About Pipe & Duct Sizing Calculator
The pipe and duct sizing calculator uses the Darcy-Weisbach equation to recommend pipe and duct sizes and compute pressure drop for HVAC hydronic and air distribution systems, in line with ASHRAE, SMACNA, and CIBSE Guide C guidance. It is used by building services engineers to check velocity limits and pump or fan head.
Enter the flow rate, fluid or air properties, material, application, and fittings; the tool finds the smallest standard size within the velocity limits and returns velocity, Reynolds number, friction factor, and the straight, fitting, and static pressure drops in real time.
How It Works
- Choose pipe or duct mode and enter the flow rate, fluid or air temperature, material, and application.
- Compute velocity v = Q / A and the Reynolds number to classify the flow as laminar, transitional, or turbulent.
- Find the Darcy friction factor (64/Re laminar, Swamee-Jain turbulent) and the Darcy-Weisbach pressure drop.
- Add fitting losses and static head, then recommend the smallest standard size meeting the application velocity limits.
Worked Example
Water at 2 L/s in a DN50 pipe (52.5 mm bore) gives a velocity v = Q / A = 0.002 / (pi/4 * 0.0525^2) = 0.92 m/s, which sits within the 0.5-3.0 m/s HVAC hydronic limit, so DN50 is acceptable.
Formulas
- Velocity
v = Q / (pi/4 * D^2)- Reynolds number
Re = v * D / nu- Darcy-Weisbach pressure drop
dP = f * (L / D) * (rho * v^2) / 2- Swamee-Jain friction factor (turbulent)
f = 0.25 / (log10(e/(3.7*D) + 5.74/Re^0.9))^2- Equivalent rectangular duct diameter
De = 1.30 * (a*b)^0.625 / (a+b)^0.25
Standards & References
- Darcy-Weisbach / Swamee-Jain
- ASHRAE / SMACNA
- CIBSE Guide C
Frequently Asked Questions
What method does the calculator use for pressure drop?
It uses the Darcy-Weisbach equation with the Swamee-Jain friction factor for turbulent flow and 64/Re for laminar flow, adding fitting (minor) losses and static head for the total.
How is the recommended size chosen?
For pipes it selects the smallest standard size whose velocity stays within the application limits; for ducts it applies the equal-friction method against the target friction rate, then offers a round size and an equivalent rectangular size.
What is the Reynolds number used for?
The Reynolds number classifies the flow as laminar (below about 2300), transitional, or turbulent (above about 4000), which determines whether the laminar or Swamee-Jain friction factor is applied.
How are rectangular ducts handled?
A rectangular duct is converted to an equivalent round diameter using De = 1.30 * (a*b)^0.625 / (a+b)^0.25, so its friction and velocity can be compared with round duct sizing.