Ductwork Pressure Loss Calculator

No. System CFM is collected as reference information for your records, but none of the pressure-loss calculations (Duct Friction Loss, Fitting Loss, Total Static Pressure, or External Static Pressure) read it — this calculator estimates pressure loss from equivalent duct length, friction rate, and fitting counts rather than from airflow velocity, so CFM does not change any output here.

External Static Pressure equals Total Static Pressure minus the filter and coil losses, which are fixed at 0.10" WC and 0.20" WC respectively regardless of any input — a constant 0.30" WC gap. Total Static Pressure is what the full air-handling system experiences; External Static Pressure excludes the internal filter/coil losses because that's the figure blower fan curves are typically rated against.

Fitting Loss is simply Number of Elbows times Loss per Elbow, plus Number of Tees times Loss per Tee — a linear relationship, so each additional elbow adds a fixed increment (typically 0.01-0.03" WC per elbow depending on radius and type) regardless of how many are already in the run. A duct run with twice as many elbows has roughly twice the fitting loss, all else equal.

No — Equipment Loss uses fixed, typical values (0.10" WC for a clean filter, 0.20" WC for a wet cooling coil, and 0.03" WC for a supply grille) rather than figures calculated from any input you provide. A dirty filter or a different coil design can add substantially more pressure drop than this baseline assumes, so treat the equipment-loss portion as a planning placeholder rather than a measurement of your actual system.