Lift the old register and measure the hole in the floor, or measure the old register's face. You get the size to buy — named by the hole, not the face — the face it shows on the floor, the body that drops in, and the air it moves, from a register maker's own chart.
The register
Buy a 4 × 10 register for a 4 × 10″ holeits face is 5½ × 11½″ on the floor · the body that drops in is 3 15⁄16 × 9⅞″ · 50–170 cfm
Quick answer: a floor register is named by the hole it fits, not by its face. A 4 × 10 register fits a 4 × 10″ duct opening, and its face on the floor is 5½ × 11½″ (Metal-Fab’s chart; Hart & Cooley’s 421 has the same face). Lift the old register and measure the hole inside edge to inside edge: that is the size to buy.
How to use the floor register size finder
- Lift the old register and measure the hole in the floor, the width and the length, inside edge to inside edge. Or measure the old register’s face if it will not come up.
- Enter the two numbers. Fractions work: 3 7/8.
- Know the room’s airflow? Enter the cfm from your HVAC contractor’s design to see how fast the air leaves the face.
You get the size to buy, the face it shows on the floor, the body that drops into the hole and the air it moves. The drawing puts the face over your hole to scale.
The rule, and where it comes from
The size is the hole. Metal-Fab’s MFFR floor register chart gives every size (E × F) with its face (A × B) and the body that drops in (C × D): the 4 × 10 has an 11½ × 5½″ face and a 9⅞ × 3 15⁄16″ body. Hart & Cooley’s 421 4 × 10 is listed at “5-1/2 in Overall Length, 11-1/2 in Overall Width”. The face overlaps the floor by about ¾″ on every side.
The air. The same chart gives each size’s free area, “Ak”, and the cfm it moves at face velocities from 300 to 1000 fpm: the 4 × 10 (Ak .18) moves 50 cfm at 300 fpm and 170 at 1000. Air speed at the face = cfm ÷ Ak.
Floor register sizes: the hole, the face and the body
| Size (the hole) | Face on the floor | Body into the hole | cfm, 300–1000 fpm |
|---|---|---|---|
| 2 × 10 | 3¾ × 11½″ | 2⅛ × 9⅞″ | 25–85 |
| 2 × 12 | 3¾ × 13½″ | 2⅛ × 11⅞″ | 30–110 |
| 2 × 14 | 3¾ × 15½″ | 2⅛ × 13⅞″ | 35–115 |
| 3 × 10 | 4½ × 11½″ | 2 15⁄16 × 9⅞″ | 40–135 |
| 4 × 8 | 5½ × 9½″ | 3 15⁄16 × 7⅞″ | 40–130 |
| 4 × 10 | 5½ × 11½″ | 3 15⁄16 × 9⅞″ | 50–170 |
| 4 × 12 | 5½ × 13½″ | 3 15⁄16 × 11⅞″ | 60–210 |
| 4 × 14 | 5½ × 15½″ | 3 15⁄16 × 13⅞″ | 70–230 |
| 6 × 8 | 7½ × 9½″ | 5⅝ × 7⅞″ | 60–195 |
| 6 × 10 | 7½ × 11½″ | 5⅝ × 9⅞″ | 70–240 |
| 6 × 12 | 7½ × 13½″ | 5⅝ × 11⅞″ | 85–285 |
| 6 × 14 | 7½ × 15½″ | 5⅝ × 13⅞″ | 100–335 |
From Metal-Fab’s MFFR Series Floor Register Specification Chart (Form L2356). Other makers’ faces differ by a fraction of an inch; the hole sizes are the same.
Worked examples
Buy a 4 × 10 register for a 4 × 10″ hole: the most common floor register. Its face is 5½ × 11½″.
Buy a 2 × 12 register for a 2¼ × 12″ hole: a narrow opening. The 2⅛″ body drops in.
Buy a 6 × 12 register for a face of 7½ × 13½″: measured from the old face; the hole under it is 6 × 12″.
Buy a 4 × 10 register for a 4 × 10″ hole: but 250 cfm through it is about 1,390 fpm, past the chart’s 1000. A 6 × 12 carries it, with a 6 × 12″ hole and boot.
Common mistakes
- Buying by the face. The old register’s face measures about 5½ × 11½″, so a “6 × 12” looks right — but its 5⅝ × 11⅞″ body will not drop into the 4 × 10″ hole. The “4 × 10” is the hole.
- Measuring over the old register. Lift it: the opening is what the new body has to drop into.
- Width and length swapped. A 4 × 10 and a 10 × 4 are the same register; the finder turns them round.
- A bigger register in the same hole. The body has to drop into the opening; more air needs a bigger hole and duct boot, which is your HVAC contractor’s call.
More on floors at Dreamy Home Style
Changing the furnace filter too? The furnace filter size finder has the same name-versus-size trap. To plan a room around its vents, draw it to scale in the free 2D floor planner.
Floor register questions
What are the standard floor register sizes?
2 × 10, 2 × 12, 2 × 14, 3 × 10, 4 × 8, 4 × 10, 4 × 12, 4 × 14 and 6 × 8 to 6 × 14 inches in Metal-Fab’s chart. Each size is the duct opening; the face is 1½–1¾″ bigger each way.
How do I measure a floor register?
Lift it and measure the hole, inside edge to inside edge. A 4 × 10″ hole takes a 4 × 10 register, whose face is 5½ × 11½″.
What size is a 4 × 10 floor register?
It fits a 4 × 10″ hole. Its face is 5½ × 11½″ and its body 3 15⁄16 × 9⅞″ (Metal-Fab); Hart & Cooley’s 421 has the same face.
What size vent cover do I need?
The size of the hole under the old one. Measure the hole, not the old cover, and buy that size.
How much air does a floor register move?
It depends on the speed: Metal-Fab’s 4 × 10 moves 50 cfm at 300 fpm at the face and 170 at 1000. Your HVAC contractor’s design gives the cfm each room needs.
What size is a 4 × 8 floor register?
It fits a 4 × 8″ hole, with a 5½ × 9½″ face and a 3 15⁄16 × 7⅞″ body in Metal-Fab’s chart.