How many mm is a 1/2 inch thread?

A 1/2 inch (G 1/2) male thread measures about 21 mm outside diameter and belongs to DN15. As a pipe size, 1/2 inch corresponds to 15 mm copper pipe.

How many mm is a 3/4 inch thread?

A 3/4 inch (G 3/4) male thread measures about 26.4 mm outside diameter and belongs to DN20. As a pipe size, 3/4 inch corresponds to 22 mm copper pipe. The washing machine tap is the best-known example of 3/4 inch.

How do I measure which thread size I have?

Use a calliper to measure the outside diameter of a male thread, or the opening of a female thread, and look the size up in a thread table. Note: the inch designation is historical and does not match the size you actually measure. A 1/2 inch thread measures not 12.7 but about 21 mm.

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Knowledge base

Thread sizes: inch to mm

Why does a 1/2 inch thread measure 21 mm? Because inch sizes are historical. Here is the complete table, plus a tool: enter what you measure and see which thread size you have.

Measure your thread: which size do I have?

Measure with a calliper and the tool tells you which thread size it is. Male thread: measure the outside diameter across the thread. Female thread: measure the opening.

Measured diameter in mm

male thread female thread

Enter a size, for example 21 for a radiator connection.

Thread size table (inch to mm)

The inch designation is historical: it refers to the inside diameter of steel pipe once used, not to what you measure. That is why a 1/2 inch thread measures not 12.7 but about 21 mm.

Inch	DN	Male thread ø	Female thread ø	Copper pipe	Typical use
1/8"	DN 6	9.7 mm	8.6 mm	-	pressure gauges, small valves
1/4"	DN 8	13.2 mm	11.5 mm	-	filling valves, pressure gauges
3/8"	DN 10	16.7 mm	15.0 mm	12 mm	tap tails, flexible connector hoses
1/2"	DN 15	21.0 mm	18.6 mm	15 mm	radiators, taps, stop valves
3/4"	DN 20	26.4 mm	24.1 mm	22 mm	washing machine tap, water heater, pumps
1"	DN 25	33.2 mm	30.3 mm	28 mm	main lines, manifolds, pumps
1 1/4"	DN 32	41.9 mm	39.0 mm	35 mm	heating manifolds, larger pumps
1 1/2"	DN 40	47.8 mm	44.9 mm	42 mm	larger installations
2"	DN 50	59.6 mm	56.7 mm	54 mm	main lines in commercial buildings

Sealing threads: a tapered thread (R) seals on the thread itself with hemp or PTFE tape, a parallel thread (G) seals on a flat gasket or O-ring. So do not crank a gasket joint up as hard as you can, the gasket does the work.

Frequently asked questions

As a thread: about 21 mm outside diameter (DN15). As a pipe size: comparable to 15 mm copper.

As a thread: about 26.4 mm outside diameter (DN20). As a pipe size: comparable to 22 mm copper. The washing machine tap is the best-known example.

DN (diamètre nominal, nominal diameter) is the standardised inside-size designation of a pipe component. DN15 belongs to 1/2 inch, DN20 to 3/4 inch.

Yes, there are adapter fittings for that: press on one side (for example TH or V), female or male thread on the other. That is how you connect taps, radiators and appliances to pressed pipework.

More in the knowledge base

Which pipe diameter?Tables for water and heating. Make a press joint in 8 stepsThe complete step-by-step guide. Which press profile?TH, U, M or V: read it off your fitting. All press knowledgeGlossary and selection guides.

See the complete sets →