How this fan specific power and motor calculator works
Stopping at the fan power makes systems hard to compare and does not pick a motor. This page gets the air power and the fan shaft power from the flow and the static pressure, then the specific fan power with the drive efficiency included and the motor horsepower required with the margin added, and picks a rating by rounding up through the list of nameplate sizes you enter. Flow is entered in cfm and static pressure in inches of water column by default, with m³/h and pascals available in the unit menus.
The relations applied are air power = flow x static pressure, fan shaft power = air power / fan efficiency, motor shaft power = shaft power / drive efficiency and specific fan power = motor shaft power / flow. The required power is that figure multiplied by the margin. Inputs are converted to SI internally (1 cfm = 0.000471947 m³/s, 1 in. w.c. = 249.0889 Pa, 1 hp = 745.7 W), so brake horsepower lands within about 0.2% of the familiar cfm x in. w.c. / (6,356 x efficiency) rule. Results show horsepower first with kilowatts in parentheses, and specific fan power in W/cfm with W per m³/s alongside. Figures reflect October 2026.
The motor margin, the drive efficiency and the list of ratings were not confirmed from an authoritative source, so no figures are supplied and all three are entered by you. For the rating list, type the motor sizes you can actually buy, for example the common NEMA horsepower steps 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20 and up. Duct pressure loss, speed changes through the fan laws and inverter savings are not covered. This calculator is for reference and does not replace design. Review by a qualified engineer is required.
Frequently asked questions
It puts the electricity used to move a given volume of air into one number, so systems can be compared directly. Shaft power rises with static pressure and falls with efficiency, and dividing by the flow makes those effects comparable.
It depends on the maker selection guide or your own design standard. No authoritative margin was confirmed, so none is supplied here and you enter the value from the standard you apply.
From the horsepower list you enter, the smallest rating at or above the required horsepower is picked, rounding up. For example 7.34 hp required picks 7.5 hp. Leave the list blank and nothing is picked, and if the required power is above the largest entry the page says so.