How do you size an HVAC louver?

HVAC louver sizing follows four steps: (1) Calculate the required free area from CFM and maximum face velocity (typically 400-600 FPM for intake louvers). Free Area (sq ft) = CFM / Velocity. (2) Determine the gross louver size using the louver free area ratio (typically 45-55% for fixed blade louvers). Gross Area = Free Area / Free Area Ratio. (3) Select a standard louver size that meets or exceeds the calculated gross area. (4) Verify that the pressure drop at the design airflow is within the duct static pressure budget (typically 0.05-0.15 in. w.g. for louvers).

What is the free area ratio of an HVAC louver?

The free area ratio is the percentage of the louver face area that is open to airflow (not blocked by blades, frames, or screens). Fixed-blade louvers typically have a 45-55% free area ratio. Drainable louvers have 40-50% due to the deeper blade profile needed for water drainage channels. Storm-proof louvers have 25-35% because the tighter blade spacing blocks wind-driven rain. Always use the manufacturer-published free area ratio, not a generic estimate — the actual ratio depends on blade angle, blade spacing, and frame width.

What is the maximum face velocity for HVAC louvers?

Maximum face velocity depends on application: 400-500 FPM for fresh air intake louvers (prevents rain ingestion), 500-700 FPM for exhaust louvers (no rain concern), and 800-1000 FPM for transfer louvers in interior walls. Exceeding these velocities on intake louvers causes water carryover into the ductwork, even with drainable blade profiles. The AMCA Louver Performance Rating includes a "beginning point of water penetration" velocity — always design below this published value.

What AMCA rain class should I specify?

AMCA 550 defines rain classes A through D based on water penetration resistance: Class A allows 0.01 oz/sq ft or less at the rated velocity — specify for critical intake applications (hospitals, data centers, clean manufacturing). Class B allows 0.03 oz/sq ft — suitable for standard commercial buildings. Class C allows 0.07 oz/sq ft — acceptable for warehouses and non-critical spaces. Class D has no water penetration limit. Most commercial projects specify Class B as the standard, with Class A for mechanical rooms directly connected to AHUs.

Do HVAC louvers need bird screens?

Yes, all exterior louvers should include bird screens (minimum 1/2" mesh). Without screens, birds, insects, leaves, and debris enter the duct system, clog filters, contaminate coils, and create health hazards. Bird screens add approximately 0.02-0.05 in. w.g. pressure drop when clean, increasing to 0.10+ in. w.g. when clogged with debris. Include screen cleaning access in the maintenance plan. Some jurisdictions require insect screens (1/4" mesh) in addition to bird screens — check local building codes.

HVAC Louver Sizing: Calculator, CFM Formulas & AMCA Rain Class Guide (2026)

An undersized louver starves the building of fresh air. An oversized louver wastes wall space and money. A louver with the wrong rain class lets water into the ductwork. HVAC louver sizing is one of those mechanical engineering tasks that looks simple but has real consequences when done wrong. This guide covers the complete louver sizing and selection process — from CFM-to-area calculations through AMCA ratings, rain protection, pressure drop, and material selection — with the practical data that contractors and engineers need on the job.

Louver Types and When to Use Each

Before sizing, you need to select the right louver type. Each type has a different free area ratio and rain resistance, which directly affects the sizing calculation:

HVAC Louver Types — Selection Guide
Louver Type	Free Area Ratio	Rain Resistance	Best Application
Fixed blade (standard)	45-55%	Low	Sheltered locations, exhaust, interior transfer
Drainable blade	40-50%	Medium (AMCA Class B-C)	Standard fresh air intake — most common
Storm-proof / wind-driven rain	25-35%	High (AMCA Class A)	Exposed locations, coastal, high-wind areas
Combination (louver + damper)	35-45%	Varies	Where isolation is needed (AHU intake plenums)
Acoustic louver	20-30%	Medium	Near noise-sensitive receptors (hospitals, schools)
Sand trap louver	15-25%	High	Desert/sandy environments (Middle East, Australia)

Default choice: For standard commercial fresh air intake in temperate climates, a drainable blade louver with AMCA Class B rating covers 80% of applications. Start here and upgrade to storm-proof only when the building is exposed to wind-driven rain (coastal, high-rise, or south/west-facing walls in storm-prone regions).

Step 1: Calculate Required Free Area

The fundamental louver sizing formula:

Free Area (sq ft) = CFM / Maximum Face Velocity (FPM)

Maximum Face Velocity by Application

Recommended Louver Face Velocity Limits
Application	Max Velocity (FPM)	Why
Fresh air intake (sheltered)	500	Prevent rain water carryover
Fresh air intake (exposed)	400	Wind-driven rain protection
Exhaust (general)	700	No rain concern, outward airflow
Exhaust (kitchen/grease)	500	Code-limited for fire safety
Transfer louver (interior wall)	800-1000	No weather concern, noise is the limit
Penthouse / AHU intake	400	Low velocity for pre-filter protection

Worked Example: 5,000 CFM Fresh Air Intake

A commercial building AHU requires 5,000 CFM of outdoor air. The louver is on an exposed wall (not sheltered by an overhang). Target velocity: 400 FPM.

Free Area = 5,000 / 400 = 12.5 sq ft

Step 2: Calculate Gross Louver Size

The free area is the open area through the louver blades. The gross (total face) area is larger because the blades, frame, and mounting hardware block part of the opening:

Gross Face Area (sq ft) = Free Area / Free Area Ratio

Continuing the Example

Using a drainable blade louver with 45% free area ratio:

Gross Area = 12.5 / 0.45 = 27.8 sq ft
Possible louver dimensions: 60" W x 67" H (27.9 sq ft) or 72" W x 56" H (28.0 sq ft)
For a storm-proof louver (30% free area): Gross Area = 12.5 / 0.30 = 41.7 sq ft — significantly larger

This is why louver type selection matters before sizing. A storm-proof louver needs 50% more wall area than a drainable blade louver for the same CFM. If wall space is limited, you may need to split the louver into multiple openings or accept a drainable blade with careful overhang design for rain protection.

Louver Sizing Calculator

Enter your airflow requirements below. The calculator applies the formulas from Steps 1 and 2 and recommends standard louver dimensions.

Airflow (CFM)

Max Face Velocity (FPM)

Free Area (%)

Default values: 400 FPM = exposed fresh air intake (ASHRAE recommendation). 45% free area = standard drainable blade louver. Adjust based on your louver type using the table above.

Need help selecting the right louver?

Our engineers can help with custom sizing for your specific project.

Get a Quote

Step 3: Check Pressure Drop

Every louver creates a pressure drop that the AHU fan must overcome. This pressure drop must be included in the duct system static pressure budget:

Typical Louver Pressure Drop at Design Velocity
Louver Type	Pressure Drop at 400 FPM	Pressure Drop at 600 FPM	With Bird Screen
Fixed blade	0.03-0.05 in. w.g.	0.07-0.12 in. w.g.	Add 0.02-0.04
Drainable blade	0.05-0.08 in. w.g.	0.10-0.15 in. w.g.	Add 0.02-0.04
Storm-proof	0.08-0.15 in. w.g.	0.18-0.30 in. w.g.	Add 0.03-0.05
Acoustic	0.15-0.25 in. w.g.	0.30-0.50 in. w.g.	Add 0.02-0.04
Sand trap	0.20-0.35 in. w.g.	0.40-0.60 in. w.g.	Typically integral

Rule of thumb: Budget 0.10-0.15 in. w.g. for a standard drainable louver with bird screen at 400-500 FPM face velocity. If your total external static pressure budget is tight (under 0.50 in. w.g.), the louver pressure drop is a significant fraction — consider upsizing the louver to reduce velocity and pressure drop.

Step 4: Select Material and Finish

Louver Material Selection Guide
Material	Cost (Relative)	Corrosion Resistance	Weight	Best For
Extruded aluminum (6061-T6)	1.0x (baseline)	Good (natural oxide layer)	Lightest	Standard commercial — most common
Galvanized steel (G90)	0.7-0.8x	Moderate (zinc coating)	Heavy	Budget projects, industrial
Stainless steel (304/316)	3-5x	Excellent	Heavy	Coastal, chemical plants, food processing
Anodized aluminum	1.3-1.5x	Very good	Light	Architectural visibility, coastal
Painted aluminum (Kynar)	1.2-1.4x	Very good	Light	Color-matched to facade

Default choice: Extruded aluminum is the industry standard for commercial HVAC louvers — it is lightweight, corrosion-resistant, and available in the widest range of sizes. Galvanized steel is chosen primarily for cost-sensitive industrial projects. Stainless steel is reserved for corrosive environments where the 3-5x premium is justified by the extended lifespan.

AMCA Rain Ratings Explained

The Air Movement and Control Association (AMCA) Standard 550 provides a standardized test for louver rain resistance. Understanding these ratings is critical for louver selection in exterior applications:

AMCA 550 Rain Classes

AMCA 550 Rain Classification
Class	Max Water Penetration	Test Condition	Typical Applications
Class A	0.01 oz/sq ft	29 mph wind, 8 in/hr rain	Hospitals, data centers, critical AHU intake
Class B	0.03 oz/sq ft	29 mph wind, 8 in/hr rain	Standard commercial buildings
Class C	0.07 oz/sq ft	29 mph wind, 8 in/hr rain	Warehouses, non-critical spaces
Class D	No limit	N/A	Exhaust only, sheltered intake

Practical note: The AMCA test uses a 29 mph wind speed with 8 inches/hour rainfall intensity — a moderate storm. In hurricane-prone or high-wind coastal areas, even a Class A louver may allow some water penetration during extreme events. In these locations, combine a Class A louver with a downstream drain pan and motorized isolation damper that closes during severe weather.

Louver Placement: Code-Required Distances

Building codes specify minimum distances between fresh air intake louvers and potential contaminant sources. These are not suggestions — they are hard requirements that affect louver placement on the building envelope:

Minimum Distances for Fresh Air Intake Louvers (IMC / ASHRAE 62.1)
Contaminant Source	Minimum Distance	Code Reference
Building exhaust outlet	10 ft (3 m)	IMC 401.4
Kitchen exhaust hood	10-15 ft (3-4.5 m)	IMC 401.4
Vehicle loading area / parking garage	15-25 ft (4.5-7.5 m)	ASHRAE 62.1
Plumbing vent stack	10 ft (3 m)	IMC 401.4
Cooling tower	15-25 ft (4.5-7.5 m)	ASHRAE 62.1
Trash/dumpster area	15 ft (4.5 m)	ASHRAE 62.1
Street / sidewalk grade level	Min 8 ft (2.4 m) above	IMC 401.4

For more on louver placement, including overhang design and architectural integration, see our Fresh Air Intake Louver Guide.

Complete Sizing Example: Hospital AHU Intake

A hospital AHU requires 8,000 CFM outdoor air. The louver is on an exposed west-facing wall (storm exposure). The project requires AMCA Class A rain resistance.

Select louver type: Storm-proof louver, AMCA Class A, aluminum frame. Free area ratio: 30%.
Target velocity: 400 FPM (exposed wall, critical application).
Free area: 8,000 / 400 = 20.0 sq ft.
Gross face area: 20.0 / 0.30 = 66.7 sq ft.
Louver size: Two louvers at 48" x 60" each (2 x 20.0 sq ft = 40.0 sq ft), or one large louver at 96" x 84" (56.0 sq ft). Since 56.0 sq ft is less than 66.7 sq ft, the single louver is undersized. Go with 96" x 96" (64.0 sq ft) — still slightly under. Best option: two louvers at 72" x 72" each (2 x 36.0 = 72.0 sq ft, providing 8% margin).
Pressure drop: At 400 FPM through a storm-proof louver, expect 0.10-0.15 in. w.g. Add 0.03-0.05 for bird screen. Total: approximately 0.15-0.20 in. w.g.
Verify system: Include 0.20 in. w.g. in the AHU external static pressure calculation.

Common Louver Sizing Mistakes

Mistake 1: Using Gross Area Instead of Free Area

The most common error. A 48" x 48" louver has 16 sq ft of gross face area but only 7.2-8.8 sq ft of free area (depending on type). Using 16 sq ft in the velocity calculation gives an apparent velocity of 312 FPM for 5,000 CFM — but the actual velocity through the blades is 568-694 FPM. This miscalculation leads to water penetration that the designer did not anticipate.

Mistake 2: Ignoring Bird Screen Pressure Drop

Bird screens add 0.02-0.05 in. w.g. when clean, but can exceed 0.15 in. w.g. when clogged with debris. In locations with heavy insect or pollen loads, the effective louver pressure drop can double within a single season if screens are not cleaned. Include screen maintenance in the building O&M manual and specify cleanable/removable screen panels.

Mistake 3: Specifying One Large Louver When Two Smaller Ones Are Better

A single 96" x 96" louver requires a massive wall opening and significant structural support. Two 48" x 96" louvers provide the same free area with smaller openings that are easier to frame, seal, and maintain. Split louvers also allow servicing one side while the other remains in operation.

Related HVAC Guides

HVAC Fresh Air Intake Louver Guide — Detailed louver placement, overhang design, and architectural integration for exterior applications.
Weather-Proof Louver Guide — Storm-proof and drainable louver specifications for high-wind and coastal environments.
Plenum Chamber: Types, Sizing & HVAC Applications — How AHU intake plenums connect to louvers and the complete HVAC air distribution system.
HVAC Duct Fittings Types Guide — Ductwork components that connect louvers to AHU intake plenums.
HVAC Damper Types Guide — Isolation and control dampers used behind fresh air intake louvers.
HVAC Air Balancing Guide — System balancing procedures that depend on correctly sized louvers for outdoor air volume.