Find the recommended pipe diameter for a desired flow rate and velocity, and see the nearest standard pipe size.
How to Use the Pipe Size Calculator
Enter the "Desired Flow Rate (L/min)" you need the pipe to carry.
Enter the "Flow Velocity (m/s)" or leave the default value of 1.5 m/s.
Click "Calculate" to see the calculated diameter and the nearest recommended standard pipe size.
What Happens When You Click Calculate
The flow in L/min is converted to cubic metres per second by dividing by 60,000.
Dividing that flow by the velocity gives the cross-sectional area of water needed.
The area is turned into a diameter with D = √(4 × A ÷ π) and shown in millimetres to one decimal place.
The tool then picks the first size in its list (15, 20, 25, 32, 40, 50, 65, 80, 100, 125 and 150 mm) that is at least as large as the calculated diameter.
This is a velocity-based sizing method. It does not check how much pressure is lost along the pipe, so for long runs or low-pressure gravity supplies, also check the expected flow with the Pipe Flow Rate Calculator.
Worked Example: Sizing a Branch Line
Suppose a bathroom branch must supply a shower drawing about 10 L/min and two taps at about 8 L/min each at the same time, a total of 26 L/min.
At 1.5 m/s the calculated diameter is 19.2 mm, so the tool recommends 20 mm.
At a gentler 1.0 m/s the diameter rises to 23.5 mm, and the recommendation becomes 25 mm.
At 2.0 m/s it drops to 16.6 mm, giving 20 mm again.
Lower design velocity means a bigger, quieter pipe with less pressure loss; higher velocity saves on pipe cost but increases noise and the risk of water hammer when taps close quickly.
How Much Each Pipe Size Can Carry
Working the formula in reverse shows the flow, in litres per minute, that each inside diameter carries at three common velocities:
Inside diameter
1.0 m/s
1.5 m/s
2.0 m/s
15 mm
10.6
15.9
21.2
20 mm
18.8
28.3
37.7
25 mm
29.5
44.2
58.9
32 mm
48.3
72.4
96.5
40 mm
75.4
113.1
150.8
50 mm
117.8
176.7
235.6
65 mm
199.1
298.6
398.2
Before You Buy Pipe
The sizes in the result are treated as inside diameters. In shops, CPVC, uPVC, GI and HDPE pipes are labelled by nominal size, inch size or outside diameter, and the actual bore differs between types and pressure classes. Compare the calculated diameter with the bore in the manufacturer's chart.
Not every fixture runs at once. Plumbing codes use diversity factors or fixture units to estimate the realistic peak flow for a building; adding every fixture's full flow oversizes main lines.
If the calculated diameter is larger than 150 mm, the tool shows "Above 150 mm" because no size in its list is big enough. Mains that large should be designed by a plumbing or public health engineer.
Frequently Asked Questions
It uses the continuity equation Diameter = √(4 × Flow Rate ÷ (π × Velocity)), which relates flow rate, cross-sectional area, and flow velocity.
1.0–1.5 m/s is a common safe range for domestic cold water supply pipes to limit noise, erosion, and water hammer. Higher velocities (up to 2–3 m/s) may be used for short runs or larger commercial systems.
Pipes are manufactured in standard nominal sizes (15mm, 20mm, 25mm, etc.). The calculator rounds your exact calculated diameter up to the next available standard size, since you can't buy a custom diameter.
Add up the flow of the fixtures you expect to run at the same time on that pipe. For a single fixture you can measure its flow by timing how long it takes to fill a bucket of known size, then convert to litres per minute.
Mainly cost. Oversized supply pipes cost more and hold more standing water, so hot water takes longer to reach a tap. For cold-water supply, going one size up is usually a safe choice when you are unsure.