Pipe Sizing Charts and Flow Rates


Feeder Fish
MFK Member
Mar 9, 2017
i have been trying find a break down of the flow rates to make it easier to figure plumbing for my sumps

Black Manta

MFK Member
Mar 3, 2017
Memphis, Tennessee
Thank you!!!
Building a trickle filter I believe this chart will be useful for my calculations and when buying hoses and fittings. I was also thinking of constructing my own hob. But I may have sourced a 900 gal box of craigslist for cheap.

Mike Thorn

MFK Member
Jun 4, 2017
Just wanted to say thanks C CHOMPERS for putting these numbers and charts together. This is data I didn't know I needed and I can't wait to start doing the math on my tank build.

Invaluable stuff!!


Feeder Fish
MFK Member
Nov 30, 2017
Wow, lot of data.

I'm using a using a 1" PVC pipe as a drain, so I can expect 600 GPH, and thus need a rated for 600GPH MAX, less would work, just less flow?


Jack Dempsey
MFK Member
Apr 15, 2012
I made a really nice chart in Excel but it turns into a mess when copying it here, so here are smaller charts and their explanations:

Cross Sectional Area (in square inches) - Pipe sizes vs. their cross sectional area. Useful for dividing flow between pipes.
size __area
1/2 = .1963
3/4 = .4418
1.0 = .7854
1.25= 1.227
1.5 = 1.767
2.0 = 3.142

Maximum Gravitational Vertical Flow - This is what we look for when sizing drains or DIY overflows. The flow under the power of gravity reaches a maximum in the same way an object reaches Terminal Velocity as it falls through the air. The gravitational force is countered by the waters viscosity (resistance to flow) and the frictional resistance of the pipe. The viscosity creates a minimum vertical length to acheive the maximum flow. If the vertical pipe length is less than the minimum, the flow rate will be somewhat less than the pipes maximum potential.
size _GPM __GPH
1/2 = 2.50 = 150
3/4 = 5.63 = 337.5
1.0 = 10.0 = 600
1.25=15.6 = 937.5
1.5 = 22.5 = 1350
2.0 = 40.0 = 2400

Min. Vertical Length
1/2 = 2"
3/4 = 5"
1.0 = 9"
1.25= 14"
1.5 = 20"
2.0 = 36"

Frictional Head Loss per 90 (measured in Feet of Head)
size_ head loss
1/2 = .00970
3/4 = .00220
1.0 = .00386
1.25= .00602
1.5 = .00878
2.0 = .00156

Anyway... Each measurement is at the pipes maximum flow rate in the Vertical Flow Rate chart. When calculating the loss for the pump side, these values are slightly more.

There is a very conservative rule when estimating head loss. It is one foot of head per fitting. This rule does not take into account flow rates, pressure, actual frictional losses, etc. It does not have to because it is too conservative. If you multiply any of the above head losses by one thousand fittings, you will be very surprised at the actual head loss. (hint: just move the decimal to the right three places.)

Gravitational Horizontal Flow This is for horizontal applications that do not rely on a pump, or the force of a vertical pipe. These flow rates are considerably less than the vertical rates because gravity does not offer a significant horizontal force. It is a balance of gravity and the viscosity vs. the cross sectional area of the pipe.
size_ GPM
1/2 = 1.563
3/4 = 3.517
1.0 = 6.253
1.25= 9.769
1.5 = 14.07
2.0 = 25.0

In this application, you would use the chart for Vertical Flow.

In this application, you would use the chart for Horizontal Flow.

Need to read this when. Not at lunch having issues with flow on my 180 set up


Feeder Fish
MFK Member
Oct 4, 2018
I have 2” ABS drain pipe (horizontal) for fish rack with P-trap, that drains connects to my laundry drain. Based on the information in this thread, would it be correct to expect a performance of being able to dump at least 25GPM before water overflows out the top of my drain pipes?


MFK Member
Aug 14, 2014
Fish Tank
The flow rates are for full pipes (full siphon). If air is introduced, the flow rates will be less because that water volume is replaced with air.
I'm pushing more than 2k on my 1.5" over flow.

The difference between what he is running and what the chart says is based on siphon. Full siphon flow will pull a lot more that just gravity draining. The numbers arebased strictly on the cross sectional numbers of the pipe. Other factors can play into it, but the numbers provided are a good"safe" number.
I realize that this is an old sticky thread but interesting read during "lock down" nonetheless. Thanks OP for posting.

What's not clear to me is that the vertical flow calculated numbers represent "full siphon" based on what OP confirmed above. It was mentioned that the numbers were not representative of full siphon and that OP's numbers were fairly accurate and there are various other factors responding to another's claim - which I do understand. However, the flow listed for say, 1.5" is around 1350gph, and most of the times folks running 1.5" PVC pipes are claiming around the 2000+gph. That is quite a difference. If you accept the calculations here, then most of us, including me, have much less turnover than we think. Moreover, I would say many of us are NOT running our 1.5" drains (in this example) wide open without reducing a valve - which then reduces flow even more.

Anyone care to elaborate and call out what I'm missing? TIA.