.30933.2491.309Tapered (R)
R 1 1/41-1/4″112.30941.9101.650Tapered (R)
R 1 1/21-1/2″112.30947.8031.882Tapered (R)
R 22″112.30959.6142.347Tapered (R)
R 2 1/22-1/2″112.30975.1842.960Tapered (R)
R 33″112.30987.8843.460Tapered (R)
R 44″112.309113.0304.449Tapered (R)

DIN ISO 228 / DIN 259 – Parallel Pipe Threads (Similar to BSPP)

DIN SizeNominal SizeThreads per Inch (TPI)Pitch (mm)Major Diameter (mm)Major Diameter (in)Thread Type
G 1/81/8″280.9079.7280.383Parallel (G)
G 1/41/4″191.33713.1570.518Parallel (G)
G 3/83/8″191.33716.6620.656Parallel (G)
G 1/21/2″141.81420.9550.825Parallel (G)
G 3/43/4″141.81426.4411.041Parallel (G)
G 11″112.30933.2491.309Parallel (G)
G 1 1/41-1/4″112.30941.9101.650Parallel (G)
G 1 1/21-1/2″112.30947.8031.882Parallel (G)
G 22″112.30959.6142.347Parallel (G)
G 2 1/22-1/2″112.30975.1842.960Parallel (G)
G 33″112.30987.8843.460Parallel (G)
G 44″112.309113.0304.449Parallel (G)

10. R Threads (ISO 7-1)

R Pipe Thread Size Chart (MM & Inch Dimensions) – Tapered External Thread – ISO 7-1

Thread SizeThreads per Inch (TPI)Pitch (mm)Major Diameter (mm)Major Diameter (in)Effective Thread Length (mm)Tapered Thread Type
R 1/16280.9077.7230.3044.5Tapered (R)
R 1/8280.9079.7280.3836.5Tapered (R)
R 1/4191.33713.1570.5188.5Tapered (R)
R 3/8191.33716.6620.6569.0Tapered (R)
R 1/2141.81420.9550.82510.5Tapered (R)
R 5/8141.81422.9110.90211.0Tapered (R)
R 3/4141.81426.4411.04111.5Tapered (R)
R 1112.30933.2491.30913.0Tapered (R)
R 1 1/4112.30941.9101.65014.0Tapered (R)
R 1 1/2112.30947.8031.88214.0Tapered (R)
R 2112.30959.6142.34716.0Tapered (R)
R 2 1/2112.30975.1842.96017.5Tapered (R)
R 3112.30987.8843.46019.0Tapered (R)
R 4112.309113.0304.44921.0Tapered (R)

Differences in Pipe Thread Standards & Dimensions

StandardRegionThread AngleTypeSeal MethodNotes
NPT (National Pipe Tapered)USA60°TaperedThread interference + sealantCommon in North America.
NPTF (Dryseal)USA60°TaperedMetal-to-metalTighter tolerance than NPT.
BSPT (British Standard Pipe Tapered)UK / EU / Asia55°TaperedThread interferenceSimilar to ISO 7-1 “R”.
BSPP (British Standard Pipe Parallel)UK / EU / Asia55°ParallelGasket or O-ringISO 228 equivalent.
ISO 7-1 (R, Rc, Rp)Global55°R = Tapered (ext), Rc = Tapered (int), Rp = Parallel (int)Thread interferenceWidely used in Europe & Asia.
ISO 228 (G Threads)Global55°ParallelGasket/O-ringMechanical seal only.
JIS (Japanese Industrial Standard)Japan55°PT = Tapered, PF = ParallelPT = Thread seal, PF = O-ringEquivalent to BSPT and BSPP.
SAE (Society of Automotive Engineers)USA / Global60°Straight (parallel)O-ring or face sealUsed in hydraulics.
DIN (German/European)EU55°Tapered & ParallelGasket or thread sealantSuperseded by ISO standards.
API (American Petroleum Institute)Global (Oil & Gas)60°TaperedThread interference + compoundFor casing, tubing, wellheads.
Metric Threads (ISO 261 / 724)Global60°ParallelO-rings / Face sealsMachine threads, not pressure-tight.

How to Identify (Determine) Pipe Thread Type?

Identifying the correct pipe thread type is an essential first step when working with plumbing or industrial fittings. Threads vary by standard, region, shape, and sealing method, and using the wrong type can lead to leaks or failed connections.

Here’s a step-by-step breakdown to help you determine the thread type:

1. Check the Thread Shape

Look at the crest (top) and root (bottom) of the threads. The threads with a rounded profile and a 55° angle are likely BSP (British Standard Pipe) or ISO. And threads with a flatter crest and a 60° angle are usually NPT/NPTF (National Pipe Thread) or Metric.

2. Determine the Thread Taper

Tapered threads become narrower toward the end and are designed to seal by tightening, and they are common in NPT, NPTF, BSPT, and ISO R threads. Straight (parallel) threads have a uniform diameter, can be found in BSPP (G threads), NPS, and Metric fittings. You can test taper by placing calipers at both ends of the thread and comparing the measurements.

3. Identify Thread Gender

4. Consider Regional Use

5. Application Clues

Equipment origin can indicate thread type. For example, Italian appliances often use BSP, while American systems use NPT.


How to Measure & Calculate Pipe Thread Sizes?

Measuring pipe threads can be confusing – especially since the nominal size doesn’t always match the physical diameter. Here’s how to accurately measure and determine thread size:

1. Measure the Outer or Inner Diameter

For male threads, measure the outside diameter (OD) of the thread. For female threads, measure the inside diameter (ID). You can use a caliper for better accuracy.

2. Count the Threads per Inch (TPI) or Measure the Pitch (mm)

TPI is used in imperial thread standards like NPT and BSP, and pitch (in millimeters) is used in metric threads. Use a thread gauge for accurate measurement.

3. Compare with Standard Charts

Once you’ve measured the thread diameter and pitch/TPI, compare your values with a pipe thread size chart for the matching standard. For example, a thread that measures ~0.84″ OD and has 14 TPI is likely a 1/2″ NPT.

4. Understand Nominal Size vs Actual Size

The nominal size refers to the approximate internal diameter of the pipe—not the thread OD. For instance, a “1/2 inch” pipe thread has an actual outer diameter of 0.84 inches.

5. Use the Right Tools

Calipers (for diameter), thread pitch gauge (for TPI or metric pitch), reference charts (NPT, BSP, ISO, Metric, JIS, etc.)

By checking the thread profile, measuring the diameter and pitch, and using charts for comparison, you can accurately identify the pipe thread type and size. This ensures better compatibility, fewer leaks, and less frustration during installation.

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