CO₂ Ventilation Estimate Calculator

Concepts behind this calculation

Estimate ventilation using the original occupied steady CO₂ balance,or infer a source-free decay rate from two readings or a log-excess time-series regression.

Occupied steady-state CO₂ balance estimates dilution from a known generation rate and the indoor concentration above outdoor background. It requires stable occupancy, generation, and mixing. An indoor reading taken while the concentration is still rising does not satisfy the same balance.

Source-free decay uses the decline in concentration above a measured constant background. Two readings give a logarithmic decay rate; a series fits log-excess concentration with a free intercept and retains all valid observations. ACH describes model dilution, and room volume converts it to equivalent CFM. Neither a smooth fit nor a high R² verifies the background, mixing, or absence of continuing sources.

How to use this calculator

1. Choose CO₂ ventilation method.
2. For steady balance, enter CO₂ generation per person, Occupants, Indoor steady CO₂, and Outdoor CO₂.
3. For decay, document Decay interval, sensor and background reference, confirm Source-free single-zone decay basis, and enter Measured constant background CO₂ and Room air volume.
4. Enter Initial indoor CO₂, Final indoor CO₂, and Elapsed time between readings, or the complete CO₂ decay readings series; inspect the fit without using it as an exposure target.

Calculation method and applicability

Steady: Q = G / (Cindoor − Cbackground), with compatible units. Two-point decay: ACH = 60 × ln[(Cinitial − B) / (Cfinal − B)] / elapsed minutes. Series: fit ln(C − B) = a + bt by OLS with a free intercept; ACH = −60b. Model CFM = ACH × room volume in ft³ / 60.

G = total generation after accounting for occupants (compatible volume/time); C and B = indoor and background concentration (ppm, converted as needed). In decay fitting b is the log-excess-concentration slope per minute; ACH = −60b.

Input definitions and units

CO₂ ventilation method

Use an occupied steady generation balance or an independently established source-free decay interval. Decay does not require an assumed per-person generation rate. Choices: Occupied steady-state generation balance; Source-free two-point decay; Source-free log-excess regression.

CO₂ generation per person (L/s·person)

An illustrative value only. Activity, body size and population affect generation; supply your own justified rate. Applies to CO₂ ventilation method: Occupied steady-state generation balance.

Occupants (people)

Positive whole-number population under stable, occupied conditions. Applies to CO₂ ventilation method: Occupied steady-state generation balance.

Indoor steady CO₂ (ppm)

A concentration for the diagnostic, not an acceptable or safe target. Applies to CO₂ ventilation method: Occupied steady-state generation balance.

Outdoor CO₂ (ppm)

Measure background outdoors; the example is not a claim about current atmospheric concentration. Applies to CO₂ ventilation method: Occupied steady-state generation balance.

Decay interval, sensor and background reference

Identify the interval, sensor/calibration/placement, measured background, room volume and stable operating conditions. Establish no continuing source or unresolved sink/interzone contribution; the tool records this reference without authenticating it. Applies to CO₂ ventilation method: Source-free two-point decay or Source-free log-excess regression.

Source-free single-zone decay basis

Establish an approximately well-mixed single zone, no continuing CO₂ generation or other sinks, stable measured replacement/background CO₂ and unchanged ventilation throughout the interval. No gas-release procedure or exposure target is provided. Choices: Not established; Source-free interval, mixing and stable background/operation established. Applies to CO₂ ventilation method: Source-free two-point decay or Source-free log-excess regression.

Measured constant background CO₂ (ppm)

Measured replacement-air/background concentration applicable throughout the interval. The 400 ppm example is a constructed assumption, not a current atmospheric value or automatic fallback. Every indoor reading must exceed it. Applies to CO₂ ventilation method: Source-free two-point decay or Source-free log-excess regression.

Room air volume (ft³)

Air volume of the same approximately mixed zone. Used only to convert inferred ACH to model-equivalent CFM; volume uncertainty is not included in regression statistics. Applies to CO₂ ventilation method: Source-free two-point decay or Source-free log-excess regression.

Initial indoor CO₂ (ppm)

First representative indoor concentration after the source-free interval starts. Must exceed measured background; an occupied trace with continuing generation is unsuitable. Applies to CO₂ ventilation method: Source-free two-point decay.

Final indoor CO₂ (ppm)

Must be below the initial concentration and above background. Values at or below background are rejected rather than clamped to create a logarithm. Applies to CO₂ ventilation method: Source-free two-point decay.

Elapsed time between readings (min)

Strictly positive actual elapsed minutes, not clock-hour notation. A flat or rising trace does not yield positive decay ACH under this model. Applies to CO₂ ventilation method: Source-free two-point decay.

CO₂ decay readings (minutes, ppm)

Enter 3–200 rows: elapsed minute, indoor CO₂ ppm. Times must strictly increase and each reading must exceed background. Nonuniform spacing and a nonzero time origin are allowed. All valid rows remain; local rises or plateaus are counted, not removed. Applies to CO₂ ventilation method: Source-free log-excess regression.

How to interpret the result

Interpret steady results through the generation assumption and decay results through the source-free interval. Inspect residuals and operating conditions before comparing inferred dilution with independent airflow measurements.

Worked examples

Assumptions and limits

• The original steady mode requires stable generation/occupancy and a well-mixed single zone. Per-person generation is supplied by the user; transient storage and other sources, sinks or interzone transport are not resolved.
• Decay requires no continuing CO₂ generation, stable measured replacement/background concentration, constant room volume and unchanged ventilation. No 400 ppm fallback or exposure threshold is adopted.
• Every decay reading must exceed background and lie within 0–999,999 ppm. These concentration-domain guards are not health or acceptable-CO₂ criteria. Near-background differences strongly amplify error.
• Two-point decay requires a strict concentration decrease and positive elapsed minutes. It does not produce R², residual variance or regression standard error from only two readings.
• Series mode uses all 3–200 valid rows, strictly increasing nonnegative times and an unweighted free intercept. It does not sort, discard outliers or force the first observation onto the curve. Nonuniform spacing and local rises/plateaus remain if the overall log slope is negative.
• Flat/increasing trends and unusable or out-of-domain fitted states are withheld; this does not establish zero outdoor ventilation. R², conditional standard error and residuals do not certify assumptions or measurement validity.
• OLS standard error assumes independent, equal-variance log residuals. Sensor calibration/lag, serial correlation, background/volume uncertainty, spatial mixing and varying flows are excluded. This is not complete field uncertainty, a confidence band or ASTM test certification.
• Outdoor air exchange can be inferred only when replacement/background, source and interzone assumptions support it. The tool cannot separate mechanical supply, infiltration or exhaust pathways. Ordinary particle filters and their CADR are not measured by CO₂ decay.
• CO₂ alone cannot certify air safety, infection risk, odor control, current-code compliance or control of all contaminants. No minimum ventilation rate, gas-release test procedure or exposure instruction is provided.

Sources

• NIST: Quick Indoor CO₂ (QICO₂) ToolGovernment context for CO₂ ventilation modeling and its limitations. The original steady method is retained; this site does not implement full QICO₂ or validate measurement conditions.
• Harvard Healthy Buildings: How to assess classroom ventilation (2020 guide)Printed page 20: 2,278 to 1,053 ppm, background 400 ppm, over 17 min 20 s. The continuous 3.656719 ACH matches the rounded 3.7. No clinical/code target, gas-release procedure or source-room CFM is implemented.
• California Department of Public Health: ventilation/filtration technical guidancePrinted page 29 gives decay ACH and volume-derived CFM. Page 32 lists outdoor readings of 443/449 ppm but an average of 441. The discrepancy and separate explicit-background cases are documented; historical code limits and background fallback are not adopted.
• USGS: Statistical Methods in Water Resources, Chapter 9Printed page 227, Table 9.1: free-intercept OLS, R², residual variance and slope standard error. Used as mathematical regression basis, not a gas-test validity criterion or full sensor uncertainty method.
• Healthwavy: CO₂ Decay Ventilation AnalyzerActual timestamp/background/volume form and regression task observed. Custom output was not confirmed: the first capture showed the default 3 ACH result and a second request timed out. No custom peer numerical validation is claimed.