Table of Contents
The PSI measures the pressure of water in pounds per square inch. The GPM measures flow rate in Gallons per minute. EasyFreeCalculator's PSI-to-GPM and GPM-to-PSI calculators provide quick conversions. It uses examples and equations to demonstrate how to determine flow rate and pressure.
Just enter the values in the respective columns: pressure (PSI), pipe diameter, material, and length. The calculator will provide an adequate water flow by using the Hazen-Williams formula. Our pipe flow online calculator is fast, easy, and useful for irrigation, plumbing setups, and water supply.
Direct conversion of GPM from PSI is not possible. As water flow capacity also depends on the real-world factors:
For example, a water hose and a fire water main could both operate at 40 PSI, but their flow rates differ due to real-world factors.
It is widely used in industry to calculate the flow rate in gallons from pressure in psi. It considers the pipe size, length, and material ( C Factor).
Where:
Q = Flow rate in GPM
C = Hazen-William roughness coefficient
D = Inside diameter of pipe in inches
L = Length in feet
hf = head loss due to friction
(Here, hf in feet of head is calculated from PSI by hf = PSI/ 0.433)
The standard Freeman Flow formula used for calculating the gpm from psi reading:
Where:
This is used by firefighters or engineers to quickly estimate the water flow from a nozzle or orifice for firefighting purposes. It is widely used in fire protection and hydraulics. For professional use, the formula includes the Coefficient of discharge(C) to include friction loss percentage and orifice geometry. Its value is 0.9 for fire hydrant Nozzels.
This does not apply to practical use for irrigation systems or plumbing setups. As there are other factors affecting the GPM, such as pipe material, length, or diameter.
The Bernoulli equation is based on the principle that relates pressure reading to flow rate in an incompressible fluid like water. It shows that in ideal conditions, pressure can convert into velocity when friction and elevation changes are ignored. It is commonly used for theoretical purposes.
Where:
P = pressure
ρ = fluid density
v = fluid velocity
g = gravitational acceleration
h = elevation head
To find out the GPM from PSI using this method, you have to find the velocity first. Then you can calculate the flow rate of water in gallons per minute by the cross-sectional area of the pipe.
Where
A = cross-sectional area in ft² (A = π × r²)
448.83 = the conversion factor from ft³/s to GPM
Hazen-Williams formula is recommended for practical use as it account on real world factors. It is more suitable for calculating water flow than the Bernoulli equation and the Freeman flow formula, which assumes perfect conditions without friction losses.
Calculate the water flow capacity in a 1" PVC of 150 feet at 60 psi for a garden irrigation system.
PSI = 60
d = 1-inch
L = 150
Material = PVC, C = 140
Step 1: Convert PSI to hf
hf = PSI× (1/0.433)
= 1/0.433 = 2.307
hf = 60 × 2.307 = 138.47
Step 2: Calculating hf/L
hf = 138.47
L = 150
hf/L = 138.47/ 150
= 0.923
Step 3: Apply Formula
Q = 0.442 × C × D²·ā¶³ × (hf/L)ā°·āµā“
Q = 0.442 × 140 × 1²·ā¶³ × (0.923)ā°·āµā“
= 59.25
The irrigation flow rate is 59.25 GPM.
Q = 4 GPM
D = 0.5"
C = 140
L = 100 ft
Step 1: Calculate the inner term
4 / (0.442 × 140 × 0.5²·ā¶³) = 0.40015
Step 2: Find head loss
hf = (0.40015) ¹/ā°·āµā“ × 100 = 18.34 ft
Step 3: Convert to PSI
PSI = 18.34 / 2.307 = 7.95 PSI
A pressure of 7.95 PSI is required to push 4 GPM through 100 feet of 0.5" PVC pipe.
Pressure = 50 PSI
d = 2.5 hydrant nozzle
Step 1: Square the diameter
2.5² = 6.25
Step 2: Find the square root of PSI
√50 = 7.07
Step 3: Multiply values
= 29.84 × 6.25 × 7.07
GPM = 1319
The GPM is 1319 under ideal conditions.
The Hazen-William roughness coefficient indicates how smooth the inside of the pipe is. A higher C value means a smoother pipe with less friction and higher flow. The C value decreases in old and used pipe, as there can be an accumulation of deposits inside. In that case, you should use a lower C value to avoid undersizing your pump. The value of C for commonly used pipe material is:
|
Pipe Material |
C Factor |
|
PVC |
140 |
|
HDPE |
150 |
|
Steel |
120 |
|
Cast Iron |
110 |
|
Concrete |
100 |
The estimated values of GPM at common pressures for a standard pipe size with no change in elevation are shown below. These are maximum pipe capacity values. Real-world flow will be lower due to fittings, elevation, and longer runs
|
PSI |
0.5 Inch |
0.75 Inch |
1 Inch |
1.5 Inch |
2 Inch |
|
20 |
6.58 |
19.12 |
40.75 |
118.38 |
252.27 |
|
40 |
9.57 |
27.80 |
59.25 |
172.12 |
366.79 |
|
60 |
11.91 |
34.61 |
73.76 |
214.25 |
456.57 |
|
80 |
13.92 |
40.43 |
86.15 |
250.26 |
533.30 |
|
100 |
15.70 |
45.60 |
97.18 |
282.30 |
601.60 |
Values are calculated using Q = 0.442 × C × D²·ā¶³ × (hf/L)ā°·āµā“ with C= 140, L=100 feet, andd pressure converted to head using 1 PSI = 2.307 feet. For values at different pipe materials, lengths, and conditions, use the calculator above.
This calculator is commonly used for:
Most people overlook the factors mentioned below, even though they can significantly affect water flow accuracy. Avoid these common mistakes while calculating flow rate.
Yes. Smaller pipes restrict flow as they cause higher friction and turbulence, which will reduce the gallons of water moving through the system.
A discharge coefficient is the efficiency of the water flow from a nozzle, faucet, or fixture. It improves the accuracy of pressure and flow calculations.
Yes. Higher water pressure generally increases flow rate if pipe size and other conditions remain the same.
Fluid properties like density and viscosity play a big role in how turbulence happens or pressure drops in pipes. They also influence the whole flow setup in industrial systems.
Fire sprinkler systems have no fixed values. The requirements depend on hazard level and design density per NFPA 13, with residential systems needing as little as 13–26 GPM. Fire hydrants require a minimum residual pressure of 20 PSI and a GPM of 500-1,500+.