Free calculator

Ventilation & carbon filter CFM calculator

Fans are rated in free air, but a carbon filter and every duct bend push the fan up its pressure curve where it moves less. Size from the room, then derate for what's bolted to the fan.

Space & restrictions

LengthftWidthftHeightft

Minutes per full air exchangemin1–3 minutes is the usual grow-space target; heat-heavy rooms aim for 1.

Carbon filter inline (+25%)Long or bendy ducting (+20%)High heat load / hot climate (+10%)

Result

Room volume 104 ft³

Fan rating to shop for

43CFM

= 74 m³/h

Base exchange requirement

35CFM

One full exchange every 3 min

Pick a carbon filter rated at or above the fan's CFM — an undersized filter chokes the fan and lets odor slip through at high flow.

Percentages are rules of thumb, not duct engineering. A speed-controlled fan one size up beats an exact-size fan at full throttle — quieter, and you keep headroom for a clogged filter (they load up over months). Running CO₂ enrichment? Exhausting during lights-on throws that money away.

Sealed room instead of vented? Size the AC → and the CO₂ system →