CO2 Grow Room Calculator
Enriching carbon dioxide only pays once light is no longer the limiting factor.
The formula
DLI = PPFD * hours * 3600 / 1,000,000
Intensity times time
A light meter reads instantaneous intensity in micromoles of photons per square metre per second. What a plant responds to is the total over a day, and multiplying by the seconds of light and dividing by a million converts one to the other. The convenient number to remember is that 11.57 µmol/m²/s maintained for a full 24 hours gives a DLI of exactly 1.
You can trade intensity for hours, up to a point
The same daily total can come from bright light for a short photoperiod or dimmer light for longer, and for many crops the result is similar. The limits are biological rather than arithmetic: short-day plants flower on photoperiod regardless of total light, and every plant needs a dark period.
What the numbers mean outdoors
Full summer sun in a temperate climate delivers a DLI around 30 to 60. A bright windowsill might manage 4. That gap — a factor of ten or more — is why houseplants are almost all shade species from forest floors, and why growing tomatoes indoors takes serious lighting rather than a lamp.
Rules of thumb, and the field
Every figure here describes an average season on an average site. Soil type, aspect, drainage, variety and weather each move the answer by more than the arithmetic does, and a grower who has kept records for a few years on one piece of ground knows it better than any formula. Use these to plan an order or size an implement, then trust what the ground has actually done before.