Thread Sizes

Table of Contents

How to measure a thread size

All (or most) thread sizes used in water fittings in the home and garden are measured in BSP (British Standard Pipe Thread). BSP can sometimes be referred to as BSPF or the Whitworth Pipe Thread. Pipe thread can be parallel or tapered depending on the use that the fitting is going to be used for.

This standard is used in the UK and it ensures that items that are used in the gas, water and Hydraulics industry are compatible. For example when you connect your washing machine to the mains water the adapter will use a standard BSP ¾” thread. This standard is used in many parts of the World outside of the United States.

Each thread can be measured using the very simple table below and there is no need to look at the number of threads per inch

To summarize the simplest way to identify which thread you have is to measure the outside diameter (male thread) or the inside diameter for a female thread

BSP Thread	1/4"	3/8"	1/2"	3/4"	1"	11/4"	11/2"	2"
Male Thread O/D mm	13	17	21	26.5	33.6	42	48	60
Female Thread I/D mm	11.5	15	18.6	24	30	38.5	45	57
Threads per inch (Take no Notice Of This)	19	19	19	14	14	11	11	11
Example Of A Male Thread	Example Of A Female Thread

O/D = Outside diameter

I/D = Inside diameter