Outside Air Percentage Calculator
OA fraction of an AHU from the classic three-temperature field formula.
Three thermometers prove whether the AHU is actually bringing in its design ventilation — or whether the economizer damper is stuck. Works best when |T_out − T_return| > 5 °C.
Formula
Outside Air Percentage Calculator is a free outside air percentage for HVAC designers, technicians and energy auditors — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Outside Air Percentage Calculator
OA fraction of an AHU from the classic three-temperature field formula. The calculation implements %OA = (T_mix − T_return)/(T_out − T_return) × 100 (ASHRAE 111; commissioning field practice). Three thermometers prove whether the AHU is actually bringing in its design ventilation — or whether the economizer damper is stuck. Works best when |T_out − T_return| > 5 °C.
How to use Outside Air Percentage Calculator
- 1Enter Mixed air temperature in °C.
- 2Enter Return air temperature in °C.
- 3Enter Outdoor air temperature in °C.
- 4Read Outside air fraction instantly — no submit button needed.
Why use Outside Air Percentage Calculator?
- ✓Implements the standard formula — %OA = (T_mix − T_return)/(T_out − T_return) × 100
- ✓Reference cited on-page: ASHRAE 111; commissioning field practice
- ✓Live worked example: the substitution recomputes from your numbers
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Outside Air Percentage Calculator use?+
It computes %OA = (T_mix − T_return)/(T_out − T_return) × 100, per ASHRAE 111; commissioning field practice. The formula is displayed under the result along with a worked example substituted with your own inputs.
What should I keep in mind when using this calculator?+
Three thermometers prove whether the AHU is actually bringing in its design ventilation — or whether the economizer damper is stuck. Works best when |T_out − T_return| > 5 °C.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Outside Air Percentage Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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