BSP Thread Sizes Explained

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BSP known as British Standard Pipe can be extremely confusing, especially when measuring existing fittings in need of replacement.

Many people think that BSP Thread size is the dimension of the outer diameter of the male threads, however this is not the case. To determine the thread size you will need to subtract ¼” (0.25”) from the outer measurement to specify the correct fitting. You can easily do this using the chart below.

Thread Type

BSPP Parallel (Straight) Threads - These are parallel to the centreline. These normally seal on with a washer or o-ring.

BSPT Taper Threads - These are at an angle in comparison to the centreline. Sealed by two pairing threads.

Thread per Inch

To also make sure the fitting is BSP, you can count the number of threads over ¼” length then multiply by 4 to get the amount of threads per inch.

For instance if there were 7 thread crests in ¼” you would do 7 (thread crests) x 4 (conversion to inch) = 28 (Threads per Inch).

Additionally you can use a screw pitch gauge to check.

Typical BSP sizes and uses are:

• ¼” - Compressed air connectors.
• ⅜” - Central heating.
• ½” - Household taps, radiator connections and shower hoses.
• ⅝”- Old outside taps.
• ¾” - Bath taps, outdoor taps and washing machines.
• ⅞” - Outdoor taps.
• 1” - Boilers and cylinders.
• 1 ¼” - Sinks.
• 1-1/2 Inch BSP - Kitchen and bathroom wastes, toilet syphons and central heating pumps.
• 2-1/4 Inch BSP - Immersion water heaters.

Male

Female

How to measure BSP Thread Sizes

• Firstly you will need either a ruler or vernier to measure either across the thread O/D if male and inside threads I/D if female.
• Get your ruler and place it across to measure the diameter threads. (Take note this is not the BSP thread size)
• When you have your measurement, for example Approx 1” this will not be the BSP size. Checking the table below, you can see that you will actually need 3/4” BSP Thread Size.

BSP Thread Size	Male O/D mm	Female I/D mm	TPI
1/8 Inch BSP	9.7 mm	8.9 mm	28
1/4 Inch BSP	13.16 mm	11.9mm	19
3/8 Inch BSP	16.66 mm	15.2 mm	19
1/2 Inch BSP	20.99 mm	19.1 mm	14
5/8 Inch BSP	22.99 mm	20.3 mm	14
3/4 Inch BSP	26.44 mm	24.6 mm	14
1 Inch BSP	33.25 mm	31.0 mm	11
1-1/4 Inch BSP	41.91 mm	39.6 mm	11
1-1/2 Inch BSP	47.80 mm	45.5 mm	11
2 Inch BSP	59.61 mm	57.4 mm	11