Louver Size Calculator
Enter the airflow in CFM or m³/h, the velocity you want through the free area and the louver's free-area ratio. The calculator gives the free area needed, the gross louver size and a square size to order.
- Free area
- Gross louver size
- Square size
Use about 500 ft/min (2.5 m/s) for intakes and up to 700 ft/min (3.5 m/s) for exhausts.
A louver only lets air through its free area, the gaps between the blades. Free area = airflow ÷ velocity. For 2,000 CFM at 500 ft/min you need 4 sq ft of free area. With 50% free area, the louver itself is 8 sq ft, about 36 × 36 in.
Louver size chart
500 ft/min, 50% free area. Search for an airflow.
| Airflow (CFM) | Free area (sq ft) | Gross (sq ft) | Square (in) |
|---|---|---|---|
| 250 | 0.5 | 1 | 12 |
| 500 | 1 | 2 | 18 |
| 1000 | 2 | 4 | 24 |
| 1500 | 3 | 6 | 30 |
| 2000 | 4 | 8 | 36 |
| 3000 | 6 | 12 | 42 |
| 5000 | 10 | 20 | 54 |
| 8000 | 16 | 32 | 72 |
| 10000 | 20 | 40 | 78 |
Square sizes rounded up to 6 in.
Airflow and louver
500 ft/min, 50% free area. Slide the airflow.
The louver grows with the air it must pass.
How to size a louver
Airflow to size. Step through it.
Airflow
CFM or m³/h.
Keep the velocity low on intakes to stop rain being drawn in.
Intake, exhaust or drainable
Louver uses. Compare them.
Fresh air.
Stale air.
Wet places.
Drainable blades have gutters that carry water away, allowing higher velocity without rain getting in.
Before you order
Louver checks. Tick each item.
Tick each item as you go.
Add a bird or insect screen, and check its effect on free area.
What you get
Louver sizing. Each tile explains one result.
Needed.
Louver size.
To order.
Check.
Check.
Free-area ratio is often 40 to 60%, depending on blade shape and frame.
Frequently Asked Questions
No question matches that search.
How do I size a louver?
Divide the airflow by the free-area velocity to get free area, then divide by the free-area ratio.
What is louver free area?
The open area between the blades that air can actually pass through.
What velocity should a louver have?
About five hundred feet a minute for intakes, to keep rain out.
What is a typical free-area ratio?
About forty-five to fifty-five per cent for most louvers.
Does a bird screen reduce free area?
Yes, by a few per cent, so allow for it.
Why is my louver letting in rain?
The velocity through it is probably too high.
Can I use metric units?
Yes. Use cubic metres an hour and metres a second.
Air in, rain out
Size the louver for the airflow at a low velocity, and the system breathes easily without letting weather in.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.