Figure

HVAC

Velocity pressure

Calculate air speed from velocity pressure, or reverse the calculation, with the stated formula and units.

These examples use V = 4005 × √VP, with air speed V in feet per minute and velocity pressure VP in inches of water column. The factor assumes standard-density air at 0.075 lb/ft³. Static pressure is a different quantity.

Velocity pressure to FPM

Take the square root of velocity pressure in in. w.c., then multiply by 4005.

Start a new calculation

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Second, then Select air speed units.

Enter velocity pressure

  1. Velocity pressure

    Press .049, then Square root.

Multiply by 4005

  1. Multiply

    Press Multiply.

  2. Enter 4005

    Press 4005, then Equals.

  3. Press the keys

    Press feet per minute.
  1. Example · Air speed (FPM)886.5445 FPM

Keep in mind

Use velocity pressure in in. w.c., not static pressure. The 4005 factor assumes standard-density air (0.075 lb/ft³). Use 2nd if FPM is above its button.

FPM to velocity pressure

Divide air speed in FPM by 4005, then square it. The result is velocity pressure in in. w.c.

Start a new calculation

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select pressure units.

Enter air speed

  1. FPM

    Press 500, then Divide.

  2. Enter 4005

    Press 4005, then Equals.

Square and label the result

  1. Press the keys

    Press Square.

  2. Press the keys

    Press inches of water column.
  1. Example · Velocity pressure (inches of water)0.01558601 in. w.c.

Keep in mind

This finds velocity pressure, not static pressure. It assumes standard-density air (0.075 lb/ft³). Use 2nd if in. w.c. is above its button.

Keep the guide beside your calculation.

These examples are built into Figure. Open Help while you calculate, then return to the same guide and reading position.

See Help in Figure →