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The Pressure Budget Along the Run
All quantities are live and update with your inputs.
Ceiling. The supply was read at rest. Actual flow will be lower as the supply sags under flow; read it flowing, or twice, to see the sag.
High velocity. 15.7 ft/s in the 1½ in run, over 8. Noise and erosion, and a less certain estimate.
- Supply
- 75.0 psi static
- Pipe
- 1½ in Steel, 10 to 20 yr
- Run
- 120 ft + 28.5 ft
- Devices
- 0.0 psi
- Velocity
- 15.7 ft/s
- Regulator
- none
Key Results
Primary outputs and operational context based on your inputs.
This calculator estimates deliverable flow. Measuring beats estimating: if the line exists, open the valve that normally draws on it and clock the meter.
99.8 gpm
≈ 84.8 to 99.8 gpm (ceiling)
Estimated deliverable flow ≈ 84.8 to 99.8 gpm (ceiling: actual will be lower as the supply sags under flow), bare pipe, no devices counted.
99.8 gpm
no devices
What you have entered: no devices counted.
187.4 gpm
at 2 in
The same run, every foot one nominal size up.
99.8 gpm
at 0.0 psi of devices
Enter a device lump to feel the Q² sensitivity.
75.0 of 75.0 psi
friction 66.5 · devices 0.0 · elevation 3.5 · destination 5.0
Of 75.0 psi available, 75.0 psi is spent getting water to the destination. 15.7 ft/s in the pipe.
What is each device in this line actually costing you? The pro room identifies each device from what you can see and iterates its loss at the flow it actually sees. The worked example in the Table tab is its output for this line.
Open the True Pipe Flow CalculatorThis is the flow the pipe can deliver. It is almost never the flow you get.
Real supply lines have things in them (a backflow preventer, a fill valve, a softener, a strainer, a meter) and in the field those devices, not the pipe, are usually the dominant restriction. A line that calculates at 200 gpm bare commonly delivers 100.
Two reasons your number here will read high, even if you filled in the device box:
- Device losses grow with flow, roughly as Q². The table above gives losses at each device’s rated flow. If your line is running above that, the real loss is much larger: a 24 inch softener rated at 31 gpm loses about 15 psi there, but roughly 67 psi at 72 gpm.
- It’s circular. You need the flow to know the losses, and the losses to know the flow. One pass can’t resolve that.
You can iterate it by hand, and you should: run it once, look at the flow you got, re-read the table at that flow (scale each device’s rated loss by (your flow ÷ its rated flow)²), re-enter the total, and run it again. Two or three passes converges. The True Pipe Flow Calculator does exactly this automatically, per device, from things you can see, but the method is yours either way.