Thread & Tube Size Identification

PushFit.com Technical Support

Identify tube sizes and threads

Use the tube outside diameter, thread diameter, thread pitch and thread form together. A single measurement rarely identifies a connection safely.

Important: tube size and thread size are different. A ¼″ tube is 6.35 mm outside diameter. A ¼″ pipe thread is a nominal pipe size and measures about 13–14 mm across the male thread.

1. Identify the tube size

Measure across the outside of the tube with callipers. Measure on a clean, round section, not over a cut end that has been crushed or burred. Tube size normally means its outside diameter (OD).

Common metric and imperial tube outside diameters
Metric tube OD	Nearest imperial size	Exact imperial conversion
4 mm	5/32″	3.97 mm
6 mm	1/4″	6.35 mm
8 mm	5/16″	7.94 mm
10 mm	3/8″	9.53 mm
12 mm	1/2″	12.70 mm
15 mm	19/32″	15.08 mm
16 mm	5/8″	15.88 mm
18 mm	23/32″	18.26 mm
22 mm	7/8″	22.23 mm
25 mm	1″	25.40 mm

Imperial conversions are exact inch-to-millimetre conversions, rounded to 2 decimal places. “Nearest imperial size” is a comparison only: for example, 6 mm tube and ¼″ tube are different sizes and are not interchangeable.

2. Recognise the thread type

BSPP — parallel

Also called G thread. The thread sides stay parallel. It commonly seals with an O-ring, bonded washer or gasket at the face.

BSPT — tapered

Also called R thread for an external taper. The thread narrows towards its end. BSP uses a 55° thread form.

NPT — tapered

Common on North American equipment. NPT uses a 60° thread form and a different pitch/profile from BSP; the same nominal size does not make them interchangeable.

Metric — usually parallel

Marked with M, the thread’s outside diameter in millimetres, and its pitch. For example, M10 × 1 has a nominal 10 mm diameter and 1 mm crest-to-crest pitch.

The drawings are simplified side-profile schematics, not to scale. Use thread gauges and the fitting specification to confirm a match.

3. Measure male and female threads

Male thread

Use callipers across the widest points of the thread crests to measure the outside diameter (OD).
Check the OD near the first and last full thread. A clear change suggests a tapered thread; similar readings suggest a parallel thread.
Use a thread-pitch gauge to match the crest spacing. Record threads per inch (TPI) for pipe threads or millimetres per pitch for metric.
Compare the diameter, pitch and profile with a thread chart or known fitting.

Female thread

Measure the opening with the inside jaws of callipers. This is an approximate clue, not the female thread’s full major diameter.
Check pitch with a thread-pitch gauge, keeping the gauge square to the thread. Use a suitable thread plug gauge where available.
Look for parallel or tapered sides and note any shoulder, seal face or O-ring seat.
Confirm against the equipment maker’s drawing or thread specification before ordering.

On female threads, the calliper reading is smaller than the matching male thread OD, so do not identify the thread from the opening measurement alone.

4. Common pipe thread reference

These are approximate male major diameters for quick identification. Pipe sizes are nominal: the fraction in the thread name is not the measured thread diameter.

Common BSP and NPT male thread sizes
Nominal size	BSP male OD	BSP pitch	NPT male OD	NPT pitch
1/8″	9.73 mm	28 TPI	10.29 mm	27 TPI
1/4″	13.16 mm	19 TPI	13.72 mm	18 TPI
3/8″	16.66 mm	19 TPI	17.15 mm	18 TPI
1/2″	20.96 mm	14 TPI	21.34 mm	14 TPI
3/4″	26.44 mm	14 TPI	26.67 mm	14 TPI

BSP taper and parallel versions share the basic diameter and pitch for a given nominal size; the taper and sealing method differ. The values above are identification guides, not manufacturing tolerances.

Further reference

This guide is an initial aid only. Confirm the fitting thread, seal and application with the equipment manufacturer before assembly.