What velocity pressure is, and where 4005 comes from
Moving air carries pressure in two forms: static pressure, which pushes outward in all directions and is what a duct system resists, and velocity pressure, which exists only because the air is moving in a direction. Velocity pressure is what a Pitot tube traverse actually measures at a point in a duct, and it converts cleanly to velocity through Pv = (V/4005)2, or the other way, V = 4005 × √Pv. The constant 4005 comes from standard air density, the same standard-air assumption (ρ = 1.204 kg/m³) used throughout the duct sizing pages on this site, it isn't a separate or arbitrary number, it's derived from the same physical constant everything else here already relies on.
Velocity, static, and total pressure
Total pressure in a duct is the sum of static and velocity pressure. Static pressure is what a system's resistance (duct friction, fittings, filters) fights against, and it's what most duct sizing and fan selection is built around. Velocity pressure only matters where you actually need to know or measure air speed, most commonly during a Pitot traverse to verify actual CFM in an existing system, or when working backward from a measured pressure to figure out what velocity produced it. The two aren't interchangeable, but they're related by the same simple formula in both directions.
The fan affinity laws, in plain terms
A fan connected to a fixed duct system behaves predictably when only its speed changes: airflow scales directly with RPM (first power), static pressure scales with the square of the speed ratio, and power scales with the cube. Doubling a fan's speed doubles its airflow, but quadruples the static pressure it develops and requires eight times the power. That cubic power relationship is the one worth remembering: a fan running at 80% speed only moves 80% of the air, but draws only about half the power (0.83 ≈ 0.51), which is why modest speed reductions, via a variable-frequency drive or a multi-speed motor, produce disproportionately large energy savings.
The fan affinity laws predict a new operating point only when the system the fan is pushing against hasn't changed, the same ductwork, the same fittings, the same damper positions, between the baseline point and the new speed. This is the most common way the laws get misapplied: if you also open a damper, add a duct run, or otherwise change the system's resistance, the fan doesn't just move along the same relationship, it settles at a genuinely different point on its performance curve, one these three formulas alone can't predict. Use the affinity laws to answer "what happens if I only change the speed," not "what happens if I change the speed and something else too."
How these numbers are derived
Velocity pressure comes directly from standard air's density, the ratio that produces the 4005 constant is the same relationship engineers have used for Pitot-tube airflow measurement for decades, computed here rather than looked up. The fan affinity laws are classical turbomachinery scaling relationships, first, second, and third power in the speed ratio for flow, pressure, and power respectively, and are public, standard, and hand-verifiable: doubling RPM in the calculator above should always show exactly 2x airflow, 4x pressure, and 8x power, which is a quick sanity check you can run yourself. Neither tool depends on any manufacturer's specific fan curve, both compute the same standard relationships that apply broadly, actual fan performance still varies by model and should be checked against a real fan curve before finalizing a selection.
Frequently asked questions
What is velocity pressure in HVAC?
Velocity pressure (Pv) is the pressure component of moving air, caused by its velocity, distinct from static pressure. It is what a Pitot tube traverse actually measures at a point in a duct, and it is related to velocity by Pv = (V/4005)^2 for standard air, where V is in feet per minute and Pv is in inches of water column.
How do you calculate velocity pressure?
Divide velocity in feet per minute by 4005 (the standard-air velocity-pressure constant) and square the result: Pv = (V/4005)^2. To go the other way, from a measured Pv to velocity, take the square root of Pv and multiply by 4005.
What are the fan laws?
The fan affinity laws describe how a fan's output changes when its speed changes but the duct system it's connected to stays the same: airflow (CFM) scales directly with RPM, static pressure scales with RPM squared, and power (horsepower) scales with RPM cubed. They let you predict a new operating point from a known one without re-measuring from scratch.
How much does fan power increase with speed?
Power scales with the cube of the speed ratio, so a modest speed increase costs proportionally much more power. Doubling RPM only doubles airflow, but it takes eight times the power (2 cubed); a 25% speed increase raises airflow 25% but power by roughly 95%. This cubic relationship is why small speed reductions can produce large energy savings.
Do the fan laws work for any duct system change?
No, only when the system curve stays fixed, the same ductwork, fittings, and damper positions between the two points. If you change the system itself, adding runs, opening or closing dampers, the fan will actually operate somewhere else on its curve, and the affinity laws alone will not predict that new point correctly.