BSP Thread Chart

BSPP, BSPT, G, R, Rp and Rc explained

Parallel does not always mean G. An Rp female is also parallel, but it belongs to the ISO 7-1 thread-sealing system. A G female should not be substituted on that basis alone. Match the fitting specification and sealing arrangement, not just whether a male thread can be screwed into it.

The ISO 228-1 summary distinguishes non-thread-sealing parallel threads; ISO 7-1 covers pressure-tight joints made on the threads. Micro Components’ ISO 7-1 guide identifies R, Rp and Rc.

How to measure a BSP thread

1. Start with the marking. Look for G, R, Rp or Rc, a nominal size and the manufacturer’s part identification.
2. Measure the correct diameter. For an external thread, measure across the crests with a caliper. The table’s major diameter is not the bore or the female opening.
3. Check parallel versus tapered. Compare measurements at two positions along the thread. A single measurement cannot establish taper.
4. Measure pitch. Use a suitable thread-pitch gauge. Match both pitch and the 55-degree Whitworth thread form.
5. Inspect the seal. Identify the seat, washer, O-ring or thread-sealing arrangement before choosing the mating fitting.
6. Confirm the specification. Use the part drawing, appropriate gauges or a reliable fitting supplier when identification remains uncertain.

On a tapered R thread, the diameter changes along its length. Identify its measuring plane from the applicable ISO 7-1 data; the G/BSPP table above does not establish tapered-thread gauge dimensions or tolerances. A caliper is useful for identification; it does not replace a thread gauge for inspection. Swagelok describes the measurement and identification process.

Why 1/2 BSP measures about 21 mm

The fraction in a BSP designation is a nominal pipe-size label. It is not the measured outside diameter of the thread. For example, the basic major diameter of G 1/2 is 20.955 mm, which is about 0.825 inch. The 1/2-inch name therefore does not mean that the thread should measure 12.7 mm.

The same principle explains G 1/4 at 13.157 mm, G 3/4 at 26.441 mm and G 1 at 33.249 mm. Use both diameter and pitch to identify a candidate size. The reference values are published in the Emsal supplier table; smaller common sizes are independently listed by Thürmer Tools.

Do not infer a fitting’s pressure rating, bore or outside body diameter from the thread designation. Those are separate product specifications.

BSP versus NPT: similar sizes can still be incompatible

BSP uses a 55-degree Whitworth form; NPT uses a 60-degree form. Their diameters and pitches also differ. Some sizes can feel deceptively close, so partial engagement by hand is not proof of compatibility.

Use a correctly specified adapter when joining different systems. Sealant does not turn mismatched thread forms into a rated connection. See Swagelok’s thread identification guidance and Parker’s BSPP/NPT adapter dimensions.

Pitch conversion and the 1:16 taper

Thread pitch in millimeters is calculated as 25.4 ÷ TPI. That gives approximately 0.907 mm for 28 TPI, 1.337 mm for 19 TPI, 1.814 mm for 14 TPI and 2.309 mm for 11 TPI. The table rounds the calculated pitch to three decimals.

The R/Rc taper is 1:16 on diameter: a 16 mm change in axial position corresponds to a 1 mm change in basic diameter. This is not a 1:16 slope on each side of the thread. It explains why two measurements taken at different points can differ even on the same nominal thread.

The basic minor-diameter column describes thread geometry; it is not a tap-drill specification. The table is an identification reference with basic dimensions. Production limits, thread engagement, gauging and acceptance belong to the applicable standard and product drawing. For machining, choose a tap-drill recommendation for the exact thread type, material and process; do not calculate it by subtracting an arbitrary amount from the nominal size.

BSP Thread Chart FAQ

Sources and data basis

The main dataset is an ISO 228-1 G/BSPP basic-dimension reference. Its 24 basic-size entries are the 24 G sizes listed in the cited Emsal reference. It includes intermediate G sizes that are not all part of the ISO 7-1 R/Rp/Rc size series. It does not reproduce full tolerances or approve a pressure connection. Derived values use 25.4 mm per inch and pitch = 25.4/TPI. The candidate finder uses a visibly described proximity heuristic and does not claim that heuristic is a thread tolerance.

• ISO 228-1:2000 — parallel pipe threadsG-thread scope and sealing outside the threads.
• ISO 7-1:1994 — thread-sealing pipe connectionsStandard scope for R/Rp/Rc pipe-thread joints.
• Emsal Vana — ISO 228-1 basic dimensionsManufacturer tabulation of nominal sizes, basic diameters and pitch.
• Thürmer Tools — thread reference tablesIndependent manufacturer check for common small sizes.
• Swagelok — identifying thread size and pitchMeasurement process, 55° Whitworth form and compatibility limitations.
• Micro Components — ISO 7-1 tapered pipe thread guideR, Rp and Rc designations and 1:16 taper.
• Parker — female NPT to male BSPP adaptersManufacturer dimensions show BSPP/NPT pitch differences for 1/8 through 1 in sizes.

Source check: October 27, 2026. Verify the current manufacturer document and the requirements governing your work.