Pipe Size Calculator
Choose the right pipe diameter for a given water flow so the velocity stays safe and pressure loss is low.
Inputs
1.0–2.0 m/s for water pipes
Formula
- Area = flow ÷ velocity
- Diameter = √(4 × area ÷ π)
- Head loss ≈ 10.67 × L × Q^1.85 ÷ (C^1.85 × D^4.87)
Water pipes are sized by velocity. Too slow and the pipe is oversized and expensive; too fast (above about 2.5 m/s) and you get noise, water hammer and heavy pressure loss. Divide the flow by the target velocity to get the required area, then convert to a diameter and round up to the nearest standard pipe.
Step-by-step calculation
- 1
Area
Q ÷ V
667 mm²
- 2
Diameter
√(4A/π)
29.1 mm
- 3
Standard size
next size up
32 mm
Worked example
- 160 lpm = 0.001 m³/s at 1.5 m/s → area 0.000667 m².
- 2Diameter = √(4 × 0.000667 ÷ π) = 0.0291 m = 29 mm.
- 3The next standard size is 32 mm (1 inch) PVC.
Tips
- Use 25 mm for a single tap line, 32–40 mm for a house main, 50 mm+ for pumps above 2 HP.
- Every 90° elbow adds roughly 1 m of equivalent pipe length.
Frequently asked questions
What size pipe for a 1 HP pump?
Usually 32 mm (1 inch) suction and delivery; go to 40 mm on runs longer than 30 m.
Is a bigger pipe always better?
Hydraulically yes, but cost rises fast. Sizing for 1.5 m/s is the sensible balance.
Why does flow drop at the top floor?
Friction in an undersized pipe plus the extra lift. Increasing the main pipe size usually fixes it.
Related calculators
Keep going with more engineering tools.
Pipe Volume Calculator
Calculate how many litres of water a pipe holds and how long it takes to flush or fill it.
Water Pump HP Calculator
Find the pump horsepower needed to lift water to your tank from the flow rate, total height and pipe losses.
Pump Flow Rate Calculator
Convert a pump's flow between litres per minute, litres per hour, m³/h and gallons, and see how long a tank takes to fill.
Tank Filling Time Calculator
Find out how long your pump takes to fill a water tank, and the electricity it uses each time.
Air Compressor Calculator
Check whether your air compressor's tank and CFM match your tools, and how long the tank takes to refill.
Fuel Consumption Calculator
Work out your vehicle's mileage, the fuel needed for a trip and what it will cost.