Louver Size Calculator: Designing for Velocity & Water Protection
Sizing an intake louver seems simple, but it is the most common cause of "Rain in the Duct" complaints. The mistake? Sizing based on Face Velocity instead of Free Area Velocity.
Standard louvers are only ~50% open. If you size a 48"x48" louver for 500 FPM face velocity, your actual air velocity through the blades doubles to 1000 FPM—guaranteeing that rain will be sucked into your building.
The Design Standard
- Maximum Intake Velocity: To prevent water penetration, the velocity through the Free Area should typically not exceed 500 FPM (feet per minute).
- Standard Free Area: Most architectural louvers provide 35% - 50% free area.
🧮 Interactive Louver Calculator
*Recommended: 400-500 FPM to stop rain.
Conclusion
Never trust the "Face Velocity" listed in a generic catalog. Always calculate based on the Free Area provided by the manufacturer data sheet. If you don't, you aren't designing an intake; you're designing a rainwater collection system.
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