Duct Velocity Pressure

Air velocity from velocity pressure or the inverse via V = 4005 x sqrt(VP) for standard air.

Run the calculator

Example

You enter

Solve for velocity
Velocity pressure (in. w.c.) 0.25

You get

Velocity (fpm) 2002.5

Details, formula, and sources

Per ACCA Manual D / ASHRAE Fundamentals.

V = 4005 * sqrt(VP) and VP = (V / 4005)^2, for standard air (0.075 lb/ft^3 at sea level, 70 F). The 4005 constant embeds the standard-air density; at altitude or elevated temperature a density correction applies.

ACCA Manual D / ASHRAE Fundamentals duct-design velocity-pressure relation, by name.

ACCA / ASHRAE licensed; the velocity-pressure identity is public and reproduced in any duct-design text.

Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

Field names used by the API: solve_for, vp_inwc, velocity_fpm

Air density standard air 0.075 lb/ft^3 (sea level, 70 F); the 4005 constant is density-dependentASHRAE Fundamentals