Convert millimetres to inches and inches to millimetres – also with fractions such as 3/8", 1 1/4" or 1 1/2". In pipe inch mode you can check the real diameter of a 1/2", 3/4", 1" or 2" pipe and thread and its DN size.
Standard inch vs pipe inch
A standard inch is exactly 25.4 mm. A pipe inch is a nominal size derived from the old inner diameter of steel pipes – which is why a 1/2" pipe has an outer diameter of 21.3 mm, not 12.7 mm. BSP pipe threads (G and R) have a diameter close to the pipe OD.
Sanitary tubes and distillation columns
Stainless tubes used in the food industry and in distillation columns (e.g. 2" = 50.8 mm, 1.5" = 38.1 mm) have an outer diameter equal to the stated number of inches. When buying fittings, check whether it is a threaded pipe (pipe inch) or a sanitary tube.
How we calculate
1 inch = 25.4 mm (exactly). The fraction is matched to the nearest 1/64". A pipe inch is a nominal size – it does not correspond to any pipe dimension: a 1/2" pipe has an outer diameter of 21.3 mm, and a G 1/2" thread 20.955 mm. Sanitary (dairy, food) tubes are measured differently: their outer diameter is exactly the stated number of inches, e.g. 2" = 50.8 mm.
Frequently asked questions
How many mm is 1 inch?
Exactly 25.4 mm. 1/2" is 12.7 mm, 1/4" – 6.35 mm, 3/8" – 9.525 mm, 2" – 50.8 mm.
How many mm is a 1/2 inch pipe?
A 1/2" (DN15) threaded steel pipe has an outer diameter of 21.3 mm, and a G 1/2" thread about 20.96 mm. It is not 12.7 mm – the pipe inch is a nominal size based on the old inner diameter of the pipe.
How many mm is a 1 inch pipe?
A 1" (DN25) threaded steel pipe has an outer diameter of 33.7 mm, and a G 1" thread 33.25 mm. A 1" sanitary tube is 25.4 mm.
How many mm is 2 inches?
Exactly 50.8 mm. That is the size of a 2" sanitary tube, often used to build columns. A 2" threaded pipe (DN50), on the other hand, is 60.3 mm.
How do I enter an inch fraction?
Type e.g. 3/8, 1 1/4 or 1-1/2. The calculator also recognises decimals, e.g. 1.25.