Estimate water flow rate through a pipe using the Hazen-Williams equation, based on pipe diameter, length, pressure, and material.
How to Use the Pipe Flow Rate Calculator
Enter the pipe "Diameter (mm)" and "Length (m)" for the pipe run you want to estimate.
Enter the available "Water Pressure (bar)" and select the "Pipe Material" from the dropdown.
Click "Calculate" to see the estimated flow rate in L/min and US GPM.
How the Flow Rate Is Calculated
The calculator uses the metric form of the Hazen-Williams equation: Q = 0.2785 × C × D^2.63 × S^0.54, where Q is the flow in cubic metres per second, D is the inside diameter in metres, C is the roughness factor, and S is the head loss per metre of pipe.
The pressure you enter is converted to a head of water: 1 bar ≈ 10.197 m.
That head is divided by the pipe length to get the slope S. The tool assumes all of the pressure is used up overcoming friction along the pipe, as when water runs freely out of an open end.
The C-factor comes from your material choice: 150 for PVC/CPVC, 140 for copper, 120 for steel/GI and 100 for old cast iron.
The answer is converted to litres per minute (× 60,000) and US gallons per minute (× 0.264172).
Worked Example: Overhead Tank to a Bathroom
An overhead tank sits about 3 m above a bathroom outlet, which gives roughly 0.3 bar. The supply is a 20 mm CPVC line, 15 m long.
Head = 0.3 × 10.1972 ≈ 3.06 m, so S = 3.06 ÷ 15 ≈ 0.204
The same pipe in old cast iron (C = 100) would give about 24.09 L/min, a third less, which is why replacing corroded GI or cast-iron lines often restores water flow noticeably.
Flow by Pipe Size and Material (30 m run, 0.3 bar)
Inside diameter
PVC / CPVC
Copper
Steel / GI
Cast Iron (old)
15 mm
11.66
10.88
9.33
7.77
20 mm
24.85
23.19
19.88
16.57
25 mm
44.69
41.71
35.75
29.79
32 mm
85.54
79.84
68.44
57.03
40 mm
153.84
143.58
123.07
102.56
Figures in litres per minute. Going up one pipe size roughly doubles the flow because diameter is raised to the power 2.63.
Getting a Realistic Figure
Enter the inside diameter. Indian CPVC and uPVC pipes are often sold by nominal size such as ½ inch or ¾ inch, and the bore is smaller than the outside size, so check the manufacturer's dimension chart.
Each elbow, tee and valve adds resistance. A common approach is to add an equivalent length of pipe for fittings before entering the length.
Taps, mixers and shower heads restrict flow far more than the pipe itself, so the flow at a fixture will usually be lower than this pipe-only estimate.
Hazen-Williams is designed for water at ordinary temperatures in full pipes. It is not suited to other liquids, hot oil, gas, or partly full drainage pipes.
Frequently Asked Questions
It uses the Hazen-Williams equation, a widely used method for estimating water flow through pipes based on diameter, length, pressure (head loss), and a roughness coefficient (C-factor) for the pipe material.
The C-factor represents pipe roughness — smoother pipes (like PVC/CPVC, C≈150) allow higher flow for the same pressure than rougher ones (like old cast iron, C≈100). Choosing the correct material gives a more accurate estimate.
No — it's a close engineering estimate. Actual flow can vary due to fittings, bends, valves, pipe age/corrosion, and elevation changes not accounted for here.
For a gravity supply, measure the height from the water level in the tank down to the outlet. Every 10 m of height gives about 1 bar, so a tank 4 m above a tap gives roughly 0.4 bar. Ground-floor taps get more pressure than top-floor taps fed from the same tank.
In the Hazen-Williams equation, flow depends on the head loss per metre raised to the power 0.54. Doubling the length halves the slope, which reduces flow by a factor of about 0.69, not 0.5. Diameter has a much stronger effect than length.