Inline Fan CFM Calculator

A fan's rated CFM is measured in open air, but pulling exhaust through activated-carbon filter media adds real airflow resistance that reduces how much air the same fan actually moves. This calculator adds a flat 25% to Required CFM when a Carbon Filter is selected so you size a fan powerful enough to still hit your target exchange rate despite that added resistance, rather than assuming the fan compensates on its own.

Ducting Length adds both a straight friction-loss factor (5% per 10 feet) and an additional bend-resistance factor (20% per 10 feet, assuming roughly one 90-degree bend per that length) since longer runs increase resistance and most real installations aren't perfectly straight. If your actual duct run has fewer bends than that assumption, treat the Required CFM figure as a conservative upper estimate rather than an exact calculation for your specific layout.

Recommended Fan Size reports the duct diameter (4, 6, 8, or 10 inches) that inline fans in your calculated CFM range typically come in, based on common manufacturer sizing tiers -- it's a shopping starting point, not a rating for a specific fan model. Always check a specific fan's actual rated CFM against your Required CFM figure before buying, since output varies by brand and model even within the same duct diameter.

A fan's actual airflow output drops as the static pressure it works against rises, so its rated CFM (measured at zero resistance) overstates what it will deliver once connected to ducting and a carbon filter. Est. Static Pressure gives you a rough figure to check against a fan's performance curve (if the manufacturer publishes one) to confirm it can still hit your Required CFM target under real load, not just on paper.

Sizing modestly above the calculated figure is generally safer, since Required CFM already includes estimated resistance factors that may not match your exact setup, and a fan's real-world output under static pressure typically falls short of its open-air rating. Buying to the exact number leaves no margin if your ducting has more bends or your filter has more resistance than this calculator's estimates assume.