I've never used PVC piping (or any other piping for that matter) before and am completely confused about how the various diameters are illustrated. I've spent numerous hours in DuHome literally checking hundreds of PVC connections, 90° elbows, etc. to see if they fit together or not.

I simply don't understand the markings/measurements.

Surely there must be a system to this madness?! Any hints?

The pipe's listed size is its outside diameter. This helps to choose the correct fittings, etc. The actual inside diameter can vary with the wall thickness of the pipe. Do all flow calculations based on an inside diameter which you can measure. There are lots of calculators on the web.

I always upsize at least 25% due to friction in the pipe and direction changes at some fittings.

And too make matters worse and add to your confusion plumbing is one of those traditional trades where they have metric and imperial sizing.

So you can buy 40mm or the imperial may be 1.5inch. (actually 38mm)

I do the same as you....take a fitting with me and try them all.

Thank you gentlemen. I'll give you an example, I have a pipe with the following printed on it: 55 (2") PVC 8.5. What do these numbers mean? The pipe (inside) is wider than 2"... I find it extremely difficult to find pieces that fit together "snugly". Any info re the sizes/markings on the pipe would be much appreciated. Ta!

Thank you gentlemen. I'll give you an example, I have a pipe with the following printed on it: 55 (2") PVC 8.5. What do these numbers mean? The pipe (inside) is wider than 2"... I find it extremely difficult to find pieces that fit together "snugly". Any info re the sizes/markings on the pipe would be much appreciated. Ta!

maybe this helps ….

Somewhat off subject... But if you are concerned with flow capacity when installing PVC make sure to remove the burrs on the inside of the pipe after you cut the pipe.

Thank you gentlemen. I'll give you an example, I have a pipe with the following printed on it: 55 (2") PVC 8.5. What do these numbers mean? The pipe (inside) is wider than 2"... I find it extremely difficult to find pieces that fit together "snugly". Any info re the sizes/markings on the pipe would be much appreciated. Ta!

maybe this helps ….

So looking at this chart it seems that Neversure's answer is incorrect, because 2" is neither 55 nor 60 mm, but is actually 50.8 mm.

So even with the thickest 13.5 Bar pipe, the inner diameter of a 2" pipe is still larger than 2". 60 mm - 2x 4.30 mm = 51.4 mm

With a thinner wall pipe the difference is more obvious.

So in fact the number written on the pipe doesn't make sense in any respect, because in no case does it correspondent to to either inside or outside diameter.

Interesting thread

I never really did measurements of those PVC things.

Just bought the fitting parts and glued them together taking 1/2" for granted.

Now I see this table stating:

Nominal size: 18 (1/2")

I just did the measurement on a piece of pipe and indeed they have an inner diameter of 18 mm (bigger than 1/2").

Outer diameter 22 mm which fits the inner diameter of the connecting parts.

What all this has to do with 1/2" ?

Hope someone comes up with the explanation. Historic?

Note: for 1 inch and all sizes up from 4 inch, the nominal sizes in mm at least approximately match the size in inch.

Approximate: 2.5 mm instead of 2.54 mm.

Thank you gentlemen. I'll give you an example, I have a pipe with the following printed on it: 55 (2") PVC 8.5. What do these numbers mean? The pipe (inside) is wider than 2"... I find it extremely difficult to find pieces that fit together "snugly". Any info re the sizes/markings on the pipe would be much appreciated. Ta!

maybe this helps ….

Thank you Motdaeng for the very helpful chart. At least now I understand the colour coding.

I'm still at a lost to understand why the measurements aren't precisely accurate - they seem to be always +/-, but I assume that's something I'll have to put up with.

The problem with inaccuracies in PVC pipe measuring comes from the conversion between the metric system and the US system (SAE). To be correct one would have to use fractions, and the manufacturers don't want to label their pipe with fractions. That is why the chart above uses the term Nominal Size in the first column

For a definition of nominal size see this reference:

http://en.wikipedia.org/wiki/Nominal_size

The problem with inaccuracies in PVC pipe measuring comes from the conversion between the metric system and the US system (SAE). To be correct one would have to use fractions, and the manufacturers don't want to label their pipe with fractions. That is why the chart above uses the term Nominal Size in the first column

For a definition of nominal size see this reference:

http://en.wikipedia.org/wiki/Nominal_size

Nominal size for a 2" pipe would be 50 mm, not 55.

http://www.theatlantic.com/national/archive/2012/05/yes-america-we-have-executed-an-innocent-man/257106/

The problem with inaccuracies in PVC pipe measuring comes from the conversion between the metric system and the US system (SAE). To be correct one would have to use fractions, and the manufacturers don't want to label their pipe with fractions. That is why the chart above uses the term Nominal Size in the first column

For a definition of nominal size see this reference:

http://en.wikipedia.org/wiki/Nominal_size

Nominal size for a 2" pipe would be 50 mm, not 55.

http://www.theatlantic.com/national/archive/2012/05/yes-america-we-have-executed-an-innocent-man/257106/

What link is a that? Mishap?

"Yes, America, We Have Executed an Innocent Man"

Or hidden irony?

The problem with inaccuracies in PVC pipe measuring comes from the conversion between the metric system and the US system (SAE). To be correct one would have to use fractions, and the manufacturers don't want to label their pipe with fractions. That is why the chart above uses the term Nominal Size in the first column

For a definition of nominal size see this reference:

http://en.wikipedia.org/wiki/Nominal_size

Good find. Thats the explanation I was looking for why 1/2" and 3/4" don't have any matching sizes (post #8).

Just "names"/"trade customs", not really something for the sliding caliper.

The problem with inaccuracies in PVC pipe measuring comes from the conversion between the metric system and the US system (SAE). To be correct one would have to use fractions, and the manufacturers don't want to label their pipe with fractions. That is why the chart above uses the term Nominal Size in the first column

For a definition of nominal size see this reference:

http://en.wikipedia.org/wiki/Nominal_size

Nominal size for a 2" pipe would be 50 mm, not 55.

http://www.theatlantic.com/national/archive/2012/05/yes-america-we-have-executed-an-innocent-man/257106/

What link is a that? Mishap?

"Yes, America, We Have Executed an Innocent Man"

Or hidden irony?

Can't edit the post anymore, so below is the correct link.

http://en.wikipedia.org/wiki/Nominal_Pipe_Size

Thanks for pointing out my error, it was a link that still was in the PC memory from yesterday.

I can not see what the problem is ,it helps if you speak some Thai like here,all sold in imperial size

See( four)-Hoon = 1/2 inch

Hock ( six)-Hoon =3/4 inch

New Nung= 1 inch

Song New =2 inch

Sam New = 3 inch

See New = 4 inch

Been buying pipe and fitting for years been using the above ,I go on my own most times never a problem.

The More expensive pipe will be better quality and thicker .

I have brought some fitting from my local market ,cheap with no label on ,some are of dubious quality and do not always fit on right.

The old saying ,get what you pay for.