Air Balance Calculator
Calculate building air balance, pressurization status, and required adjustments from supply, return, exhaust, and outside air flows.
About this calculator
This calculator checks whether a building's ventilation system is balanced by comparing air entering the space (Supply Air + Outside Air + Transfer Air) against air leaving it (Return Air + Exhaust Air). Net Air Balance is the difference between the two, and Pressurization is a stepped classification of that difference: positive when Net Air Balance exceeds +50 CFM, negative when it's below −50 CFM, and neutral in between. Supply Air has the largest effect on Net Air Balance and Pressurization -- it drives the balance more strongly than Return Air, Exhaust Air, or Outside Air, since it's the largest airflow at default values and enters the "air in" side directly. At the default flows (5,000 supply, 4,500 return, 500 exhaust, 500 outside, 0 transfer), Net Air Balance is +500 CFM, well past the +50 CFM threshold, so Pressurization reads Positive; trimming Return Air up to 5,200 CFM and Exhaust Air to 800 CFM flips Net Air Balance to −500 CFM and Pressurization to Negative, while a smaller adjustment that lands Net Air Balance at +20 CFM keeps it Neutral.
Transfer Air defaults to zero and has no effect on any output until it's set above zero, since it's simply added into the "air in" total like Supply and Outside Air. Balance Ratio is airIn divided by airOut (Return Air plus Exhaust Air), so 100% means the two sides match exactly; if Return Air and Exhaust Air are both left at zero while Supply Air, Outside Air, or Transfer Air is positive, there is no measured path for air to leave at all, and No Return/Exhaust Configured (1=Yes) flags that case rather than letting Balance Ratio default to a misleadingly "balanced" 100%. This is a simplified commissioning check -- it doesn't model duct leakage, stack effect, or wind-driven infiltration that a full test-and-balance report would capture.
Inputs
ASHRAE 62.1 Table 6.2.2.1: offices 5 CFM/person+0.06 CFM/ft²; classrooms 10 CFM/person+0.12 CFM/ft²
Results
Net Air Balance
500 CFM
Pressurization
1 (1=Positive, 2=Negative, 3=Neutral)
How to Use This Calculator
- Enter Supply Air, Return Air, and Exhaust Air.
- Set Outside Air and Transfer Air.
- Review Net Air Balance (CFM) and Pressurization ((1=Positive, 2=Negative, 3=Neutral)).
- Use Balance Ratio (%) and Adjustment Needed (CFM) to inform your decision.
- If No Return/Exhaust Configured (1=Yes) is flagged, Return Air and Exhaust Air are both zero — Balance Ratio is not meaningful and the space has no measured path for air to leave.
How the result changes with Supply Air
| Supply Air | Net Air Balance | Pressurization |
|---|---|---|
| 2,500 | -2,000 CFM | 2 (1=Positive, 2=Negative, 3=Neutral) |
| 3,750 | -750 CFM | 2 (1=Positive, 2=Negative, 3=Neutral) |
| 7,500 | 3,000 CFM | 1 (1=Positive, 2=Negative, 3=Neutral) |
| 12,500 | 8,000 CFM | 1 (1=Positive, 2=Negative, 3=Neutral) |
What each input means
- Supply Air
- Total supply air delivered to the space from AHU(s).
- Return Air
- Total return air drawn back to the AHU(s) from the space.
- Exhaust Air
- Total exhaust air removed from the building (restrooms, kitchen hoods, etc.).
- Outside Air
- Fresh outside air introduced through the AHU or dedicated unit per ASHRAE 62.1 minimum OA requirements. Offices: 5 CFM/person + 0.06 CFM/ft²; classrooms: 10 CFM/person; retail: 7.5 CFM/person + 0.12 CFM/ft².
- Transfer Air
- Air transferred from adjacent zones through ducts, door undercuts, etc.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSupply Air = 5000, Return Air = 4500, Exhaust Air = 500, Outside Air = 500 = 5 input(s) provided
- Calculate Net Air BalanceNet Air Balance500 = 500
- Calculate PressurizationPressurization1 = 1
- Calculate Balance RatioBalance Ratio110 = 110
- Calculate Adjustment NeededAdjustment Needed-500 = -500
Engine last updated . Checked against 4 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What input has the biggest effect on the air balance?
Supply Air has the largest effect on Net Air Balance and Pressurization among the five flow inputs, since it's the largest airflow at default values and feeds directly into the "air in" side of the balance -- Return Air, Exhaust Air, and Outside Air all move the result too, but by smaller amounts at these defaults.
What counts as positive, negative, or neutral pressurization?
Pressurization reads Positive when Net Air Balance (air in minus air out) exceeds +50 CFM, Negative when it drops below −50 CFM, and Neutral for anything in between. At the default flows, Net Air Balance is +500 CFM, comfortably past the Positive threshold.
Does transfer air always affect the balance?
Transfer Air is added directly into the "air in" total alongside Supply Air and Outside Air, so it always raises Net Air Balance when set above zero -- but it defaults to zero and has no effect on any output until a nonzero transfer airflow is entered, since adding zero changes nothing.
Is this the same math a certified test-and-balance technician uses to sign off a building?
No -- a certified TAB report under NEBB or AABC procedures requires field traverse readings taken at each duct and diffuser with calibrated instruments, multiple readings averaged per point, and documented tolerances the technician signs off on. This calculator takes the summed CFM figures you already have and runs the arithmetic instantly, which makes it useful for a quick planning-stage gut check before a formal balance or for sanity-checking a completed TAB report, but it isn't a substitute for the certified field procedure itself.
Why does Balance Ratio show 0% instead of 100% when Return Air and Exhaust Air are both zero?
Because reporting 100% in that situation would misleadingly suggest the system is balanced when it's actually in the worst possible state: air is entering through Supply Air, Outside Air, or Transfer Air with no measured path recorded for it to leave the space at all. No Return/Exhaust Configured (1=Yes) flags this condition explicitly so it reads as the severe imbalance it is, rather than blending into a percentage that looks healthy at a glance.
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