Register geometry
HVAC Register Opening Area Calculator
Separate gross face area from effective net opening area using an explicit free-area assumption.
Register inputs
Free area is product-specific. Use the manufacturer's actual published value when the selected register is known.
Enter dimensions, quantity and free-area factor.
Register opening result
Net opening area
—
—
—Gross face area
—Net opening
—Area reduction
—Registers
—
Gross versus net register area
Gross face area uses the outside rectangular dimensions. Effective opening area then applies the selected free-area percentage.
Gross area = width × height × quantity
Net opening = Gross area × free-area factor ÷ 100
Net opening = Gross area × free-area factor ÷ 100
Worked example
A 300 × 200 mm register has a gross face of 0.06 m². Six registers have 0.36 m² gross face area. At 85% free area, the effective opening is 0.306 m².
Performance boundary
Net opening is not a complete airflow-capacity calculation. Product geometry, face velocity, pressure drop and acoustic behavior require separate data.
Frequently Asked Questions
Should I always use 85%?
No. Treat it as an example/default assumption, not a universal product value.
Can free area be 100%?
Mathematically yes, although real products may have frames, blades or other restrictions.