Calculate louver free area from width, height and the manufacturer’s free-area percentage, size a louver for a required free area, or check air velocity.

Apply the supplied manufacturer free-area percentage to the gross face area.

Use the published free area for the exact louver model, size and screen configuration. Area and average velocity alone do not establish pressure drop, rain performance or airflow capacity.

Use the percentage for the intended model, size and screen; it is not a universal louver constant.

Optional airflow velocity

Blank omits velocity. Enter 0 for zero flow. Flow divided by net free area gives average free-area velocity.

Louver Free Area Formula

The following equation calculates free-area percentage. Net free area itself equals gross face area multiplied by that percentage divided by 100.

LFR = (TOLA) / (WOA) × 100
  • Where LFR is the louver free area (%)
  • TOLA is the total net free (open) area through the louver (excluding blades, frame, and supports) (any consistent area unit, e.g., ft² or m²)
  • WOA is the gross wall/louver opening (face) area (any consistent area unit, e.g., ft² or m²)

To calculate louver free-area percentage, divide total net free (open) area by gross face area on the same manufacturer dimension basis, then multiply by 100.

What is Louver Free Area?

Definition:

The free area of a louver (also called net free area) is the net open area available for airflow through the louver after accounting for blades, frame, and supports. It is usually expressed as a percentage of the louver’s gross (nominal) face area (often the wall/rough opening).

Free area is used primarily to estimate airflow capacity and pressure drop. Solar control, privacy, and rain resistance depend mostly on blade geometry and louver type rather than free-area percentage alone.

For a given airflow, a higher free-area percentage generally lowers air velocity through the louver and can reduce pressure drop (potentially lowering fan energy). However, requirements such as wind-driven-rain performance, sight-proofing, security, or acoustics can require designs with lower free area.

Louvers are commonly categorized by blade operation (fixed vs. operable) and performance features (non-drainable vs. drainable, storm-resistant/wind-driven-rain rated, sight-proof, acoustic, etc.), and they are made from materials such as aluminum, steel, stainless steel, wood, or composites.

Most HVAC/architectural wall louvers use opaque blades (often metal). These can provide shading/visual screening and weather protection while still allowing airflow through the openings between blades.

Some designs incorporate perforated/expanded-metal elements or separate bird/insect screens. These can provide airflow with additional screening, but typically reduce free area and increase pressure drop compared with the same louver without a screen.

Free area is published for a specific louver model and size. Use the manufacturer’s face-area basis and confirm whether screens are included. Percentage changes with size, so inverse area sizing at a fixed percentage is preliminary. Average free-area velocity equals airflow divided by net free area; pressure drop and rain performance require separate manufacturer test data.

How to Calculate Louver Free Area?

Example Problem:

The following example outlines the steps and information needed to calculate Free Area.

First, determine the net (free) open area through the louver after accounting for blades, frame, and any screens/obstructions. For this example, this is found to be 10 ft^2.

Next, determine the total wall (gross) opening area. In this case, the wall opening has an area of 20 ft^2.

Finally, calculate the louver free area using the formula above:

LFR = (TOLA / WOA) * 100

LFR = (10 / 20) * 100

LFR = 50%