How do I calculate gas black iron pipe capacity when I have pipes of a bunch of different sizes connected together? Every single chart I found just assumes a long length of a single size.

In particular the "Longest Length Method" seems to me that is assumes pipes are always laid out from largest to smaller.

In particular I'd like to know if the 10' pipe (marked in red) should be upsized? And if I have enough capacity to add patio heaters and grill (also marked in red)?

diagram of gas pipes

  • 2
    This question will probably answer, but the 1 inch pipe from the meter is the choke point. Larger pipes past that point do not increase flow. diy.stackexchange.com/questions/268260/…
    – crip659
    Jun 7, 2023 at 23:28
  • 1
    Combined BTU of x2 heater and grill? 3/4" @1/2psi is 450,000 BTU. But if "??" is half inch, that's 235,000. It's less than 75' total so in 1/2" it's fine as long as you're under that BTU. Half inch = quarter million. Three quarters = half a million. Prob fine no mater what; it's 3/4 most of the way and starts in inch. Places run entirely in 3/4s; I've never known a problem. 100' runs on 1/2 starting from 1/2 is questionable. You have a modern service or it's so old that it's already over sized. should be upsized? yeah, when the grill goes out rocking both heaters at full blast. Otherwise no.
    – Mazura
    Jun 8, 2023 at 3:24

1 Answer 1


In particular I'd like to know if the 10' pipe (marked in red) should be upsized?

Probably, yes. Depends on your PSI and whether you're meter supports a larger diameter.

That initial 10' @ 1" pipe is a choke point. I'd upgrade it to 1.5".

I'd also recommend a tee where those first two pipes join and lead it outside using 1.5" to make sure you don't run into pressure loss issues due to length.

And if I have enough capacity to add patio heaters and grill (also marked in red)?

You have not supplied enough information.

  • How much gas pressure is your meter configured for?
  • How many BTUs are your patio heaters?
  • How many BTUs is your grill?
  • How many gas appliances could potentially be in use while you're enjoying the outdoors at full blast?
  • I'm upgrading the line to 1.5", although the meter bar is 1", so it will have a tiny distance of 1". I have not bought the patio heaters or grill, so if I have a number to stay below, I'll know what to buy. (Also what size line should I run to them for 25'?) I suppose everything could be on at the same time, except the boiler. I can't imagine enjoying the outdoors this way in the dead of winter. It's a modulating boiler, so at most if it was cold enough to need the heaters, it would be running on low flame (20% flame, maybe a bit more). The meter says 5PSI on it. Also 275CFH.
    – Ariel
    Jun 9, 2023 at 5:23
  • @Ariel It sounds like the meter can max out at 5 PSI but is likely configured for 1/4 PSI based on the 275 CFH. If that's the case then your supply is roughly 285K btu/h
    – MonkeyZeus
    Jun 9, 2023 at 14:50

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