📐 Free Gage Block Calculator — no login. Open it →

← AskMachinist · Guides · Calculator · 🔎 Search

BSPP / G Thread Size Chart (ISO 228-1)

Guide · Updated October 28, 2026

BSPP, written G on drawings and catalogs, is the British Standard parallel pipe thread defined by ISO 228-1. It uses the 55° Whitworth form with rounded crests and roots, the same pitch and diameters as the tapered BSPT (ISO 7-1, R and Rc) threads, but with no taper. Because the flanks never wedge, a G joint does not seal on the thread: the seal comes from a bonded washer, an O-ring, or a sealing face. The table below lists the standard sizes with diameters from ISO 228-1, checked against the published thread tables from Perfor and Radii and the Wikipedia ISO 228 summary.

Need an exact reference size?

Build any gage block stack with the free Gage Block Calculator. Free, no login.

Step by step

  1. Measure the major diameter: Use calipers on the male thread crests. A G 1/4 reads about 13.1 mm (0.518 in), far larger than the nominal 1/4 because the size refers to an old pipe bore, not the thread.
  2. Count the pitch: Use a Whitworth (55°) thread pitch gage. G sizes are 28, 19, 14 or 11 TPI only.
  3. Check for taper: Lay a straightedge along the crests or measure diameter at both ends. Constant diameter means G (parallel); a 1:16 taper means BSPT R.
  4. Check the thread angle: A 60° gage will not seat cleanly on a 55° Whitworth profile. If 60° fits and the pitch is 27, 18, 14 or 11.5 it is NPT/NPS instead.
  5. Look at the sealing method: A flat face, bonded seal or O-ring groove near the thread almost always means G. Tape or sealant on the thread itself points to tapered threads.
  6. Match the table: Pick the row whose TPI and major diameter both match within a few hundredths of a mm.

ISO 228-1 G thread dimensions (mm)

Diameters are basic sizes. External threads are made in class A or class B tolerance on the pitch diameter (class B is wider and is the general commercial grade); internal threads have one tolerance class. Sizes G 5/8 and G 7/8 are listed in the standard but are rarely stocked.

Formulas behind the table

The Whitworth profile has height H = 0.960491 P and a thread depth h = 0.640327 P. The pitch diameter is d2 = d − 0.640327 P and the minor diameter is d1 = d − 1.280654 P. Pitch is simply 25.4 / TPI.

Worked example for G 1/2: P = 25.4 / 14 = 1.8143 mm. h = 0.640327 × 1.8143 = 1.162 mm. d2 = 20.955 − 1.162 = 19.793 mm and d1 = 20.955 − 2.323 = 18.632 mm, matching the table to the third decimal.

Worked example for G 1: P = 25.4 / 11 = 2.3091 mm, h = 1.479 mm, d2 = 33.249 − 1.479 = 31.770 mm, d1 = 33.249 − 2.957 = 30.292 mm.

Tapping drill sizes

For a G internal thread, drill close to the minor diameter. The Wikipedia ISO 228 table, compiled from tap-maker charts, lists roughly 80% thread depth drills of 8.8 mm for G 1/8, 11.8 mm for G 1/4, 15.3 mm for G 3/8, 19.1 mm for G 1/2, 24.6 mm for G 3/4 and 30.9 mm for G 1. Check your tap maker’s chart before drilling a production port.

Because G threads are parallel, no taper reamer is needed, unlike Rc (BSPT) ports.

G vs BSPT vs NPT

Checking a G thread in the shop

Use G plug and ring gages for production work. For a spot check, three-wire measurement works on the 55° form using the Whitworth best-wire size of 0.5637 P and the 55° wire constant; for a G 1/4 that wire is about 0.754 mm. Measure the major diameter with a micrometer and compare to the table, allowing for the tolerance class.

Remember that the inch name is a trade size. G 1/8 is about 3/8 in across the crests, and G 1 is about 1.31 in.

FAQ

Is BSPP the same as G thread?

Yes. BSPP is the common name and G is the ISO 228-1 designation for the same parallel 55° pipe thread.

What is the OD of a G 1/4 thread?

13.157 mm (0.518 in) basic major diameter, 19 threads per inch.

Can I use thread tape on BSPP?

BSPP is not designed to seal on the thread. Use the bonded washer, O-ring or face seal the fitting was made for.

Is G 1/2 the same as 1/2 NPT?

No. Both are 14 TPI, but G is 55° and parallel while NPT is 60° and tapered, and the diameters differ slightly.

Related guides and calculators

Build your stack in seconds

The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.

Questions? Ask a machinist.

General shop information only — follow your drawing, tool maker's data and your quality system.