Round sheet metal
11 in diameter
- Section area
- 95.0 sq in
- Calculated air speed
- 606 fpm
- Equal-friction diameter
- 11.00 in
- Straight-run friction rate
- 0.054 in. w.c./100 ft
- Loss across entered path
- 0.107 in. w.c.
Supply, return, blower. One pressure budget.
Duct size is only one part of an airflow problem.
That far room may need more branch area. It may also be starved by the return path, a high-pressure filter, the coil, or the blower setup. Use the room check for a first pass, or enter field measurements.
- Local browser calculation
- Inputs stay on this device
- Limits shown with the result
HVAC airflow worksheet
Begin with the information you actually have.
Homeowners can screen one room without guessing at static pressure. Technicians can work from CFM, pressure allowance, and total effective length. Both views are a starting point for field verification.
Start from the symptom
Map the room before blaming the branch.
We compare a rough airflow requirement with a reference layout that has one 6-inch round branch for every supply register. The result is conditional. Another installed diameter needs its own measurement.
No branch-size flag
The reference branch is not the leading suspect.
Measure return-side pressure, filter and coil drop, damper position, and delivered airflow before changing duct size.
- Estimated room demand
- 200 CFM
- Demand per supply
- 100 CFM
- Screened round branch
- 6 in round, per supply
- Reference-area difference
- No shortfall in the reference
Field checks before a duct change
Measured system inputs
Test a metal section against the pressure allowance.
Use the design airflow for this section, the pressure left after component losses, and the total effective length of the longest path. Whole-inch results must stay under both friction and velocity limits.
Pressure-based friction allowance 0.100 in. w.c. per 100 ft Controlling check: air speed
Rectangular sheet metal
14 x 7 in
- Section area
- 98 sq in
- Calculated air speed
- 588 fpm
- Equal-friction diameter
- 10.66 in
- Straight-run friction rate
- 0.063 in. w.c./100 ft
- Loss across entered path
- 0.125 in. w.c.
Calculation trail
Equations behind the dimensions
- Friction allowance from pressure
friction allowance = duct pressure / effective length x 100- Air speed through area
air speed = airflow x 144 / cross-sectional area- Rectangular-to-round equivalent
round equivalent = 1.30 x (a x b)^0.625 / (a + b)^0.25- Straight-section pressure drop
pressure drop = friction factor x (length / diameter) x dynamic pressure. Darcy-Weisbach with a Swamee-Jain approximation determines metal-duct friction.
Model basis: air near 68 F, galvanized sheet metal, full-open whole-inch sections, and no leakage or flex compression. The entered effective length must already account for every fitting. Results describe one constant-flow section; an installed trunk changes airflow after each takeoff.
Use this output as a design check, not a final schedule. A licensed HVAC contractor must complete final sizing under ACCA Manual D with measured static pressure, verified blower data, total effective length, and actual fitting losses.
Contractor view
Airflow belongs to the complete air path.
Load defines the airflow target.
A branch has no correct diameter until the room's heating and cooling load sets its required airflow. Floor area is an early screen; orientation, glass, insulation, ceiling height, and infiltration can shift the target.
The return side can starve the supply.
The blower has to pull air back before it can send air out. An undersized return, restrictive filter, or loaded coil can consume static pressure across every branch, including one with adequate area.
Equipment may set the ceiling.
A duct change cannot create airflow beyond the air handler's fan curve or equipment setup. Blower speed, coil pressure drop, zoning position, and total external static have to agree with the requested CFM.
Diagnostic questions
Separate a duct symptom from a system fault.
If the far room is weak, does that mean its branch is too small?
No. A partially closed damper, torn or compressed duct, high filter drop, weak return path, coil restriction, or blower setting can produce the same complaint. Diameter becomes a conclusion only after airflow and pressure readings.
Why does return air matter when the supply register feels weak?
Supply airflow depends on the blower receiving air with reasonable resistance. A tight return route or restrictive filter raises return-side pressure, reduces total system airflow, and can leave distant supplies with very little air.
Can the indoor equipment be the actual bottleneck?
Yes. The selected fan speed, the blower's published performance, coil pressure drop, filter setup, and zoning position can cap airflow before the branch does. Duct area cannot recover pressure that the equipment and components already used.
What belongs in total effective length?
Start with the longest supply and return route, then add the published equivalent length for every elbow, takeoff, transition, boot, grille, and other fitting. The result can be several times the measured straight-line distance.
Is this a completed ACCA Manual D design?
No. The worksheet exposes early sizing arithmetic for one section. Final ACCA Manual D work needs room-by-room loads, measured static pressure, the equipment fan table, actual duct material, fitting losses, and a licensed HVAC contractor.
Bring field readings, not a guess.
Diagnosis sets the scope.
We measure delivered airflow and total external static, then check the return, filter, coil, blower setup, dampers, and duct path. Any repair scope and estimate comes after that diagnosis. A calculator result alone is not enough to price a duct change.