Room airflow before round duct size
HVAC Duct Size Calculator & Room Airflow Planner
Build an editable room plan, divide known system airflow, and see preliminary branch and trunk sizes based on air velocity. The animated map makes the distribution visible without claiming that house square footage alone can design an HVAC system.
- Editable room areas
- Known CFM mode
- Animated duct map
- No signup required
Interactive design sandbox
Build the airflow plan
Current plan summary
Change the mockup
Rooms and load clues
Exposure is a teaching adjustment—not a load calculation. Use it to see why equal-size rooms may not need equal airflow.
Animated concept map
Where the air goes
Air handler1,200 CFM
- Lower clue
- Average
- Higher clue
Velocity-only starting points
Room airflow and branch table
| Room | Area | Load clue | Target airflow | Outlets | Each outlet | Round branch start | Resulting velocity |
|---|---|---|---|---|---|---|---|
| Plan total | — | Return system must carry the same design airflow. | |||||
What the math actually does
From airflow to a preliminary diameter
The planner weights each room's entered area by its exposure clue, divides the total airflow among those weights, and splits larger targets across multiple outlets. For each outlet it uses airflow divided by the selected velocity to find duct cross-sectional area, then converts that area to a round diameter and rounds up to a common nominal size.
That is useful for learning relationships: more CFM requires more area, and a lower velocity target requires a larger duct. It is not enough to choose installed duct because pressure loss accumulates through straight runs, elbows, transitions, boots, grilles, filters, and coils.
The real residential design sequence
What must happen before installation
- JRoom-by-room load
Calculate sensible and latent loads using the building, climate, orientation, windows, infiltration, and other actual characteristics.
- SEquipment selection
Choose equipment using its published performance at the design conditions—not nominal tonnage alone.
- DDuct design
Use blower data, available static pressure, total effective length, fittings, materials, friction rate, and room airflow targets.
- TAir outlets and returns
Select and place grilles and registers for throw, spread, pressure drop, sound, and return-air balance.
- ✓Commission and balance
Measure total and room airflow, static pressure, leakage, temperature change, and equipment operation after installation.
Why there is no honest “duct size by house square feet” answer
Two 1,800-square-foot houses can have very different loads. One may be tight, shaded, well insulated, and built over conditioned space. The other may have leaky ducts in a hot attic, large west-facing glass, weak insulation, and high ceilings. Even rooms of equal area can need different airflow because their exterior exposure and load differ.
The square-foot input here is useful for building a room mockup, checking whether room areas roughly match the house, and comparing CFM per square foot. It intentionally does not convert floor area into required tons or BTU/h.
Educational planning tool only. It is not ACCA-approved design software and must not be used as the sole basis for equipment selection, duct fabrication, installation, code compliance, or balancing. Use current standards, manufacturer data, local code, and qualified HVAC design professionals.