I looked at other questions and could not find answer. Maybe because my question is to simple and I should know the answer.

I need to replace a main valve and it’s 600 Psi but I am not 100% whether it’s 1 inch or 1 1/4 inch , etc. There is a large “1” on the side of the valve but does this mean 1 inch? If I order a 1 inch valve off Amazon will it work?

I have not removed the valve yet but have another old cracked valve that side by side looks the same size and also has a “1” on its side. Yet when I measure the inside diameter of valve (thread edges) it is 1 1/4 inches. When I wrap a string around the pipe it’s attached to divide its length by 3.14159 it’s also approx 1 1/4 inches.

I am so confused. What does the “1” on the side of valve mean?

So can the “1” indicate the ball valve is 1 inch while my thread diameter is 1 1/4 inches? Looking at pics can anyone tell me what size valve to buy?

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  • Please consider merging your unregistered and registered accounts, which will allow you to edit, comment on any of your posts and most importantly accept an answer on your question. Thanks, and welcome to the site!
    – Niall C.
    Jul 9 at 19:14
  • I would gather several gallons of water in buckets, jugs, whatever. Shut off the water at the curb. Remove the main shut off valve. Take it to a plumbing supply house or big-box store. Find a match (they'll do it for you at the supply house, on your own at big-box), take it home, install the valve, turn the water back on. The jugs/buckets are in case anyone needs water at home (flushing) while you've got it shut off.
    – FreeMan
    Jul 11 at 16:10
  • @FreeMan That works fine if the curb shut off works. In my situation (many years ago, plumber did the work), my main shut off was not quite 100% and the curb shut off was not quite 100%. With both shut off, plumber and assistant worked fast to cut the main pipe after my old shut off and sweat in a new quarter-turn ball valve. Jul 11 at 16:22
  • 1
    Fair point, @manassehkatz-Moving2Codidact, hence a comment, not an answer. ;)
    – FreeMan
    Jul 11 at 17:21

1 Answer 1


The 1 is the inside of the pipe diameter.

You are measuring the outside of the pipe diameter.

Just wait till you start comparing pipe size to hose or tubing sizes.

  • No I am measuring the inside diameter. Edge of threads to edge or threads on inside. See pics.
    – John
    Jul 9 at 16:52
  • Of the valve. The valve goes over the outside of the pipe. Pipe sizes are based on the internal diameter of the pipe, not the outside, which would be the inside of the valve.
    – crip659
    Jul 9 at 17:04
  • 1
    You are measuring the inside of the female threads. The 1" is the hole in the pipe, inside the MALE threads on the OUTSIDE of the pipe. 1" pipe has an outside diameter of 1.315" engineersedge.com/hardware/taper-pipe-threads.htm
    – Ecnerwal
    Jul 9 at 19:03

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