Outdoor and return air fractions from airflow, or the outdoor air for a target percentage.
| System Type | Typical OA% | Notes |
|---|---|---|
| Minimum ventilation (VAV) | 10–20% typical | Set by the ASHRAE 62.1 OA CFM, not a percentage; the fraction rises as VAV supply turns down |
| Economizer, partial | 20–70% | OA cooler than return; free cooling assists the coil |
| Economizer, full | Up to 100% | Cool, dry OA; high-limit control locks out warm or humid OA |
| 100% OA (DOAS, most labs) | 100% | No recirculation; lab air is normally exhausted, not returned |
| Heating mode | Minimum OA | Hold OA at the ventilation minimum to limit heating load |
| Dedicated OA unit | 100% | Supplies only ventilation air; separate recirculation |
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →This OA / return air percentage calculator splits a supply airstream into its outdoor and recirculated fractions. Enter outdoor air CFM and total supply CFM to get the percentages, or enter a target OA percentage and total CFM to find the required outdoor air CFM. Optional outdoor and return temperatures add the resulting mixed air temperature and an economizer check.
Outdoor air percentage drives ventilation compliance, coil loads, and free-cooling potential. The tool shows the split on a gauge, flags low outdoor air against typical ASHRAE 62.1 minimums, and highlights when cooler outdoor air offers an economizer opportunity.
| Quantity | Equation |
|---|---|
| OA fraction | OA% = (OA CFM ÷ Total CFM) × 100 |
| Return fraction | RA% = 100 − OA% |
| OA CFM from target | OA CFM = (target% ÷ 100) × Total CFM |
| Mixed air temp | MAT = OA‑frac · T‑OA + (1 − OA‑frac) · T‑RA |
Outdoor air percentage is a simple flow ratio. Mixed air temperature is the flow-weighted average of the outdoor and return streams — the adiabatic mixing relationship from ASHRAE Fundamentals Ch. 1. Strictly the streams mix by mass, so the CFM values are treated as standard-air flows (proportional to mass); the small difference in moist-air specific heat between the streams is neglected (well under 0.1 °F). If your CFMs are actual airflows at very different temperatures — subzero outdoor air, for example — the true mixed temperature is a few degrees colder than this result; use the Mixed Air Temperature tool, which can mix actual CFM by mass. Minimum outdoor air is governed by the ASHRAE 62.1 breathing-zone requirement (Vbz = Rp·Pz + Ra·Az), not a fixed percentage, so treat the 15 percent flag as a prompt to verify, not a code limit.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.