Register Size Calculator

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Find the right supply register, return grille, or filter grille size for a given airflow (CFM).Learn more ▾Show less ▴
Registers and grilles are sized by free area — the open area air actually passes through — not by their nominal face dimensions. This tool works back from the face-velocity guidance in ACCA Manual D (3rd Edition): enter the airflow, choose supply, return, or filter, and read off the smallest standard sizes that fit, each with its actual face velocity and how much of its capacity you are using. A quieter option targets a lower velocity when noise matters more than size.
CFM
Required free area
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Target face velocity
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Manual D limit
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Sizes that fit

Nominal size Free area Face velocity Capacity used
Split across several registers or grilles
Supply outlets are also selected for throw — how far the air carries into the room. ACCA Manual D sets a 700 FPM upper face-velocity limit; check throw in the manufacturer's data.
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About This Tool

This calculator answers a common duct-design question: which nominal register or grille size handles a given airflow? Enter the CFM a run delivers, pick the application, and it returns the smallest standard sizes that keep the face velocity within ACCA Manual D guidance. It covers supply registers, return grilles, and filter grilles, using typical free areas for standard nominal sizes, and falls back to a multi-register split when one opening is not enough.

Sources: ACCA Manual D

How to Use

  1. Enter the airflow the duct run delivers, in CFM
  2. Choose supply register, return grille, or filter grille, and a standard or quieter velocity target
  3. Pick one of the suggested sizes, checking its face velocity and how it fits your install

How to Use

  1. Enter the airflow the duct run delivers, in CFM
  2. Choose supply register, return grille, or filter grille, and a standard or quieter velocity target
  3. Pick one of the suggested sizes, checking its face velocity and how it fits your install

Methodology

Sizing works backwards from face velocity: required free area (sq in) = CFM × 144 ÷ target velocity (FPM). The calculator compares that requirement against typical free areas of standard nominal sizes and lists the smallest ones whose rated capacity covers your airflow. Velocity targets follow ACCA Manual D, Table A1-1: supply outlets are sized for throw with a 700 FPM upper limit — 500 FPM is the usual residential comfort target — while return grilles are limited to 500 FPM and filter grilles to 300 FPM. The quieter option lowers the target to 400 FPM for supply and 300 FPM for return, because a lower face velocity produces less register noise (Manual D, Appendix 13).

Sources: ACCA Manual D — Table A1-1 & Appendix 13

Understanding Your Results

Each candidate shows the actual face velocity at your airflow and how much of the size's rated capacity you are using. Lower velocity means quieter operation, but a much larger supply register also reduces throw — the distance conditioned air carries into the room — so the smallest fitting size is usually the best starting point. Several shapes share the same free area (a 6×10 equals a 10×6), so pick the proportion that suits your joist bay, wall cavity, or floor opening. Free areas are typical catalog values. Confirm the actual free area and throw in the manufacturer's data before ordering.

Sources: ACCA Manual D

Practical Examples

A bedroom run delivers 120 CFM of supply air. At the 500 FPM comfort target the required free area is 120 × 144 ÷ 500 = 34.56 sq in — the smallest fitting sizes are 6×10 or 10×6 (42 sq in of free area), with an actual face velocity of about 411 FPM. A 900 CFM return at 500 FPM needs 259.2 sq in — more than any single listed grille. The calculator suggests two 24×8 grilles handling about 450 CFM each at roughly 484 FPM; for a quieter system, size the same airflow at the 300 FPM target instead.

Register Sizing Tips

Match the shape to the install: long, shallow sizes (such as 4×10 or 4×12) suit floors along windows and joist bays, while squarer sizes suit walls and ceilings. When a room needs a lot of air, two smaller supply registers on opposite sides usually distribute it more evenly — and more quietly — than one large register. Returns are the cheap win for a quiet system: generously sized return and filter grilles cost little extra and keep face velocity well under the limits. Filter grilles need the most area — at 300 FPM a filter grille passes 40% less air than the same size used as a plain return at 500 FPM. This tool sizes the opening; pair it with the duct calculator to size the run that feeds it and with the room airflow calculator to get each room's CFM.

Sources: ACCA Manual D

Sources

All calculations are performed locally in your browser. No data is sent to any server.

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Frequently Asked Questions
How do I size a register or grille from CFM?
Divide the airflow by the target face velocity to get the free area the opening needs: free area (sq in) = CFM × 144 ÷ velocity (FPM). A 120 CFM supply run at 500 FPM needs 34.56 sq in, so the smallest standard size with enough free area — such as a 6×10 — fits. This calculator does that lookup across a table of typical nominal sizes and shows the two or three smallest options.
What is face velocity and why does it matter?
Face velocity is the speed of the air passing through the grille opening, measured in feet per minute (FPM). It drives two things: noise and throw. ACCA Manual D notes that a properly installed supply outlet avoids objectionable noise at face velocities up to 700 FPM, and a return grille up to 500 FPM. Velocity also sets how far supply air carries into the room, which is why supply outlets are selected for throw as well as noise.
Why do supply, return, and filter grilles use different velocity limits?
ACCA Manual D, Table A1-1 sets noise-based limits per component: supply outlets are sized for throw with a 700 FPM upper limit, return grilles should stay at or under 500 FPM, and filter grilles at or under 300 FPM. Returns get a lower limit because they have no throw requirement, so there is no reason to accept the noise of high velocity. Filter grilles are lowest of all: air must also pass through the filter media, and the pressure drop across the filter grows as velocity rises.
What is free area?
Free area is the open portion of the grille face that air can actually pass through — the total face area minus fins, louvers, and frame. It is always smaller than the nominal width × height: a 6×10 register is not 60 sq in of opening but typically around 42 sq in of free area. The free areas in this tool are typical values for standard stamped-face registers. Actual free area varies by model and manufacturer, so check the product's engineering data for the exact figure.
What if no single register is large enough?
For large airflows the calculator suggests splitting across several units of the largest listed size — 900 CFM of supply air, for example, becomes two 24×8 registers at about 450 CFM each. Multiple smaller outlets often work better than one large one anyway: they distribute air more evenly through the space, and each one runs at a lower, quieter face velocity.
When should I use the quieter velocity option?
Choose it for noise-sensitive rooms — bedrooms, offices, media rooms. It targets a lower face velocity (400 FPM for supply, 300 FPM for return), which means a larger opening moving the same air more slowly. That is exactly how Manual D's noise guidance works: the lower the face velocity, the less register noise. The trade-off is simply a physically larger grille. Filter grilles have no separate quieter setting because their 300 FPM limit is already the lowest target.
Is my data private?
Yes. The calculator runs entirely in your browser: the airflow you enter and the sizes it suggests never leave your device, and no server requests are made for the calculation. If you share a link with your inputs, those values travel only inside that link to whoever you send it to.
How accurate are the recommendations?
Treat them as a solid starting point, not a final selection. Nominal sizes and free areas vary by manufacturer, and a specific register's throw, spread, and noise ratings come from its engineering catalog. For final selections — especially supply outlets, which Manual D sizes for throw — verify the chosen size against the manufacturer's performance data and your overall duct design.