Measure the metal hole in your floor, not the register sitting in it. Then match it to one of six standard sizes.
The six standard sizes
| Size | Fits a duct opening of | Faceplate | Typical use |
|---|---|---|---|
| 2″ × 10″ | 2 × 9.75″ | 3.5 × 11.5″ | Narrow ducts, kickspaces |
| 2″ × 12″ | 2 × 11.75″ | 3.5 × 13.5″ | Narrow ducts, older homes |
| 2″ × 14″ | 2 × 13.75″ | 3.5 × 15.5″ | Long narrow runs |
| 4″ × 8″ | 4 × 7.75″ | 5.5 × 9.5″ | Small rooms, bathrooms |
| 4″ × 10″ | 4 × 9.75″ | 5.5 × 11.5″ | The most common size |
| 4″ × 12″ | 4 × 11.75″ | 5.5 × 13.5″ | Living rooms, larger supplies |
If you are replacing like for like in a typical US house, 4×10 is the first thing to check. See all six sizes and prices.
Why your tape reads under the nominal size
Duct openings are cut with clearance, so a nominal 10-inch length usually measures about 9.75″. That is not a problem to solve. The sizes are built expecting it. Match to the nearest standard size and move on.
The failure case is the opposite: a measurement noticeably over nominal, say 4.5″ × 10.5″. That is either a non-standard duct or an opening that has been enlarged, and it is worth emailing before ordering.
The check most people skip
The faceplate overhangs the hole by about three-quarters of an inch per side. That footprint has to land on flat, clear floor.
Floor ducts are very often cut right against a wall, which means the overhang runs into the baseboard or shoe moulding. Before ordering, measure from the edge of the opening to the baseboard. Under three-quarters of an inch and the register will sit proud on that edge and rock underfoot.
Same check applies near door casings, cabinet toe-kicks, and carpet-to-tile transition strips.
Damper or no damper
Floor registers are supply outlets, so they should have a damper. The lever that opens and closes a plate underneath the face. It lets you throttle a room that runs hot without touching the thermostat.
What matters mechanically is whether the damper holds position. Thin-stock registers use a friction fit that loosens, and the damper gradually falls open. An opposed-blade damper in heavier steel keeps the setting you give it, which is most of the reason to buy a heavier register at all.
Carpet, tile or wood
The same register works on all three; what changes is how it sits. On carpet, the faceplate rests on the pile and beds in slightly. On tile or wood, it sits directly on a hard surface, so any warp in a thin faceplate shows immediately as a rocking edge. This is the other argument for heavier gauge on hard floors.
Not sure between two sizes?
Email the two numbers to support@brycehardware.com. Faster than a return, and we would rather tell you we do not make your size than sell you the wrong one.
Step by step
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Lift out the old register
Floor registers are not fastened. Pull straight up by the damper lever or hook a finger through the louvers. Set it aside. You will not be measuring it.
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Measure the duct opening
Tape inside the sheet-metal hole, edge to edge. Take the short dimension first, then the long one. Sweep out dust or paint chips on the duct lip while you are down there, since debris is what stops a new register sitting flat.
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Match to a standard size
Compare against the six sizes below. Your measurement will usually read slightly under nominal, 9.75 inches for a 10, for example. That is expected.
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Check the faceplate footprint
Add roughly 0.75 inch to each side of your opening and confirm that footprint lands on clear floor. Baseboard, a door casing, a cabinet toe-kick or a transition strip can all interfere.
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Drop the new one in
Faceplate flat to the floor, damper lever accessible. No screws, no caulk. Set the damper open until you are balancing the room.