TryBuildCalcRoom-by-Room CFM Calculator (Proportional Load-Based Allocation)
Allocate CFM to your rooms instantly.
🕒 Last updated: September 15, 2026
Inputs
System Design Data
Use the selected equipment's actual design airflow, and room/system loads from one consistent load calculation.
Room Loads
Add every room, or only the ones you want to check.
Room 1
Room 2
Room 3
Room 4
System Design Airflow
1,200 CFM
Room-by-Room Allocation
Cooling System Airflow
1,200 CFM
Heating System Airflow
1,000 CFM
| Room | Cooling CFM | Heating CFM | Design CFM | Governing |
|---|---|---|---|---|
| Living Room | 360 | 200 | 360 | Cooling |
| Primary Bedroom | 240 | 300 | 300 | Heating |
| Bedroom 2 | 240 | 250 | 250 | Heating |
| Kitchen / Dining | 360 | 250 | 360 | Cooling |
Room CFM values are apportioned so this table's column totals always match the assigned airflow below, even when an individual room's exact share isn't a whole number.
Assumptions Used
Allocates your entered system design airflow across rooms in exact proportion to each room's own sensible load share of the matching system total — cooling airflow follows cooling load share, heating airflow follows heating load share, independently. Room and system loads must come from the same load calculation (mixing a room-level Manual J with a different system-level estimate produces a meaningless split). Each room's whole-number CFM is apportioned (largest-remainder method) so the table's totals always reconcile with the assigned airflow shown above, rather than each room rounding independently. Each room's target is the larger of its cooling or heating airflow when both seasons are served, since the branch duct and register must handle whichever season demands more air. This is a proportional planning split, not a substitute for actual duct-by-duct friction/velocity sizing or post-installation balancing.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete HVAC Airflow / Room CFM Calculator.
Room-by-room CFM allocation
Once you know a system's actual design airflow, each room's target CFM is that system airflow multiplied by the room's own share of the whole-system load — cooling airflow follows cooling load share, heating airflow follows heating load share, independently.
This page defaults to a 4-room, 1,200/1,000 CFM cooling/heating system — edit the inputs above to match your own load calculation and equipment.
HVAC Airflow Formulas
Each mode uses its own independent formula — pick the one that matches what you already know, not the one that seems simplest.
Sensible Load to CFM
CFM = Sensible Load (BTU/hr) ÷ (1.08 × ΔT °F)
ΔT = absolute supply-to-room temperature difference
The 1.08 factor is standard air's specific heat capacity per unit volume per hour (60 min/hr × 0.075 lb/ft³ standard air density × 0.24 BTU/lb·°F) — a textbook ACCA/ASHRAE relationship, not a rule of thumb. It assumes standard air density and does not self-correct for altitude or unusual conditions. Use sensible load only; the equation has no term for latent (moisture-removal) heat. The displayed CFM is rounded up, never to nearest, so it never understates the calculated requirement.
Equipment Capacity to CFM
Target CFM = Cooling Tons × Selected CFM/ton
Safe planning band = Tons × 350 to 450 CFM/ton
400 CFM/ton is ACCA's own cited baseline, balancing sensible and latent capacity at standard conditions (80°F/50% RH). The 350–450 CFM/ton band is bounded by real coil failure modes, not an arbitrary range: below it, the coil risks icing; above it, condensed moisture risks blowing off the coil before it drains. This is a preliminary planning target — the matched equipment's own blower performance table at its actual installed external static pressure governs the final airflow.
Room-by-Room Allocation
Cooling Factor = System Cooling CFM ÷ Total Cooling Sensible Load
Room Cooling CFM = Cooling Factor × Room Cooling Sensible Load
Heating Factor = System Heating CFM ÷ Total Heating Load
Room Heating CFM = Heating Factor × Room Heating Load
Room Design CFM = larger of Room Cooling CFM or Room Heating CFM
This is a proportional split of a known system airflow — not an independent calculation of what each room needs on its own. Room and total loads must come from the same load calculation, or the resulting percentages don't represent anything real. The larger of the two seasonal values becomes each room's branch target, since the same duct and register have to serve both seasons.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| System cooling / heating airflow | 1,200 / 1,000 CFM |
| Total cooling / heating load | 24,000 / 30,000 BTU/hr |
| Living Room cooling / heating load | 7,200 / 6,000 BTU/hr |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| Cooling factor | 1,200 ÷ 24,000 | 0.05 CFM/BTU·hr |
| Heating factor | 1,000 ÷ 30,000 | 0.03333 CFM/BTU·hr |
| Living Room cooling CFM (exact, before apportionment) | 7,200 × 0.05 | 360 CFM |
| Living Room heating CFM (exact, before apportionment) | 6,000 × 0.03333 | 200 CFM |
| Living Room design CFM (governing) | max(360, 200) | 360 CFM (Cooling) |
Room CFM values are apportioned (using a largest-remainder method) so the whole-number values in the room table always sum to the assigned CFM total shown above — a single room's exact share, before that adjustment, can differ from the table by a fraction of a CFM.
Therefore, Living Room needs 360 CFM (Cooling), out of a system total of 1,200 CFM.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.