Mechanical

Duct Pressure Drop Calculator

Calculate indicative duct pressure drop, velocity and total loss from airflow, duct size and equivalent length.

Use this calculator for preliminary ventilation pressure loss checks where airflow, duct size and equivalent duct length are known. It is intended for early-stage duct sizing and fan allowance checks rather than full detailed pressure loss calculations.

Inputs

Equivalent duct length should include allowances for bends, branches, attenuators, dampers and fittings where appropriate.

Result

Pressure Drop
-
Enter values and click calculate.

Method

Air velocity is calculated from airflow and duct area:

v = Q ÷ A

Duct area is determined from the selected geometry:

Circular: A = πd² ÷ 4
Rectangular: A = W × H

Pressure drop is then estimated from the equivalent duct length and selected friction rate:

ΔP = L × R

Velocity pressure is shown as an additional check:

Pᵥ = 0.5 × ρ × v²
  • v = air velocity (m/s)
  • Q = airflow rate (m³/s)
  • A = duct area (m²)
  • L = equivalent duct length (m)
  • R = friction rate (Pa/m)
  • ΔP = pressure drop (Pa)
  • Pᵥ = velocity pressure (Pa)
  • Calculated result = -

Reference

  • This calculator estimates straight-duct pressure loss using an entered friction rate and equivalent duct length.
  • Velocity pressure is based on an assumed air density of 1.2 kg/m³ for preliminary design purposes.
  • Relevant guidance may include CIBSE Guide B2, CIBSE Guide C and CIBSE Guide C4 for duct sizing, airflow and pressure loss relationships.
  • Where applicable, final design should also align with project specifications, ductwork standards and manufacturer data.
Design note: This calculator provides an indicative duct pressure loss only. Final design should consider fitting losses, terminals, attenuators, dampers, coils, filters, leakage, fan selection and project-specific pressure loss criteria.

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