Greenhouse Ventilation CFM Calculator

Target exhaust airflow

{{ resultsReady ? fmt(result.target_ach) + ' ACH' : '— ACH' }} {{ params.objective === 'summer' ? 'UGA summer reference' : 'Custom target' }}

Length:

Width:

Average interior height:

Use volume-equivalent average height, not the roof peak.

Ventilation objective:

Air changes per hour:{{ params.ach }}

Choose a rate supported by your crop and site requirements.

ACH

Equal fan stages:

Whole stages from 1 to 6; this splits the target, without setting motor speeds.

Candidate total exhaust:

Optional: enter tested airflow at expected static pressure. Use 0 to skip.

Intake opening count:

Optional: identical openings, 0 to skip the geometry check.

Opening width:

Measure each identical intake opening before applying its open-area percentage.

Opening height:

Measure each identical intake opening before applying its open-area percentage.

Open-area percentage:{{ params.free_percent }}

Use the manufacturer’s unobstructed area. This does not model screen resistance.

Metric	Value	Unit	Basis	Copy
{{ row[1] }}	{{ typeof row[2] === 'number' ? fmt(row[2]) : row[2] }}	{{ row[3] }}	{{ row[4] }}

Chart unavailable. The sizing sheet retains the values.

Stage	Added airflow (CFM)	Total airflow (CFM)	Air changes (1/h)	Intake (ft²)	Copy
{{ row[1] }}	{{ fmt(row[2]) }}	{{ fmt(row[3]) }}	{{ fmt(row[4]) }}	{{ fmt(row[5]) }}

Enter a whole stage count from 1 to 6 in Advanced. The primary sizing result remains available.

Moving air around the plants is not the same as replacing greenhouse air. Exhaust ventilation carries indoor air outdoors and draws replacement air through intakes. Circulation fans mix air inside the house, so their ratings cannot be added to the outdoor-air exchange capacity.

For exhaust planning, the enclosed volume connects fan airflow with the rate of air exchange. A tall greenhouse contains more air than a low greenhouse with the same floor area. Its volume-based airflow requirement is therefore larger at the same exchange rate.

Cubic feet per minute (CFM): The volume of air moved each minute. Cubic metres per hour, written m³/h, expresses the same kind of airflow in different units.
Air changes per hour (ACH): The hourly exhaust volume divided by the enclosed greenhouse volume. It describes how many equivalent house volumes pass through the ventilation system each hour.

An exchange rate of 60 ACH corresponds to one house volume per minute. That does not establish that every part of the greenhouse receives the same fresh airflow, or that all the original air has been replaced after one minute. Air entering near an exhaust fan can bypass the growing space instead of travelling through it.

Roof shape matters because the relevant height is the average height of the enclosed space, not the highest point. Treating the roof peak as the height over the entire floor overstates the volume and the airflow associated with a chosen exchange rate.

The inlet also affects whether a fan can deliver its intended flow. Frames and louvers reduce unobstructed opening area, while screens and shutters introduce resistance to airflow. A fan must work against the resulting pressure difference, expressed as static pressure. An airflow rating measured without resistance may be higher than the flow available after installation.

Ventilation alone cannot cool below outdoor dry-bulb temperature, meaning the ordinary outdoor air temperature. Sunlight, shade, crops, humidity and air distribution affect conditions inside the greenhouse. Meeting an air-change reference therefore does not establish that the crop will remain at a suitable temperature.

How to Use This Tool:

Start with the greenhouse volume and the ventilation objective, then add equipment information for the optional comparisons. Results update as you edit.

Enter Length, Width and Average interior height, choosing feet or metres for each measurement. Units may be mixed, and changing a unit converts the existing value. For a simple gable, use eave height plus half the roof rise. For a curved roof, use enclosed volume divided by floor area.
Choose the Ventilation objective. Summer · UGA 60 ACH uses the University of Georgia (UGA) reference of one exchange per minute. Custom air-change target accepts a rate from 1 to 120 ACH, but does not verify its suitability for your crop or season.
Enter Candidate total exhaust under Advanced to compare a proposed fan system with the target. Use its tested total airflow at the expected operating static pressure, in CFM or m³/h. Enter 0 to skip this comparison.
Enter an Intake opening count and the dimensions of each identical opening to check available inlet area. Set Open-area percentage to the manufacturer's unobstructed share of that opening. A count of 0 skips this geometry check.
Set Equal fan stages to a whole number from 1 to 6, then read the cumulative flows in Stage schedule alongside the fan and intake comparisons in Sizing sheet.
A Check inputs summary means a required dimension or custom air-change rate needs correction. Follow the accompanying message. Check optional inputs in a comparison affects only that optional check. An invalid stage count removes the schedule, not the primary airflow result.

Interpreting Results:

Read Fan comparison and Intake comparison separately. A candidate can meet the airflow reference while its intake geometry falls short. Even when both say Meets numeric reference, the calculation has not measured installed airflow or tested the complete fan-and-inlet system.

Intake at target follows the selected airflow target. Intake to support fans uses the larger of that target and a valid candidate airflow. A larger candidate therefore increases the supporting inlet area without changing Target exhaust. A smaller candidate does not reduce the inlet reference below the target-based area.

The Airflow comparison retains the 60 ACH summer reference when a custom target is selected. Its separate target and candidate values allow comparison without turning the custom rate into a summer recommendation. In Stage schedule, added airflow is the equal increment and total airflow is cumulative. The final stage reaches the selected target, not a larger candidate rating.

Before choosing equipment, verify the manufacturer's fan curve at the expected system resistance and the pressure losses of the actual intakes, screens and shutters. Confirm controls, electrical suitability and installed airflow with the installer. A numerical match alone is not a complete ventilation design.

Technical Details:

Air-change sizing specifies an exhaust volume rate from the enclosed volume and a chosen replacement frequency. It does not balance solar heat gain against heat removal, so neither indoor temperature nor cooling capacity can be inferred from the airflow result alone.

Greenhouse volume, exhaust and inlet area

Floor area and volume determine how much air corresponds to one exchange. Multiplying volume by ACH gives an hourly flow; dividing by 60 converts it to a per-minute flow.

\begin{array}{l} F = L \times W \\ V = F \times H \\ Q_{t} = \frac{V \times a}{60} \end{array}

L, W and H are length, width and volume-equivalent average height in feet. F is floor area in ft², V is air volume in ft³, a is the selected ACH, and Q_t is target exhaust in CFM. Summer sizing fixes a at 60, so the numerical CFM value equals the numerical volume in ft³. Custom sizing uses the entered rate instead, without an additional seasonal adjustment.

Length conversion uses exactly 0.3048 metres per foot. Consequently, one square foot is 0.09290304 m², and one CFM is 1.69901079552 m³/h. The flow factor comes from cubing 0.3048 and multiplying by 60. A candidate rating entered in m³/h is divided by this factor before the CFM comparison.

A valid positive candidate airflow, Q_c in CFM, gives an equivalent air-change rate a_c. Its surplus or shortfall is the candidate flow minus the target.

\begin{array}{l} a_{c} = \frac{60 \times Q_{c}}{V} \\ ΔQ = Q_{c} - Q_{t} \end{array}

The inlet reference is 1.5 ft² per 1,000 CFM, or 0.0015 ft² per CFM. A_t is intake area at the target and A_s is intake area to support the larger airflow, both in ft².

\begin{array}{l} A_{t} = 0.0015 \times Q_{t} \\ A_{s} = 0.0015 \times \max (Q_{t}, Q_{c}) \end{array}

With no valid candidate rating, the supporting intake uses the target alone. The same inlet coefficient is applied to custom airflow targets. It remains a summer-derived opening reference, not a separately validated winter inlet method or an evaporative-pad sizing calculation.

Available open area depends on geometry. For n identical openings, each with width w and height h in feet, multiply their combined area by the entered open-area percentage p divided by 100. B is the available open area in ft².

\begin{array}{l} B = n \times w \times h \times \frac{p}{100} \\ ΔA = B - A_{s} \end{array}

The intake surplus or shortfall, ΔA, is compared with the supporting area, not necessarily the smaller target-only area. A 75% open-area entry describes the unobstructed share of the opening. It does not mean the fan will deliver 75% of its rating, and it does not calculate pressure loss.

Equal stages divide the target into N increments. At cumulative stage i, from 1 through N, the schedule reports total airflow, equivalent ACH and matching intake area.

\begin{array}{l} Q_{added} = \frac{Q_{t}}{N} \\ Q_{i} = \frac{i \times Q_{t}}{N} \\ a_{i} = \frac{60 \times Q_{i}}{V} \\ A_{i} = 0.0015 \times Q_{i} \end{array}

This is an arithmetic allocation of target airflow. It does not select a number of physical fans, set motor speeds, calculate a fan-speed curve or choose thermostat settings. Actual inlet opening must be coordinated with the airflow of the equipment operating at each stage.

Comparison boundaries and precision

Fan and intake comparisons use unrounded quantities. Equality counts as meeting the numeric reference. To avoid unit-conversion roundoff creating a false shortfall, a difference within one trillionth of the reference is treated as zero.

G (x, r) = {\begin{cases} 0 & if | x - r | \leq | r | \times 10^{- 12} \\ x - r & otherwise \end{cases}

Here x is candidate airflow or available inlet area, and r is its corresponding reference. The reported surplus or shortfall uses G. This tolerance is only for numerical roundoff, not an allowance for deficient equipment or uncertain measurements.

Meaning and boundary conditions of the fan and intake comparison labels
Comparison label	When it appears
Not entered	Candidate airflow is 0, or intake opening count is 0, for the corresponding check. The comparison is skipped.
Check optional inputs	The candidate entry or active intake geometry is invalid. The primary airflow calculation remains available.
Below numeric reference	A valid, active comparison has an adjusted gap less than 0.
Meets numeric reference	A valid, active comparison has an adjusted gap greater than or equal to 0.

Displayed numbers normally use up to two decimal places. Nonzero values with a magnitude below 0.01 use scientific notation with up to three significant digits. Calculations are not rounded to those display values, and airflow is not rounded up to a purchasable fan size. The same relative tolerance preserves greenhouse dimension bounds through unit conversion.

Accepted input ranges

Length limits below are in feet or the equivalent metres. Endpoints are included unless the lower bound explicitly says greater than zero. These are calculation limits, not recommended greenhouse dimensions or crop ventilation rates.

Numerical ranges accepted for greenhouse dimensions and optional ventilation calculations
Quantity	Lower bound	Upper bound	Condition
Length and width	0.1 ft	3,300 ft	Required measurements.
Average interior height	0.1 ft	100 ft	Required volume-equivalent height.
Custom air-change rate	1 ACH	120 ACH	Decimal rates are accepted.
Candidate total exhaust	0 CFM	100,000,000 CFM	An equivalent m³/h rating is accepted; 0 skips the check.
Intake opening count	0	1,000	Whole numbers only; 0 skips the check.
Each opening dimension	> 0 ft	100 ft	Required when the opening count is positive.
Open-area percentage	> 0%	100%	Required when the opening count is positive.
Equal fan stages	1	6	Whole numbers only.

The opening dimension fields show a minimum of 0.0001 ft, or 0.00003048 m, although the geometry calculation accepts any positive dimension up to the stated maximum. Open-area controls show 1 to 100%, while the calculation also accepts positive percentages below 1%. Blank entries are not substitutes for the explicit 0 used to skip a candidate or opening check.

Installation and cooling limits

UGA's fan-rating reference is approximately ⅛ inch of water static pressure, about 31 Pa. The expected resistance of a particular installation may be higher because of screens, shutters or ducts. No pressure loss or automatic fan derating is calculated here. The candidate airflow must already represent the tested flow at the relevant resistance.

The dimensions and ACH do not describe solar load, shading, crop moisture release, humidity or air distribution. Long or multi-span houses require a distribution design, and a low custom ACH does not establish adequate winter humidity control. Natural ventilation, horizontal airflow (HAF) circulation and evaporative-pad sizing require separate methods. The inlet reference does not specify a cooling-pad area or predict an indoor temperature.

Worked Examples:

Summer reference for the default greenhouse

A greenhouse 30 ft long and 20 ft wide with an average interior height of 8 ft has 600 ft² of floor area and 4,800 ft³ of air volume. At 60 ACH, multiplying 4,800 by 60 and dividing by 60 gives Target exhaust of 4,800 CFM, displayed as 8,155.25 m³/h. Multiplying 4,800 by 0.0015 gives Intake at target of 7.2 ft². With candidate airflow and opening count both left at 0, neither equipment comparison has been completed. These numbers establish a reference to compare with tested equipment, not a verified installation.

A custom target with a larger candidate system

For the same greenhouse, a hypothetical custom target of 30 ACH gives 2,400 CFM and 3.6 ft² of intake at target. A candidate rated at 6,000 CFM at the expected static pressure represents 75 ACH and a 3,600 CFM surplus. Intake to support fans rises to 9 ft², approximately 0.84 m². Two identical openings, each 3 ft by 2 ft with 75% open area, provide exactly 9 ft² and meet that geometric reference. With three equal stages, each increment is 800 CFM, producing cumulative flows of 800, 1,600 and 2,400 CFM, equivalent to 10, 20 and 30 ACH. Their intake references are 1.2, 2.4 and 3.6 ft². The 9 ft² full-candidate intake requirement still applies if the proposed system operates at 6,000 CFM; the target-based stage schedule does not describe that operation.

References:

Greenhouses: Heating, Ventilation, and Cooling, University of Georgia Cooperative Extension, Bulletin 792, revised November 4, 2025.