Work out duct static pressure instantly with clear inputs, formula shown and shareable results.
External static pressure is the friction the fan must overcome. Straight duct and fittings are combined into a total effective length, multiplied by the friction rate per 100 ft, then added to the fixed losses of coil, filter and terminals. On typical residential systems fittings contribute more than the straight duct does.
Total effective length
TEL = straight length + sum of fitting equivalent lengths
Static pressure
ESP = friction rate x TEL / 100 + component losses
TEL = straight length + sum of fitting equivalent lengths. External static pressure is the friction the fan must overcome.
Most residential air handlers are rated at 0.5 in.wg total external static. Exceeding the rated value cuts airflow below design and can freeze the coil.
This calculator takes 4 inputs: Longest straight duct run, Fitting equivalent length, Friction rate, Coil, filter and grille losses. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.