Connect your indoor CO₂ target with outdoor airflow
A large airflow number on a ventilation quote does not explain whether it serves your room and occupancy assumptions.
This calculator uses a confirmed per-person CO₂ generation rate, outdoor concentration and your chosen indoor target to estimate the outdoor airflow required by a single-zone steady-state model.
Compare that requirement with two alternative measured or design outdoor airflows, and save the assumptions for a supplier discussion.
The target is your planning input; the tool does not prescribe a health threshold or a statutory minimum.
The model assumes well-mixed air and constant conditions.
Start with your own records or explicitly load the hypothetical example; the initial form supplies no recommended concentration or generation rate.
Particle-cleaner CADR and outdoor ventilation differ
Clean-air delivery for particles
Particle-cleaner CADR relates to particle removal, including filtering room air and returning it to the room.
Do not substitute that number, or a recirculating fan flow, for fresh outdoor airflow in a CO₂ balance.
Use the separate CADR purchase-planning calculator for particle-cleaner combinations, and enter outdoor air serving this zone here.
Outdoor air reaching this zone
Total fan flow and outdoor airflow may differ, especially in systems mixing outdoor and recirculated air or serving multiple rooms.
Verify the outdoor share allocated to this room under the relevant duct pressure, filter condition and operating mode.
The output preserves the distinction between design and measured evidence.
Inputs and evidence
Occupants and generation per person
Enter the simultaneous occupant count as an integer, and the gas-volume CO₂ generation rate for the stated activity conditions.
Choose L/h, L/s or m³/h to match the reference.
Record the activity and generation source, then confirm that you checked them.
If occupant groups differ in activity or characteristics, review whether one representative rate is appropriate or calculate them separately.
The tool does not generate an activity-based recommended rate.
Outdoor and target CO₂
Both values are ppmv: parts per million by gas volume.
Use an outdoor measurement or design record and your chosen target.
The difference is target minus outdoors.
A target equal to or below outdoors cannot produce a requirement under this dilution equation.
The optional note can record measurement time, location and the target document or decision maker.
Volume and sensitivity
Enter one zone in m³, typically floor area × height, rather than m² or an entire property area.
Sensitivity changes the base generation rate down and up by your percentage.
±20% creates 80%, 100% and 120% generation scenarios.
It is not a confidence interval, sensor accuracy allowance or recommended equipment reserve.
Candidates A and B
Each candidate is a separate alternative serving the same zone; their flows are not added.
Enter m³/h or L/s, select design or measured evidence and record the reference and operating conditions.
A blank flow or missing evidence withholds only that comparison; the requirement and the other complete candidate remain available.
Blank does not mean zero supply.
Supported form ranges are 0–10,000 integer occupants; 0–1,000 generation in the selected unit; 0–1,000,000 ppm; 0.01–1,000,000 m³; 0–100% sensitivity; and 0–1,000,000,000 candidate airflow in the selected unit.
These are input-processing limits, not physiological or design recommendations.
Changing a unit does not convert the entered number: re-enter the number in the selected unit.
Steady-state mass balance and unit conversion
Required outdoor airflow
G = N × g
Q = G / ((Ctarget − Cout) × 10^-6)
ACH = Q / V
Css = Cout + (G / Qc) × 10^6
N is occupant count; g is generation per person; G is total generation; Q is required outdoor airflow; V is zone volume; Qc is candidate outdoor airflow; and Css is its steady-state indoor concentration.
Use m³/h for both generation and airflow.
Multiply L/h by 0.001, or L/s by 3.6, to obtain m³/h.
A gas mass rate in kg/h cannot be entered as a volume rate.
Convert the ppm difference to a volume fraction
A 600 ppm difference is 0.0006.
Dividing 0.036 m³/h by 0.0006 gives 60 m³/h.
Dividing directly by 600 creates a factor-of-one-million unit error.
Display rounding occurs after calculation, and surplus or shortfall is assessed before rounding.
Step-by-step use
- Define one zone. Verify occupancy and volume; avoid merging separate rooms or different occupancy conditions.
- Check generation evidence. Record activity, document, number and gas-volume unit, then select the confirmation checkbox.
- Enter concentrations. Use ppm consistently, verify a positive target-minus-outdoor difference, and set your generation sensitivity.
- Enter candidate evidence. Record the outdoor airflow actually serving the zone, its design or measured basis and operating conditions.
- Read base and upper scenarios. A candidate with base surplus can still have an upper-generation shortfall.
- Save the review. Download TXT or print the assumptions and pending states, and bring the review with measurements and airflow records to your supplier.
Hypothetical example: two occupants, 0.036 m³/h total
Assume 18 L/h per person, two occupants, 420 ppm outdoors, a 1,020 ppm target and 60 m³ volume.
The 600 ppm difference requires 60 m³/h, approximately 16.667 L/s total or 8.333 L/s per person.
Required outdoor-air ACH is 1 h⁻¹.
These are demonstration inputs, not recommended generation, targets or safety thresholds.
Hypothetical ±20% generation, outdoor airflow requirement and candidate CO₂| Generation scenario | G (m³/h) | Required Q (m³/h) | A 50 m³/h CO₂ (ppm) | B 72 m³/h CO₂ (ppm) |
|---|
| Lower 80% | 0.0288 | 48 | 996 | 820 |
| ★ Base 100% | 0.036 | 60 | 1,140 | 920 |
| Upper 120% | 0.0432 | 72 | 1,284 | 1,020 |
A is a hypothetical measured flow of 50 m³/h, 10 m³/h below the base requirement.
B is a hypothetical design flow of 20 L/s, converted to 72 m³/h.
Its base surplus is 12 m³/h, but it equals the upper-generation requirement.
Design and measured labels describe the evidence type; a design surplus is not a field measurement.
Generation sensitivity and the role of volume
Generation changes the requirement
At the same concentration difference, doubling total generation doubles required outdoor airflow.
Evaluate higher activity or occupancy as separate operating conditions.
At 100% sensitivity, lower generation is zero; this does not remove other pollution sources or operational ventilation requirements.
At 0% sensitivity, all three displayed scenarios have the same values.
Volume changes outdoor-air ACH
With the same occupants, generation and concentration difference, volume does not change Q in this steady-state equation.
Doubling volume halves ACH for the same Q.
Volume and occupied duration matter to the approach toward steady state, but this tool does not calculate a concentration-versus-time curve.
Do not substitute particle-removal equivalent ACH for outdoor-air ACH.
Read flow margins and predicted concentration carefully
Margin is candidate flow minus required flow: positive means surplus under the inputs, negative means shortfall, and zero means equal.
The shortfall column shows the magnitude of a negative margin, and margin percentage divides margin by the requirement.
For a 60 m³/h requirement and 50 m³/h candidate, margin is −10 m³/h, shortfall is 10 m³/h and percentage is approximately −16.67%.
The tool does not rank equipment or decide a purchase.
Blank and zero have different meanings
A blank candidate flow means the comparison lacks information.
A documented zero means no outdoor supply; its steady concentration is not calculated, and the dash is neither current nor outdoor concentration.
A zero requirement has no margin percentage, and a zero-occupant scenario has no per-person flow.
Requirements above the supported 1,000,000,000 m³/h limit are withheld for a review of generation, units and the concentration difference.
Candidate CO₂ is the steady-state prediction if the stated generation and outdoor airflow continue.
It is not an immediate sensor reading or a prediction of a briefly observed peak.
A surplus does not assess all pollutants, health effects, infection risk or code compliance.
Home and small-office scenarios
Before replacing a home ventilator
Document the actual occupant group and activity for one room.
Compare a current measurement with a replacement design on the same unit basis.
Check room allocation and operating conditions before relying on total rated fan flow, then use the worksheet to discuss the measurement plan.
Changing meeting-room occupancy
Calculate normal and maximum occupancy separately.
Short meetings or frequent occupancy changes may not achieve steady state, so do not equate the result with a momentary measurement.
Save the activity, occupant count and outdoor concentration with the result, and review applicability against the operating record.
A and B should represent alternatives for the same zone.
When evidence comes from different rooms or operating periods, document those differences first.
A ventilation airflow search or product listing still needs actual outdoor-air data and an installation review before a purchase decision.
Measurement tips and model limits
- Record measurement time, position, calibration, occupancy and activity together.
Review whether a local reading influenced by breathing or another nearby source represents the zone.
- Record equipment mode, windows and doors, filters and duct conditions.
Verify delivered outdoor airflow separately from a nameplate rating.
- Check whether generation, occupancy, outdoor CO₂ and airflow were sufficiently constant.
Readings just after occupancy starts or during a short interval are not automatically steady-state measurements.
- Review uneven mixing, air transport between zones, combustion and process CO₂ separately.
The calculator does not add those effects or model multiple zones.
A numerical match does not establish a suitable installation or approval.
Duct pressure losses, thermal loads, noise, other pollutants and jurisdiction-specific facility requirements are outside the calculation.
Recalculate when evidence or operating conditions change.
Frequently asked questions
Is the 1,020 ppm example a safety threshold?
No.
It is a hypothetical target chosen to demonstrate a 600 ppm difference.
It is not a statutory limit or health threshold.
Verify the source of your actual target.
Can I enter particle-cleaner CADR as outdoor airflow?
No.
Particle CADR and recirculating flow are not outdoor-air intake.
Use measured outdoor airflow reaching the zone or design outdoor airflow for the relevant conditions.
Is checking the generation confirmation box enough?
Activity conditions and a generation reference are also required.
Confirmation records your review; it does not mean the tool certifies the source or recommends an activity-based rate.
Does a larger room require more steady-state airflow?
Q is unchanged when occupants, generation and concentration difference are unchanged.
A larger volume lowers ACH for the same flow.
Time-dependent concentration is outside this tool.
Can measured and design data be compared together?
Yes, with their evidence type and reference recorded.
Do not treat design surplus as measured performance.
Verify delivered outdoor airflow at actual operating conditions with your supplier.
Is ±20% a recommended equipment reserve?
No.
It is hypothetical generation sensitivity, not a confidence interval or recommended reserve.
Enter a range appropriate to the evidence you are reviewing.
Does zero occupancy mean ventilation is unnecessary?
No.
It means zero occupant generation in this model.
Other sources, operating requirements and facility rules remain separate.
If candidate airflow is also zero, no steady concentration is calculated.
Does saving the review send an enquiry?
TXT saves inputs, references, calculations and pending states as a local file.
Printing is also available.
Share it with your supplier alongside measurements; the calculator does not infer an enquiry or application completion.
Official references and your next review
Method checked 2026-10-02
The publication date and current verification date are different.
The tool does not apply US ventilation tables or Korean statutory ppm defaults.
Recheck method revisions, generation references and equipment outdoor-air records during future maintenance.
Prepare a worksheet with its assumptions
Collect generation evidence, occupancy records, outdoor CO₂, target rationale and candidate airflow together.
Review upper-generation shortfalls and pending evidence, then save TXT or print it for a supplier discussion with the measurement and airflow records.
Return to calculator inputs