Duct Velocity Pressure
Air velocity from velocity pressure or the inverse via V = 4005 x sqrt(VP) for standard air.
Example
You enter
- Solve for velocity
- Velocity pressure (in. w.c.) 0.25
You get
- Velocity (fpm) 2002.5
Checked against published worked examples from: ACCA Manual D / ASHRAE Fundamentals (velocity-pressure relation).
Details, formula, and sources
ScopePer ACCA Manual D / ASHRAE Fundamentals.
Formula or tableV = 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.
Source and editionACCA Manual D / ASHRAE Fundamentals duct-design velocity-pressure relation, by name.
Free accessACCA / ASHRAE licensed; the velocity-pressure identity is public and reproduced in any duct-design text.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namessolve_for, vp_inwc, velocity_fpm
- Air density optional (added 2026-10-09): V = 4005 sqrt(VP x 0.075/rho), the standard-air constant scaled for the entered density (the exact form is 1096.2 sqrt(VP/rho)); 0.075 lb/ft^3 leaves it unchangedASHRAE Fundamentals; air-density-correction
- Air density standard air 0.075 lb/ft^3 (sea level, 70 F); the 4005 constant is density-dependentASHRAE Fundamentals