How do I measure pipe?

I am trying to get a connector for 2 pipes, but I’m not sure how to measure them. What I measured as a 1 1/4" pipe is actually a 1 1/2" pipe. I measured the other pipe as 1 1/2", but it’s not. The guy at the hardware store told me that it may be 2", but that seems way too big. Is there a certain trick to measuring pipes that I am not getting?

IANAP, but pipes are measured I.D. (inside diameter) and O.D. (outside diameter). A set of calipers will help you out.

It sounds to me like you’re measuring the I.D., but you really need to measure the O.D. because the I.D. varies according to the pipe’s schedule. Note that the 1 1/2" figure only represents the nominal diameter, which usually doesn’t match either the I.D. or the O.D. (except for 14" and larger).

Here’s a Pipe Chart.

What kind of pipe are you measuring? Iron? PVC? Something else? For plumbing pile ID should match the nominal diameter pretty closely.

Most plumbing pipe diameters are nominal, if you are not familier with the pipe sizes the easiest way would be to bring a piece of the pipe with you or measure the exact outside diameter and bring your tape measure with you to the store.

To give you a rough idea: the thread of the coupling on a garden hose is nominally 3/4", the thread on a smaller faucet which also has threads is 1/2".

Measuring radiator pipes. Sorry, cynic, the table is greek to me.

The pipe in the floor is 1 3/4" (2"?) and the radiator pipr for sure is 1 1/4" ( a pipe labeled as such fits nicely. I don’t have any 2" pipes to check against the floor pipe. )

It’s “black pipe”, correct? I can’t use PVC on a steam radiator, can I? The radiators have been in the house for 40 years or so, and I am lacking funds to convert to something better or more modern.


I’m assuming we are talking about steel pipe (btw - you will scare people when you talk about “black pipe” because that indicates that you are dealing with a gas line).

If that is a gas line, and you cannot even id the pipe diameters, do yourself, your neighbors, and the fire department a favor and call in a pro - I have done plumbing - supply, waste, and vents, 110 and 220 electrical, walls, cabinets, flooring - whole bunches of stuff - and I will not touch gas lines.

Once you know what is in the pipe, we can discuss this further.


100%, absolutely, positively certain that it is steam. I thought it was called “black pipe” because I thought that’s what the box at the national hardware chain I go to had called it. Apologies for the misunderstanding.

OK, std threaded steel pipe is not sized per any rational (to me, at least) system - see cynic’s chart - find the OD (Outside Diameter) in the second column, and read the “nominal” size in the first column. Pipe is sold by the “nominal” size.
If all else fails, wrap a slip of paper around the pipe and mark it - you now have the circumference - the clerk can work from that.
And make sure that:
a) you get the correct grade of pipe for a steam system.
b) the steam is OFF before trying to open the line - if that is a live steam system, it can scald you to the point that you are cooked (the ER does need this) - don’t mess.

Are you sure its a pipe and not a tube ? Note that pipe and tubing sizes are different.

Sorry for beating a dead or dying horse. A few more questions.

  1. Could the thread on the floor pipe be in reverse, and that’s why I can’t get a 1 1/2" pipe on? Why would that be?
  2. What do the letters on the pipe fittings mean? I’ve seen “G”, “P”, and “SLK”.

To recap, I am moving a steam radiator to a different room, and cannot seem to get the radiator flange (?) to screw onto the floor pipe.

If I can’t figure this out, I’m selling the house…:mad: :wink:

The only threads that are reversed, AFAIK, are propane and tourniquets. Although I seem to remember certain fittings in the plumbing section of Ace Hardware couplers that can screw onto 2 male ends of pipe from different directions–someone else can prolly explain it better. A d-i-yer plumber told me once different letters and numbers on pipe (copper, iron, plastic, steel) mean different strengths, or “thicknesses”