Why are PVC Pipes not the correct size?

[Stuardo]

I'm hoping someone can explain the following;

I measured a PVC pipe I wanted a fitting for and it measured 40mm externally. When I bought a 40mm coupling it was too big?

40mm Coupling has an internal dimension of 47mm?

32mm PVC pipe has the dimensions 42mm externally and 36mm internally?

32mm Coupling that fits above is actually 42mm internally and fits my existing pipe better?

Existing pipe 40mm40mm coupling is too big32mm coupling fits 40mm pipe32mm pipe is not 32mm

Is this a plumbers secret to confuse DIY'ers?

Cheers,

Stuart

[MitchellMc]

Hi @Stuardo,

The confusion comes from the difference between nominal and actual sizes. PVC pipe sizes are labelled by their nominal size, which is an approximate internal diameter, not the exact measurements.

• 40mm Pipe: Nominal size is 40mm, but the actual external diameter is around 50mm, and the internal diameter is close to 40mm.
• 32mm Pipe: Nominal size is 32mm, with an actual external diameter around 42mm and an internal diameter around 36mm.

Fittings are sized to match the nominal size but can have larger internal diameters to fit various pipe wall thicknesses. Thus, a 40mm fitting might seem too large if you’re measuring externally, while a 32mm fitting could fit better if it matches the actual dimensions of your pipe.

I always try to take a section of pipe into a store with me to match up, rather than relying on the sizes.

Please let me know if you have any questions.

Mitchell 

[Stuardo]

Thanks @MitchellMc for that information.

It's interesting they can make them all accurately enough that they fit all the other fittings called 40mm and 32mm etc. 

Just saying.

Cheers,

Stuart

[EricL · Accepted solution]

Hi @Stuardo 

Did you by chance grab the white 40mm coupler instead of the grey one? There is a difference between the DWV -drain waste and vent and the 40PN12 which translates to 40mm size with pressure nominal to 12 bar internal pressure - pressure pipe. The indicator is that all the white plumbing fittings with thick walls are for pressure use and the ordinary grey fittings are for drain and waste. They are not interchangeable and are exclusive to their purpose.

Thus, a 40mm pipe such as the Holman 40mm x 1m PVC DWV Pipe will only fit a Holman 40mm PVC DWV Straight Coupling. 

If you need further assistance, please let me know.

Eric

[Stuardo]

Hi @EricL 

Yes I did buy the white 40mm coupling (pictured) as I assumed it would fit my existing drain pipe which I measured at 40mm. Who would have thought I needed to buy the 32mm coupling to fit an accurately measured 40mm pipe?

I guess the point of my original question still stands that in fact the size they are sold under or advertised as, is not really the size they actually are and only a nominal size. This then facilitates a return journey to the store to exchange it for a different nominal size even though it was bought in good faith at the size advertised. Not to mention the returns desk being baffled by the incorrect sizes and thinking I measured it wrong.

Yes in future I will do as @MitchellMc suggested and take a sample with me.

Thanks Guys

[MitchellMc]

Sorry, @Stuardo. Perhaps I've confused the situation further by pointing out nominal versus actual sizes.

This now appears to be a pressure pipe versus a standard pipe issue. As @EricL mentioned, the fittings are not interchangeable.

Your existing drain pipe is not a pressure pipe. You picked up a white pressure pipe fitting. You need the grey Holman 40mm PVC DWV Straight Coupling for your non-pressured pipe.

The reason for the interchangeability issue is back to the nominal versus actual sizes. The pressurised and non-pressurised pipe and fittings might both be called 40mm, but they are different sizes due to their wall thicknesses.

Mitchell

[Stuardo]

Hi @MitchellMc and @EricL 

Problem solved and lesson learnt. Thank you both very much. I went back and bought the grey 40mm coupling and it definitely fits the 40mm drain pipe.

It makes sense the pressurised pipe would have a thicker wall, and I think in future I'll stick to working with wood instead of PVC pipes!

Stuart.

[MitchellMc]

Love your work @Stuardo. Sorry about the confusion!

Mitchell