Duct Sizing Calculator

Calculate velocity, pressure drop, or auto-size duct dimensions based on target criteria.

Sizing Method:

Select Duct Type

Roughness (Material)

Flow Rate (Q)

L/s

Air Temperature

°C

Width (W)

Height (H)

Diameter (D)

Fluid Density (ρ)

kg/m³

Kinematic Viscosity (ν)

m²/s

Calculator Definitions

Calculate performance of a known duct, or auto-size unknown dimensions backward based on strict Velocity or Pressure Drop targets.

Input dimensions for rectangular or circular ducts. If auto-sizing, lock one dimension to force the algorithm to calculate the other.

The air volume moving through the duct. It's a key input that exponentially affects velocity and pressure drop.

Choose the operating temperature of the air. This dynamically updates the air's viscosity and density via Sutherland's Law.

These dictate the Reynolds number and the pressure drop through the system, reacting precisely to temperature changes.

Choose the duct material to apply the correct roughness factor (ε). This heavily influences pressure loss and internal friction.

The Ductulator uses standard fluid dynamics equations to calculate velocity, pressure drop, Reynolds number, and friction factor.

V = 0.001 \times Q / A

V = Velocity (m/s)
Q = Flow rate (L/s)
A = Cross-sectional area (m²)
Velocity helps determine if the air is moving too slowly (inefficient) or too fast (noisy or energy-intensive).

Re = (V \times D h) / ν

Re = Reynolds number (dimensionless)
V = Velocity (m/s)
D_h = Hydraulic diameter (m)
ν = Kinematic viscosity (m²/s)
Reynolds number determines whether flow is laminar or turbulent, which influences friction and energy loss.

ΔP/m = f \times (1 / D h) \times (ρ \times V² / 2)

ΔP/m = Pressure drop per meter (Pa/m)
f = Friction factor (dimensionless, via Haaland equation)
D_h = Hydraulic diameter (m)
ρ = Air density (kg/m³)
V = Air velocity (m/s)
When auto-sizing by Pressure Drop, this calculator utilizes a continuous Bisection Root-Finding Algorithm to solve the non-linear relationship backward for the missing dimension.

Values shown are maximums unless otherwise stated and lower values may be used as appropriate. Reference: AIRAH Technical Handbook 2021.

System Component	Velocity (m/s)
Ductwork – Supply (main/riser)	11.00
Ductwork – Supply (branch)	8.00
Ductwork – Return	6.00
Ductwork – Return (Behind R/A Grille)	3.00
Ductwork – Exhaust	6.00
Ductwork – Flexible – Supply	3.50
Neck Velocity for Supply Air Registers	2.50
Coils – Cooling	2.25
Coils – Heating	3.50
Air Filters	1.80 to 2.50
Louvres – Intake (Velocity through free area)	2.50
Louvres – Exhaust (Velocity through free area)	2.50
Kitchen Hood Exhaust Duct	7.50 to 10.0 (typ) 2.54 min (DCV)
Door Grille (through free area)	2.50

The National Construction Code (NCC) sets maximum requirements for pressure drops. Refer to NCC Volume One 2019 Section J5.4.

Component	Pressure Drop (Pa)
Coils – Cooling	100 to 250
Coils – Heating	50 to 100
Air relief and door grilles	Size for 15 max
Ductwork For ductwork sized by the Equal Friction Method. Ensure pressure drop between first and last take-off to air outlets is not greater than 80Pa.	1.00 Pa/m max (0.80 Pa/m for energy efficiency)