Outdoor Air Fraction Calculator
Estimate how much outdoor air an air handler is bringing in from three temperatures — return, mixed and outdoor air — and the outdoor airflow that implies.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Outdoor air fraction
- 20%
- Outdoor airflow
- 800CFM
About 20 % outdoor air (800 CFM).
- Change per 1 °F of mixed-air sensor error
- 5%
Show the working
- OAF = (MAT − RAT) ÷ (OAT − RAT)(79 − 75) ÷ (95 − 75) = 0.2
- Sensitivity = 1 ÷ |OAT − RAT| per °F100 ÷ 20 = 5 percentage points per °F
- Outdoor airflow = OAF × supply airflow0.2 × 4,000 = 800 CFM
The formula
Mixed air is a blend of return and outdoor air, so its temperature sits between them in proportion to the blend: OAF = (MAT − RAT) ÷ (OAT − RAT). This is the formula in Pacific Northwest National Laboratory’s Re-tuning Measure Guide, which recommends it over assuming the damper position equals the outdoor air share.
Multiply by the supply airflow to estimate outdoor airflow, and compare with the ventilation the space needs.
Getting a reliable reading
- Take readings when outdoor and return air differ by a good margin; the calculator shows how many percentage points each degree of sensor error is worth
- Mixed air is often stratified: average several points across the mixing box
- If the mixed air is not between return and outdoor temperatures, a sensor is wrong or misplaced
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Return air temperature (RAT)
- 75 °F
- Outdoor air temperature (OAT)
- 95 °F
- Mixed air temperature (MAT)
- 79 °F
- Supply airflow (optional)
- 4,000 CFM
Result
- Outdoor air fraction
- 20 %
- Change per 1 °F of mixed-air sensor error
- 5 %
- Outdoor airflow
- 800 CFM
About 20 % outdoor air (800 CFM).
Assumptions
- Mixing by temperature: the mixed air temperature is the airflow-weighted average of outdoor and return air (sensible energy balance), so OAF = (MAT − RAT) ÷ (OAT − RAT).
Limitations
- Mixed-air sensors are often in stratified air and read wrong; average several points.
- Use temperature readings only when outdoor and return air differ by a good margin.
Frequently asked questions
How do I calculate percent outdoor air?
Subtract return air temperature from mixed air temperature, and divide by outdoor minus return. With return 75 °F, outdoor 95 °F and mixed 79 °F, outdoor air is 20 %.
Why not just read the damper position?
Damper position rarely equals the outdoor air share; the relationship depends on the damper, linkage and pressures. The temperature method measures the result.
Can I use it on a mild day?
Only with care. When outdoor and return air are within a few degrees, small sensor errors swing the answer widely.
Sources
- Re-tuning Measure Guide (PNNL-SA-183964), April 2023 — Pacific Northwest National Laboratory. The outdoor air fraction can be calculated as OAF = (MAT − RAT) ÷ (OAT − RAT), or approximated by the damper command; use it to find each AHU’s outdoor airflow.
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
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