About

CO₂ enrichment in controlled-environment agriculture increases photosynthetic rate by 20 - 30% when concentrations rise from ambient 420 ppm to the 1200 - 1500 ppm saturation range for C3 plants. Miscalculating injection volume wastes gas (compressed CO₂ tanks at $30 - 60 per refill) or, worse, under-enriches the canopy while ventilation purges the deficit. This calculator solves for the exact volumetric flow rate Q in ft³/hr required to maintain target ppm given your room geometry, air exchange rate, plant uptake coefficient, and injection method. It accounts for the continuous loss term from exhaust fans and passive leakage. Results include compressed-tank duration and propane/natural gas burner sizing. Note: the model assumes well-mixed air volume and steady-state conditions. Stratification, canopy boundary-layer resistance, and humidity-driven stomatal closure are not modeled.

Formulas

The core enrichment equation calculates the volumetric CO₂ flow rate Q needed to maintain target concentration against continuous ventilation losses and plant absorption.

Q = V × C_target − C_ambient1,000,000 × ACH + U_plants

Where Q = CO₂ injection rate in ft³/hr, V = room volume in ft³, C_target = desired CO₂ concentration in ppm, C_ambient = incoming air CO₂ in ppm (typically 420), ACH = air changes per hour (ventilation rate), and U_plants = total plant uptake rate in ft³/hr.

U_plants = n × A_leaf × r × 3600 × 44 × 10⁻⁶ ÷ 1.842

Where n = plant count, A_leaf = estimated leaf area per plant in m², r = photosynthetic uptake rate in μmol CO₂/m²/s, and 1.842 g/L is the density of CO₂ at STP used for volume conversion. The factor 44 is the molar mass of CO₂ in g/mol.

T_tank = W_tank × 8.741Q × H_day

Where T_tank = tank duration in days, W_tank = CO₂ weight in lbs, 8.741 = cubic feet per pound of CO₂ at STP, and H_day = enrichment hours per day.

Reference Data

Growth Stage	Optimal CO₂ ppm	Uptake Rate μmol/m²/s	Light Requirement PPFD	Temp Range °F	Notes
Seedling / Clone	400 - 600	5 - 10	200 - 400	72 - 78	Minimal enrichment needed
Early Vegetative	800 - 1000	12 - 18	400 - 600	75 - 82	Rapid leaf expansion
Late Vegetative	1000 - 1200	18 - 25	600 - 800	78 - 85	High metabolic demand
Early Flowering	1200 - 1500	20 - 30	800 - 1000	78 - 85	Peak CO₂ benefit window
Late Flowering	1000 - 1200	15 - 22	600 - 800	75 - 80	Taper enrichment
Ripening / Flush	400 - 600	8 - 12	400 - 600	70 - 78	Ambient is sufficient

CO₂ Source	Output ft³ CO₂	Per Unit	Cost Range	Heat Byproduct	H₂O Byproduct
Compressed Tank (20 lb)	174.8	per tank	$25 - 45	None	None
Compressed Tank (50 lb)	437.1	per tank	$40 - 70	None	None
Natural Gas Burner	1.0	per ft³ gas	$0.01/ft³	~1000 BTU/ft³	Yes
Propane Burner	21.8	per lb propane	$2.50/gal	~21600 BTU/lb	Yes
Fermentation (Yeast)	Variable	per batch	$5 - 15	Minimal	Yes
Dry Ice (Solid CO₂)	8.741	per lb	$1 - 3/lb	Cooling effect	None

Room Size Example	Volume ft³	CO₂ to reach 1200 ppm ft³	Hourly need (1 ACH) ft³/hr
2×2×5 (Small tent)	20	0.016	0.016
4×4×7 (Standard tent)	112	0.087	0.087
5×5×8	200	0.156	0.156
8×8×8	512	0.399	0.399
10×10×8	800	0.624	0.624
10×20×10 (Commercial)	2000	1.560	1.560
20×40×12 (Warehouse)	9600	7.488	7.488

Frequently Asked Questions

Each air change replaces the entire enriched volume with ambient air at approximately 420 ppm. If your exhaust fan cycles 2 ACH, you lose twice the room volume of enriched air per hour. The calculator's ACH term directly scales injection rate to compensate. To reduce waste, run CO₂ injection only when exhaust fans are off, or use a sealed room design with air conditioning instead of exhaust ventilation.

OSHA sets the 8-hour permissible exposure limit at 5000 ppm and the short-term exposure limit at 30,000 ppm. Plant benefit saturates around 1500 ppm for most C3 species. Levels above 2000 ppm offer no additional photosynthetic gain and waste gas. Always use a CO₂ controller or monitor with an alarm set at 1500 ppm.

RuBisCO's carboxylation rate increases with temperature up to approximately 85 °F (29 °C). At higher temperatures, photorespiration increases and competes with CO₂ fixation. Enriching to 1200 - 1500 ppm suppresses photorespiration, allowing plants to tolerate 5 - 10 °F higher temperatures than at ambient CO₂. This calculator assumes steady-state temperature within the recommended range for each growth stage.

Compressed tanks produce pure CO₂ with no heat or moisture byproduct. They suit sealed rooms under 500 ft³. Natural gas and propane burners produce CO₂ cheaply at scale but add approximately 1000 BTU per cubic foot of natural gas burned, plus water vapor. In rooms over 1000 ft³, burners are more cost-effective but require additional cooling and dehumidification capacity.

Each plant absorbs CO₂ at a rate proportional to its illuminated leaf area and photosynthetic capacity (measured in μmol/m²/s). A dense canopy of 20 large plants with 0.5 m² leaf area each, uptaking at 25 μmol/m²/s, consumes roughly 0.042 ft³/hr of CO₂. This absorption term is additive to the ventilation loss term in the formula.

No. Photosynthesis halts without light. Plants respire in the dark, releasing CO₂ rather than absorbing it. Injecting CO₂ during the dark period wastes gas entirely and raises room concentration to no benefit. Configure your CO₂ controller to operate only during the photoperiod. This calculator's hours-per-day input should reflect only illuminated hours.