Understanding your result
The headline is the nominal grille size to buy, multiplied by the number of grilles, with the airflow per grille and the actual face velocity through its free area in the sub-line. The stats give the free area needed per grille, the nominal area of the grille chosen and the actual velocity, which is a little below your target because the stock size is larger than the exact one.
The table includes a rule-of-thumb check, the total grille area in square feet per ton of system, which experienced installers use at a glance: around 1.5 to 2 sq ft of grille per ton is normal, and anything under 1 sq ft per ton is a restricted system. A single grille on a system above 1,000 CFM gets a note suggesting two or more; a filter grille at more than 300 FPM gets a note to lower the velocity.
The return duct behind the grille is sized separately with the duct size calculator at the return velocity limit, and an existing return can be checked with the duct velocity calculator. The system airflow the returns must collect is the same total the CFM per room calculator distributes to the supply side; if a filter lives in the grille, the filter cost calculator budgets its replacements.
How we calculate this
Velocity is airflow divided by the area the air passes through, and for a grille that area is the free area, not the face; the free-area share is the manufacturer’s figure for the grille type. The stock sizes are the common nominal return grille dimensions from 10 × 10 to 30 × 36 inches, in order of area, and the calculator takes the smallest that provides the nominal area needed. The velocity targets are the residential quiet-operation conventions set out on the ventilation and exhaust minimums page alongside the duct limits, and the duct size chart pairs each grille with the return duct that serves it. Metric mode converts to millimetres, square centimetres and metres per second.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Face velocity | 400 FPM (300 for filter grilles) | Residential quiet-design practice; ACCA Manual D guidance for return grilles |
| Free area | 70% | Typical stamped-steel return grille; bar grilles 75 to 85% |
| Stock sizes | 10 × 10 to 30 × 36 in | Common US return grille nominal sizes |
| Rule of thumb | 1.5 to 2 sq ft of grille per ton | Installer practice at 400 CFM per ton and 70% free area |
| Filter grille limit | 300 FPM | Pressure drop of 1-in pleated filters rises steeply above this (filter makers’ curves) |
Assumptions last reviewed October 8, 2026.
The calculator sizes grilles for airflow and noise. It does not size the return duct or plenum behind them, does not model the pressure drop of a specific filter, and does not decide where returns should go, which the guide on return air covers along with transfer grilles and jump ducts. The guide to MERV filters and airflow explains why a filter grille needs the lower velocity.
Two worked examples
One central return for a 3-ton system
1,200 CFM through a single grille at 400 FPM, 70% free area, no filter in the grille.
- Free area needed: 1,200 ÷ 400 × 144 = 432 sq in
- Nominal area needed: 432 ÷ 0.70 = 617 sq in
- Smallest stock grille: 24 × 30 in (720 sq in)
- Actual face velocity: 1,200 × 144 ÷ (720 × 0.70) = 343 FPM
- Check: 1.67 sq ft of grille per ton
A 24 × 30 grille is a large opening in a hallway wall, which is why the note suggests splitting the return. Two 16 × 20 grilles (640 sq in together) carry the same air at 386 FPM each and can be placed to serve both ends of the house.
Two filter grilles for a 2-ton system
800 CFM split between two grilles that hold 1-inch pleated filters, so 300 FPM, 70% free area.
- Airflow per grille: 400 CFM
- Free area needed: 400 ÷ 300 × 144 = 192 sq in; nominal 192 ÷ 0.70 = 274 sq in
- Smallest stock grille: 12 × 24 in each (288 sq in)
- Actual face velocity: 400 × 144 ÷ (288 × 0.70) = 286 FPM
- Check: 2.0 sq ft of grille per ton
At 286 FPM a clean 1-inch pleated filter in each grille adds a modest pressure drop and the system breathes; the same 800 CFM through one 14 × 20 filter grille would run at 588 FPM, and a dirty filter there would choke the blower. The filter cost calculator budgets two filters changed every 90 days.
Where to find your inputs
Total return airflow. 400 CFM per ton of the system (350 humid, 450 dry), or the blower’s rated airflow from the air handler’s label.
Number of grilles. How many returns you will install or already have; the calculator divides the airflow equally. For unequal returns, run it once per grille with that grille’s share.
Free area. From the grille maker’s catalogue, listed as free area in square inches or as a percentage; 70% for stamped steel if unknown.
Filter. Tick the box if the grille holds the filter; the calculator then expects 300 FPM or less.
Common mistakes
- Sizing to the nominal face. A 20 × 20 grille is not 400 sq in of opening; at 70% free area it is 280.
- One small return for a big system. It is loud, it raises static pressure and it starves closed rooms.
- A filter grille at supply-register velocities. Keep filter grilles at 300 FPM or move the filter to a 4-inch cabinet at the air handler.
- Forgetting the duct behind the grille. A big grille on an 8-inch duct is still an undersized return; size the duct at the return limit.
- No return path from bedrooms. Closed doors isolate rooms from a central return; add returns, jump ducts or transfer grilles.
- Decorative grilles. Wood and pattern grilles can be under 50% open; check the free area before using them on a return.
Questions people ask
- How big should a return air grille be?
- Large enough to pass the system's airflow through its free area at 300 to 500 feet per minute. For 400 CFM per ton at 400 FPM through a 70%-open grille, that works out to about 2 square feet of nominal grille per ton, so a 3-ton system needs about 6 sq ft, for example one 24 × 30 and one 20 × 20 grille. A single 14 × 20 grille (1.9 sq ft) on a 3-ton system runs at over 1,000 FPM through its free area and is loud and restrictive.
- What is free area on a grille?
- The open area between the bars or fins through which air actually passes, as a share of the nominal face dimensions. Stamped steel return grilles are typically 65 to 75% open; bar-type and aluminium grilles 75 to 85%; decorative wood grilles sometimes under 50%. A 20 × 20 grille at 70% free area has 280 sq in of opening, not 400. Manufacturers publish the free area in their catalogues; the calculator uses 70% unless you change it.
- What face velocity should a return grille have?
- 300 to 500 FPM through the free area, with 400 a common quiet design and 300 for grilles that hold the filter. At 500 FPM a return grille starts to hum; at 700 it whistles. Supply registers run faster because throw matters there and the air is leaving the room; return grilles sit where people are and have nothing between the blower and the room but the grille itself.
- Should the filter be in the return grille or at the air handler?
- Either works if sized right. A filter grille is convenient to change, but the 1-inch filter it holds adds a pressure drop that rises steeply with velocity, so the grille should run at 300 FPM or less, which means a larger grille. A 4-inch media filter cabinet at the air handler has a lower pressure drop, lasts 6 to 12 months and lets the grilles be sized for airflow alone; it is usually the better arrangement for systems of 3 tons and up.
- How many return grilles does a house need?
- At least one central return sized for the whole system, and better a return in or near every room that has a door, or a transfer grille or jump duct from each bedroom to the hall where the central return is. A single return starves bedrooms behind closed doors, pressurises them and depressurises the hall, which pulls outside air in through every gap. The calculator sizes each grille for the share you give it.
- Why does my return grille whistle?
- Air is moving through its free area faster than about 500 to 600 FPM. The grille is too small for the system's airflow, or a filter in it is dirty, or the duct behind it is undersized so that the blower pulls hard through the restriction. Enter the system's airflow and the grille's size in the duct velocity calculator to see the velocity, and use this calculator to see what size would be quiet.