Vent Size Finder

Measure the duct opening, the bare hole in the subfloor or wall, and order that nominal size: a 4 x 10 in hole takes a 4 x 10 vent, because vent sizes are named after the duct, not the cover.

This finder needs JavaScript. Match your measurement by hand against the full size table instead — it lists the duct opening for every size in inches and millimetres.

Measure this, not that

Measure the duct opening, the bare hole in the subfloor or wall, with the old cover lifted off. Take width and length at the narrowest point, to the nearest 1/8 inch or 3 millimetres. Do not measure the old cover: every cover is built larger than its duct so the flange has something to bear on, so a cover measurement sends you a size too big. If the hole is out of square, use the smaller reading.