Greenhouse CO2 Enrichment Calculator

Most plants in a closed greenhouse run out of CO2 before they run out of light. Ambient air is roughly 420 ppm, and a healthy crop in full sun can pull that down to 250 or 300 ppm by mid-morning, at which point photosynthesis basically stalls. Bumping CO2 to 1,000 to 1,200 ppm — what greenhouse growers call enrichment — can lift biomass production 20 to 40 percent on the same square footage.

The first dose math: how much CO2 to add to bring a fixed volume from current ppm to target ppm.

CO2_kg = volume_m3 × (target_ppm − current_ppm) × 1.84e-6

The constant comes from CO2 density (1.84 g per liter at standard conditions) and the ppm-to-fraction conversion. A 100 m² greenhouse with a 3 m peak averaging 2.5 m tall is 250 m³. Going from 420 to 1,200 ppm requires 250 × 780 × 1.84e-6 = 0.36 kg of CO2 — about 800 mL liquid CO2 from a tank, or what a small generator burns in 5 minutes.

The much bigger number is the hourly maintenance dose. CO2 leaks out through the cladding, vents, doors, and any active ventilation. A tightly sealed glass greenhouse loses roughly 1 air change per hour through cracks alone. A polytunnel with vents open for cooling can hit 30 air changes per hour, at which point holding 1,200 ppm becomes a losing battle.

maintenance_kg_per_hour = volume_m3 × (target_ppm − ambient_ppm) × 1.84e-6 × air_changes_per_hour

Same 250 m³ greenhouse, holding 1,200 ppm against 420 ambient, with 1 air change per hour: 0.36 kg/h. With vents open at 20 ACH for cooling: 7.2 kg/h. That second number is the reason commercial growers do not enrich during peak summer ventilation — it would cost more in CO2 than the extra growth is worth.

Sources of CO2. Liquid CO2 from a bulk tank is the cleanest, used by most commercial greenhouses. Propane or natural gas burners produce CO2 plus some heat as a useful byproduct, so they double as a heater on cold nights. Compost piles produce CO2 too, but the rate is unreliable and the levels rarely top 600 ppm even in a small space.

Plants do not benefit equally from enrichment. C3 plants (tomatoes, peppers, cucumbers, leafy greens, most flowers) respond strongly. C4 plants (corn, sugarcane) are already efficient at low CO2 and barely benefit. Cannabis is C3 and responds well, which is why enrichment is standard practice in indoor cultivation.

Three safety notes. CO2 above about 5,000 ppm is unsafe for humans for extended exposure; do not enter a recently dosed greenhouse without ventilating first. Burner-based enrichment produces some carbon monoxide, which is acutely dangerous — install a CO detector. And shut enrichment off at night, when plants respire instead of photosynthesizing.