Frequently Asked Questions (FAQ)

How do I determine what grille size fits? Or... How can I find a standard grille size to cover my random size duct?

Formulas to help calculate duct size.

If there is no grille covering the duct, then just measure the duct size. Convert any fractional numbers (such as 7-1/2, or 8-1/4) to the nearest even whole number (6, 8, 10). Remember that ducts tend to be irregular, while grilles are standardized to fit even number duct sizes in two inch increments. So if you measure a 5-7/8" x 12-3/4" duct, round the fractional number to the next even whole number, 6" x 12".

If the grille is attached over the duct, just measure the grille and subtract the overlap, to determine the size of the duct. Generally a stamped metal grille has a 1-1/2" to 2" overlap, over the duct size. So if you measure your grille to be 7-1/2" x 13-1/2" than you have a 6 x 12 duct underneath that grille.

Duct Size

Measure the height and width of the duct. HVAC ducts are handmade and then banged out on site. Therefore, there is no such thing as a perfect duct.

Ducts may be handmade and never the same size, but grilles are standard sizes. So you must choose from a standardized grille selection to fit your non-standard duct. Grilles are standardized to the nearest even whole number in inches, like 4", 6", 8", 10", etc. So if your duct measures 7" x 14" you must determine if it will fit best with a grille made to fit a 6" x 14" or an 8" x 14". Determine which size works best by overall duct size and by proximity of any wall or baseboard molding which limits the size of the grille.

All grilles must be larger than the duct they cover. There must be at least a 3/4" border around each edge of the grille so that you can attach it to the wall or ceiling. Different manufacturers make slightly different size grilles. A grille made to cover a 6x12 duct may be 7-1/2" x 13-1/2" on the small side and 8" x 14" on the large side.

Decorative grilles tend to be larger than the typical stamped metal grille as pictured above. Many of our grilles have a tapered edge, rather than the square edge of the stamped metal grille. The tapered edge improves the look of the grille. It has a look and taper typical of casing molding found around doors and windows. Use of the tapered edge, means a larger frame size around the grille, since the tapered edge is not suitable for the pre-drilled screw hole. Since the last 5/8" inch of our decorative grilles is tapered, they can tend to be 1-1/2" wider and taller than the typical stamped metal grille.

A 6" x 12" duct is covered by a stamped metal grille which is 8" x 14". A Louis XIV decorative grille to fit the same size 6" x 12" duct, measures 9-1/2" x 16". In other words, these decorative grilles have a larger frame with a tapered edge, which adds to the width and height of the outer dimensions of the grilles' frame. The larger tapered edge is helpful in two respects, first it has a larger area to drill screws to help mount the grille securely. Secondly, the larger tapered edge helps by covering any old paint lines resulting from removing the old grille.