4x8 Metal Wall Vents: Free Area, Airflow and Duct Fit
A 4x8 wall vent has a free area of 10.8 square inches and a calculated airflow of 37 CFM, and it drops into a duct opening of 95.3 x 200 mm.
- Duct opening
- 3.75″ × 7.87″
- Metric
- 95.3 × 200 mm
- Best free area
- 10.8 sq in
- Comes in
- Flush Wall · Framed Wall
4x8 is the short wall size for tight spots above a door header, under an upper cabinet or in a stair soffit, and it is the efficiency winner of the wall line, returning 10.8 square inches from a face 20 percent smaller than the 4x10's 11.1.
Free area and airflow at this size
| Product | Free area | Open | Airflow | Price | |
|---|---|---|---|---|---|
| Flush Wall Vent [Metal] | 10.8 sq in | 34% | ≈ 37 CFM | from $39.99 | View 4x8 |
| Framed Wall Vent [Metal] | 10.8 sq in | 34% | ≈ 37 CFM | from $37.99 | View 4x8 |
Free area is the open cross-section air actually passes through, not the outside of the cover. How we calculate it.
Measuring for this size
The 4x8 is the one wall size with an uneven face gap, 11.1 mm on the short sides and 17.0 mm on the long sides, and it needs a 95.3 mm by 200.0 mm rough opening, so it will not cover an existing 4x10 cut.
Why does the 4x8 give almost as much free area as the 4x10?
Because of the slot width. The 4x8 runs an 11.1 mm gap on the short sides and 17.0 mm on the long sides, the widest long-side gap of any 3-inch or 4-inch wall size. The result is 10.8 square inches against the 4x10's 11.1, so 97 percent of the free area from a face 20 percent smaller.
Where would I use a 4x8 instead of a 4x10?
Above a door header, under a kitchen upper cabinet, in a stair soffit or in a closet wall, anywhere without 10 inches of clear run. It is the size that solves a framing problem, and because it gives up almost nothing in free area you are not paying for the shorter face in airflow.
What rough opening does the 4x8 need?
95.3 mm by 200.0 mm. That is the same width as the 4x10 and 4x12 wall vents and 47.7 mm shorter than the 4x10. It will not cover an existing 4x10 cut, so if you are downsizing to solve a clearance problem, plan to patch and re-cut rather than swap covers.
How do I measure for a replacement vent?
Measure the duct opening, not the old cover. Pull the existing vent out, measure the hole itself across the width and the length in millimetres, and match that to the duct dimension listed on this page. Vent sizes are named for the duct, so a vent called 4x10 has a face larger than 4 by 10 inches. Measuring the faceplate is the single most common reason a size arrives wrong.
What happens if I order the wrong size?
Email support@khansupply.co with your order number and the millimetre measurements of your opening, and we will work out which size actually fits and sort out the exchange. Every Khan Supply vent also carries a 2-year warranty against defects, which is separate from a fit exchange.
What does shipping cost?
Nothing. Shipping is free on every order across North America, on a single vent and on a 5-pack alike. You will get tracking by email once the order is on its way. If a vent arrives damaged, photograph the box and the part and email support@khansupply.co, Monday to Friday 9 to 5 ET, and we reply within one business day.
Every Khan Supply vent is all metal, not plastic, and ships free across North America with a 2-year warranty. If a size is wrong, email support@khansupply.co and we sort out the exchange. Mon-Fri 9-5 ET, one business day reply.
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