How to use this calculator
- Enter airflow. Use the design CFM through the checked trunk or branch, not whole-system airflow unless that run carries the whole system.
- Pick duct geometry. Choose round/spiral or rectangular and enter the actual inside duct dimensions.
- Add equivalent length. Enter straight duct length and the equivalent straight length for elbows, takeoffs, transitions, dampers and other fittings.
- Check static margin. Compare pressure drop, friction rate and velocity against the available static budget before selecting the blower or resizing the run.
How it works
The airflow and duct geometry set the flow area and velocity:
v = Q / A
Round duct uses A = πD²/4. Rectangular duct uses A = W·H
and hydraulic diameter D_h = 2WH/(W+H).
The calculator then estimates Reynolds number and friction factor:
Re = ρvD_h / μ
Laminar flow uses f = 64/Re. Turbulent flow uses the Swamee-Jain
approximation with the entered absolute roughness.
The straight-run friction rate is:
Δp/L = f · (ρv²/2) / D_h
and the total loss is Δp = (Δp/L) · TEL, where total equivalent
length is the measured straight duct plus the fitting equivalent length.
Use this after selecting a size in the duct size calculator or from the duct size worksheet. For a query-focused loss workflow, keep the duct friction loss worksheet nearby. For Manual D-style friction-rate intent, use the Manual D friction rate calculator. Then carry the run loss into the HVAC static pressure calculator with filters, coils, terminals and accessories. For reducer geometry, use the duct transition calculator.
Worked example
Verified against the live calculator
For about 530 cfm through a 10 in round or spiral duct with 50 ft of straight duct and 33 ft of fitting equivalent length, galvanized-sheet roughness gives about 1,000 fpm velocity and roughly 0.15 in. w.g. per 100 ft. Over the total equivalent length, the duct run drops about 0.13 in. w.g., leaving roughly 0.37 in. w.g. from a 0.50 in. w.g. budget.
Frequently asked questions
How do you calculate duct friction loss?
This calculator finds duct area, velocity, Reynolds number and Darcy friction factor, then uses Delta p/L = f*(rho*v^2/2)/D_h. Total pressure drop is the friction rate multiplied by straight duct length plus fitting equivalent length.
Is spiral duct different from round duct?
For a first-pass screen, spiral duct can be entered as round duct with the actual inside diameter and a sheet-metal roughness assumption. Lined duct, flexible duct, leakage, crushed flex and poor installation can lose much more pressure than clean round metal duct.
Do fittings count in this calculator?
Yes, if you enter them as fitting equivalent length. Add equivalent length for elbows, takeoffs, transitions, dampers and boots from your design method or manufacturer data, then keep the final static budget in the HVAC static pressure calculator.
Is friction rate the same as total pressure drop?
No. Friction rate is pressure loss per length, such as in. w.g. per 100 ft or Pa/m. Total pressure drop multiplies that rate by the run total equivalent length.
Method & assumptions
- Steady, fully developed air flow in one duct run at the entered airflow.
- Round and spiral duct are screened as clean round duct with the entered inside diameter and roughness.
- Pressure loss uses Darcy-Weisbach, hydraulic diameter and fixed room-temperature air viscosity of 1.81×10⁻⁵ Pa·s.
- Fittings are included only through the equivalent length you enter. Actual fittings, leakage, balancing dampers, lined duct, flex duct, acoustics and terminal selections still need the applicable design method and manufacturer data.
- Final HVAC design should follow the applicable ACCA, ASHRAE, SMACNA or engineered duct-design method and the fan table at the design airflow.