Pipe Diameter Calculator

How to Size a Pipe

Pipe diameter selection balances two competing demands: too small causes high pressure drop and velocity; too large increases cost. The optimal diameter is found by specifying a target velocity and calculating the diameter from the required flow rate.

The Formula

D = √(4Q / (π × v))

Where:

• D = internal pipe diameter (m)
• Q = volumetric flow rate (m³/s)
• v = target velocity (m/s)

Standard Pipe Sizes

Actual pipe diameters are standardised. After calculating the minimum diameter, select the next larger standard size. Common DN sizes: 15, 20, 25, 32, 40, 50, 65, 80, 100, 125, 150, 200, 250, 300mm.

How to Use This Calculator

Enter required flow rate and target velocity. The calculator returns the minimum internal diameter and recommends the nearest standard nominal bore (DN) size.

Practical Examples

Example 1: 10 L/s at 1.5 m/s. D = √(4 × 0.01 / (π × 1.5)) = √(0.00849) = 0.0922m = 92mm → use DN100 pipe.

Example 2: House with 20 GPM (1.26 L/s) at 1.5 m/s. D = √(4 × 0.00126 / (π × 1.5)) = 0.0327m → DN32 or 1¼ inch pipe.

Rules of Thumb

• Domestic water: 1–2 m/s
• Cold water mains: 1.5–2.5 m/s
• Drain lines: 0.6–1.0 m/s (minimum 0.6 for self-cleaning)
• Steam: 20–40 m/s (much higher velocity acceptable)
• Gas (natural gas): 5–20 m/s (depends on pressure)