Carbon Filter Sizing Calculator

Carbon filter sizing starts from the assumption that you want to exchange all the air in your grow space roughly once a minute, so this calculator's baseline required CFM tracks room volume directly, before any adjustment. Odor level then scales that baseline up from the Low-odor baseline across its three tiers (moderate and high odor levels multiply the base CFM requirement upward -- there is no tier that reduces it below the Low baseline, since stronger-smelling flowering plants need more carbon media contact time to fully scrub odor from the exhaust air) -- and because that multiplier applies across the entire baseline, switching odor level from low to high can move required CFM by more than a modest change in room volume would. Plant count adds a smaller flat CFM allowance on top to account for the extra volatile compounds more plants produce.

Recommended filter diameter is chosen from standard commercial filter sizes (4, 6, 8, 10, or 12 inches) at CFM breakpoints, so it only jumps to the next size up once required CFM crosses one of those thresholds -- a small change in room volume or plant count that stays within the same CFM tier won't change the recommended diameter or its replacement cost, even though required CFM itself is still moving continuously underneath. Filter life is estimated separately from filter size: it depends on odor level and plant count (both add to the load the carbon media has to absorb before it's spent) but not on room volume itself, since a bigger room exchanging more air through the same filter media doesn't by itself shorten how long that media lasts in this model. What this calculator doesn't account for: humidity's real effect on carbon media saturation rate, duct static pressure and fan curve matching (a filter rated for a given CFM needs a fan that can actually push that much air through it), and odor differences between plant strains within the same general odor-level category.