Understanding your result
The headline is the register size to buy, with the count if the room needs more than one, and the airflow per register and its actual face velocity in the sub-line. The stats repeat the count, the free area each register needs and the velocity. The table shows the room’s total airflow, how it was divided, the free and nominal areas, and the nominal area of the stock size chosen, which is why the actual velocity comes out a little under the target.
If the airflow per register would exceed the maximum you set, the calculator adds registers rather than choosing a huge one, and the notes say how to place them. For a ceiling or sidewall register that would run under about 350 FPM the notes warn about short throw, since a register that barely pushes air into the room leaves the far side stuffy.
The room’s airflow comes from the CFM per room calculator; the branch duct and boot that feed each register are sized with the duct size calculator at the branch limit; and the air that these registers deliver has to find its way back through the returns sized by the return grille calculator. An existing register can be checked with the duct velocity calculator using its free area.
How we calculate this
The arithmetic is the same as for a return grille, with register free areas and velocity targets instead. Stock sizes are the common nominal register dimensions, floor registers from 2 × 12 to 6 × 14 inches and ceiling or sidewall registers from 10 × 4 to 20 × 8, ordered by area. The velocity targets are the usual residential ranges for comfort and noise, and the maximum per register of 180 CFM is a conservative figure for avoiding drafts; raise it if the room has few places to put registers. The duct size chart pairs each register’s airflow with the branch duct that feeds it, and the ventilation and exhaust minimums page lists the velocity conventions for grilles, registers and ducts.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Face velocity | 500 FPM | Mid-range of residential practice; 400 to 600 floor, 500 to 700 ceiling and sidewall |
| Free area | 65% | Typical stamped-steel supply register with damper; bar-type 75 to 80% |
| Maximum per register | 180 CFM | Comfort practice; 100 to 150 CFM per register is common |
| Stock sizes | Floor 2 × 12 to 6 × 14 in; ceiling and sidewall 10 × 4 to 20 × 8 in | Common US register dimensions |
| Throw warning | Below 350 FPM for ceiling and sidewall registers | Manufacturer throw data; short throw leaves rooms unmixed |
Assumptions last reviewed October 8, 2026.
The calculator does not look up throw, spread or noise criteria for specific products, which differ between a stamped register and an engineered diffuser at the same size, and it does not model the damper behind the register, which when half closed raises the velocity and the noise. The guide to duct sizing basics covers registers as the end of the supply path, and the guide on oversized systems explains why a room that gets too much air is as uncomfortable as one that gets too little.
Two worked examples
A bedroom floor register
120 CFM through floor registers at 500 FPM, 65% free area, maximum 180 CFM per register.
- Registers: 120 ÷ 180 → 1
- Free area needed: 120 ÷ 500 × 144 = 34.6 sq in; nominal 34.6 ÷ 0.65 = 53.2 sq in
- Smallest stock floor register: 4 × 14 in (56 sq in)
- Actual face velocity: 120 × 144 ÷ (56 × 0.65) = 475 FPM
A single 4 × 14 under the window covers the room at a quiet 475 FPM. The common 4 × 10 (40 sq in) would run at 665 FPM, audible at night; the 6 × 10 (60 sq in) at 443 FPM would also work and is the more usual stock item in many areas.
A living room on ceiling registers
220 CFM through ceiling registers at 600 FPM for throw, 75% free area (bar type), maximum 180 CFM per register.
- Registers: 220 ÷ 180 → 2, at 110 CFM each
- Free area needed: 110 ÷ 600 × 144 = 26.4 sq in; nominal 26.4 ÷ 0.75 = 35.2 sq in
- Smallest stock ceiling register: 10 × 4 in × 2 (40 sq in each)
- Actual face velocity: 110 × 144 ÷ (40 × 0.75) = 528 FPM
Two 10 × 4 registers at 528 FPM throw well across a living room and avoid the single cold column that one 220 CFM register would produce. Placed at opposite ends of the room, aimed towards the exterior wall, they sweep the glass; the return should be on the interior side so the air crosses the room on its way back.
Where to find your inputs
Room airflow. From the CFM per room calculator, or a contractor’s Manual J room load divided by the system’s CFM per ton.
Register type. Floor registers for systems with ducts below (basements, crawl spaces, slab); ceiling and high sidewall for attic systems.
Free area. From the register maker’s catalogue, as a percentage or in square inches; 65% for a stamped steel register if unknown.
Face velocity. 500 for floor registers, 600 for ceiling registers if the room is more than about 14 ft across, lower in bedrooms.
Common mistakes
- Sizing by the nominal face. A 4 × 10 register is 40 sq in of face but about 26 sq in of opening.
- One register for a big room. Above 150 to 180 CFM the register is drafty or loud; add registers.
- Ceiling registers too large. A big register runs slow, throws short, and leaves the far side of the room unmixed.
- Floor registers under furniture. A register under a sofa delivers nothing; plan the placement with the furniture.
- Throttling with the register damper. Half-closed register dampers are noisy; balance at the trunk dampers instead.
- Mixing up supply and return sizing. Returns need more free area at lower velocity; registers need throw.
Questions people ask
- What size register do I need for 100 CFM?
- A 4 × 10 inch floor register at a face velocity around 550 FPM, or a 4 × 12 at 460 FPM if you want it quieter, assuming a 65% free area. For a ceiling or sidewall register the same 100 CFM through a 10 × 6 at 75% free area gives about 320 FPM, which is quiet but throws air only a short distance; a 10 × 4 at 480 FPM throws farther. The calculator shows the actual velocity for the stock size it picks.
- What face velocity should a supply register have?
- 400 to 600 feet per minute for floor registers, which deliver air upward along a wall or window and do not need to throw far; 500 to 700 for ceiling and high sidewall registers, which must throw the air across the room before it falls. Below about 350 FPM a ceiling register dumps air straight down and leaves the far side of the room unmixed; above 700 a register of any kind whistles and the air feels like a draft.
- How many registers should a room have?
- One for every 100 to 150 CFM, placed so that the air sweeps the exterior walls and windows. A bedroom needing 120 to 180 CFM gets one or two; a living room needing 400 gets three or four spread along its outside walls. The calculator divides the room's airflow by the maximum you set per register (180 CFM by default) and rounds up. Fewer, larger registers save duct runs but make the air arrive in one place.
- What is register throw?
- The distance the air travels from the register before its velocity falls to about 50 FPM, the point where it stops being felt. Manufacturers publish throw for each size at each airflow. A ceiling register should throw about three-quarters of the way across the room; a floor register under a window should throw up to the ceiling. Throw rises with face velocity, which is why a register that is too large for its airflow leaves the far side of a room stagnant.
- Is the free area of a register the same as a grille's?
- It is usually lower. Supply registers have adjustable louvres and a damper behind the face, and stamped floor registers are typically 60 to 70% open; bar-type and aluminium registers run 70 to 80%. Return grilles have fixed, widely spaced bars and more free area. Use the maker's catalogue figure when you have it; the calculator defaults to 65%.
- Why does the room feel drafty even though the register is the right size?
- Usually placement or balancing. A register aimed at a seating area, or one that throws cold air across a doorway, feels drafty at any sensible velocity; a register that receives more than its share because the duct dampers are open while others are closed runs fast. Check the room's share with the CFM per room calculator, adjust the dampers at the trunk, and aim the louvres along the wall rather than into the room.