# How do I measure pipe?
**URL:** https://boards.straightdope.com/t/how-do-i-measure-pipe/148340
**Category:** Factual Questions
**Created:** [January 13, 2003, 6:52pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340 "2003-01-13T18:52:13Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@Casey1505](https://boards.straightdope.com/u/Casey1505)
#### Post date: [January 13, 2003, 6:52pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/1 "2003-01-13T18:52:13Z")
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?
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### Author:  [@BF](https://boards.straightdope.com/u/BF)
#### Post date: [January 13, 2003, 7:37pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/2 "2003-01-13T19:37:13Z")
IANAP, but pipes are measured I.D. (inside diameter) and O.D. (outside diameter). A set of calipers will help you out.
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### Author:  [@cynic](https://boards.straightdope.com/u/cynic)
#### Post date: [January 13, 2003, 7:53pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/3 "2003-01-13T19:53:12Z")
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](http://www.engineersedge.com/pipe_schedules.htm).
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### Author:  [@Padeye](https://boards.straightdope.com/u/Padeye)
#### Post date: [January 13, 2003, 8:35pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/4 "2003-01-13T20:35:15Z")
What kind of pipe are you measuring? Iron? PVC? Something else? For plumbing pile ID should match the nominal diameter pretty closely.
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### Author:  [@Anachronism](https://boards.straightdope.com/u/Anachronism)
#### Post date: [January 13, 2003, 8:54pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/5 "2003-01-13T20:54:43Z")
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.
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### Author:  [@sailor](https://boards.straightdope.com/u/sailor)
#### Post date: [January 13, 2003, 9:26pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/6 "2003-01-13T21:26:50Z")
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".
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### Author:  [@Casey1505](https://boards.straightdope.com/u/Casey1505)
#### Post date: [January 13, 2003, 9:41pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/7 "2003-01-13T21:41:28Z")
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.
Thanks.
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### Author:  [@happyheathen](https://boards.straightdope.com/u/happyheathen)
#### Post date: [January 13, 2003, 10:50pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/8 "2003-01-13T22:50:12Z")
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.
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### Author:  [@Casey1505](https://boards.straightdope.com/u/Casey1505)
#### Post date: [January 13, 2003, 11:03pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/9 "2003-01-13T23:03:38Z")
**happyheathen,**
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.
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### Author:  [@happyheathen](https://boards.straightdope.com/u/happyheathen)
#### Post date: [January 13, 2003, 11:20pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/10 "2003-01-13T23:20:39Z")
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.
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### Author:  [@andy\_fl](https://boards.straightdope.com/u/andy_fl)
#### Post date: [January 13, 2003, 11:32pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/11 "2003-01-13T23:32:23Z")
Are you sure its a pipe and not a tube ? Note that pipe and tubing sizes are different.
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### Author:  [@Casey1505](https://boards.straightdope.com/u/Casey1505)
#### Post date: [January 14, 2003, 1:31pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/12 "2003-01-14T13:31:58Z")
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: 😉
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### Author:  [@SlickRoenick](https://boards.straightdope.com/u/SlickRoenick)
#### Post date: [January 14, 2003, 2:54pm UTC](https://boards.straightdope.com/t/how-do-i-measure-pipe/148340/13 "2003-01-14T14:54:35Z")
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”