Tools · Airflow

Airflow & Velocity Pressure Airflow

Both halves of the field airflow square-root: K-factor flow runs a VAV box’s CFM = K × √VP in either direction — flow from the pickup’s velocity pressure, or the K a hood reading implies. Duct velocity turns a pitot reading into FPM with V = 4005 × √VP and a duct area into CFM.

Input

K is the manufacturer’s flow coefficient for the box’s velocity-pressure pickup — the CFM it reads at exactly 1.0 in. w.c. Conventions vary across vendors and controllers, which is why the second mode exists: back-solve K from a balancer’s hood reading and the VP the controller sees, and you’ve calibrated the box no matter whose convention the paperwork used.

Output

Flow —

Worked example

A VAV box with K = 1000 reports a velocity pressure of 0.36 in. w.c. at its flow pickup.

  1. The flow equation: CFM = K × √VP.
  2. Plug in: CFM = 1000 × √0.36 = 1000 × 0.6 = 600 CFM.
  3. Now the calibration direction: the balancer’s hood says this box actually passes 800 CFM while the pickup reads VP = 0.64. Back-solve: K = 800 ÷ √0.64 = 800 ÷ 0.8 = 1000 — the configured K checks out, so the earlier mismatch was somewhere else.

A box that reads wrong against the hood usually has a wrong K, a kinked sensing line, or a pickup mounted too close to the inlet elbow. The back-solve separates the first cause from the other two in one reading.

Input

One pitot reading is a point velocity, and duct velocity isn’t uniform — it peaks at the center and dies at the walls. For a number you’d sign, traverse the duct (multiple points on an equal-area grid) and average the velocities; this tab does the per-point math.

Output

Velocity (FPM) —
Velocity (m/s equivalent) —
Duct area (ft²) —
Airflow (CFM) —

Worked example

A pitot traverse point in a 24 × 12 in. supply duct reads VP = 0.0625 in. w.c.

  1. Velocity from VP: V = 4005 × √0.0625 = 4005 × 0.25 = 1001 FPM.
  2. Duct area: A = (24 ÷ 12) × (12 ÷ 12) = 2.0 ft².
  3. Airflow: Q = 1001 × 2.0 = 2002 CFM.

Standard air, and where 4005 comes from

Both tabs assume standard air — 0.075 lb/ft³, sea level, ~70 °F. The 4005 folds that density into V = √(2 · ΔP / ρ); at altitude the air is thinner, the same VP means a higher true velocity, and this math under-reads by a few percent per few-thousand feet. Most balancing instruments correct for density internally — when yours does, trust it over this page. For altitude-corrected psychrometric work, the psych chart carries a proper altitude input.

This tool is US-native (CFM, in. w.c., FPM) — that’s the frame K-factors and pitot charts are published in on this side of the ocean. The m/s readout rides along; a metric VP mode (L/s, Pa, V = 1.291 × √Pa) is a tracked follow-up.

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