Engineering 2026-05-03 3 min read By Konvertibly Team

Pipe Sizing Calculator: Diameter, Velocity, and Pressure Drop

Size pipes correctly for water, gas, and HVAC systems. Calculate required pipe diameter for a given flow rate and velocity limit, and estimate pressure drop using Hazen-Williams.

Pipe Sizing Principles

Undersized pipes create high velocity (noise, erosion), pressure drop, and pump strain. Oversized pipes waste money. Target velocity is the starting point.

Required Diameter from Flow Rate

Q = A × v → A = Q / v
d = √(4A / π) = √(4Q / πv)

Target velocities:
  Water supply: 1.0-3.0 m/s
  Hot water return: 0.5-1.0 m/s
  Gas pipe: 5-15 m/s (low pressure)
  Steam: 25-50 m/s

Example: Q=0.005 m³/s, v=2 m/s:
d = √(4×0.005 / π×2) = 0.0564 m → use DN65 (64.9mm bore)

Hazen-Williams Pressure Drop (Water)

h_f = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87)
C = roughness coefficient (PVC=150, Cast iron=100, Steel=120)
L = length (m), d = diameter (m)

Short form: allow 1-5 Pa/m for design runs

Nominal Pipe Size vs Bore

DN15 (½") internal bore: ~15.8mm
DN25 (1") bore: ~26.6mm
DN50 (2") bore: ~52.5mm
DN100 (4") bore: ~102mm

Calculate pipe sizing: Free Pipe Sizing Calculator