CFM and air changes calculator

CFM is cubic feet of air per minute. Air changes per hour (ACH) is how many times that airflow would replace all the air in a room in an hour. They are the same idea in two units, joined by the room volume.

CFM = length × width × height × ACH ÷ 60

ACH = CFM × 60 ÷ (length × width × height)

The 60 turns minutes into hours. Volume is in cubic feet.

Air changes are the right tool for ventilation and exhaust: clearing smoke, moisture, odors or fumes from a room. They are the wrong tool for sizing heating and cooling airflow to a room. That airflow comes from the room's heat gain and loss, which depends on its windows, walls and exposure, not its volume. For that, start with the load calculator.

One ton of cooling is 12,000 Btu/h. Most air conditioners and heat pumps are designed to move about 400 cfm across the coil for each ton.

Lower airflow per ton keeps the coil colder, so it pulls more moisture out of the air. That helps in humid climates. Higher airflow gives more sensible cooling and suits dry climates. The equipment maker's expanded performance tables show the real sensible and latent capacity at each airflow; Manual S has you check those against the load rather than trusting a rule.

Each room then gets its share: room cfm = room load ÷ house load × system cfm. A bedroom that is 8% of the load on a 1,200 cfm system gets 96 cfm. The duct size calculator turns that into a duct size.

Heating airflow on a furnace is set differently, by the temperature rise the furnace is rated for. It is often lower than the cooling airflow. Check the rise range on the furnace's label.

The Home Ventilating Institute (HVI) sets out the sizing most contractors use. A fan you switch on should move at least 50 cfm; a fan that runs all the time can do the job at 20 cfm or more. Local code may set its own minimums, so check them too.

Right size. HVI recommends 1 cfm per sq ft of floor for bathrooms up to 100 sq ft with 8 ft ceilings, and never under 50 cfm. For bigger bathrooms, size by fixture: 50 cfm each for a toilet, a shower and a tub, and 100 cfm for a jetted tub. An enclosed toilet room gets its own fan.

Picking the fan once you know the CFM

The cfm from this calculator is what the fan has to deliver installed, against the resistance of its duct, damper and cap. Fan ratings are taken at a set static pressure, and real installs often have more. So:

• Read the fan curve. HVI-certified fans publish airflow at more than one static pressure. Pick a fan that delivers your target at the pressure your duct run will have, not only at the lowest rating point.
• Keep the duct short and smooth. Every elbow and every foot of flex adds resistance. A straight, short run of smooth duct to a roof or wall cap with a damper sized to match does more than a bigger fan on a bad run.
• Mind the noise. A fan that is too loud gets switched off. Sones ratings let you compare; quieter fans tend to get used.
• Use controls. A timer or humidity sensor that keeps the bath fan running after a shower clears moisture the switch-off-on-exit habit leaves behind. HVI recommends running bathroom ventilation at least 20 minutes after each use.

HVI recommends sizing a range hood by width: 100 cfm per linear foot for a hood against a wall, and 150 cfm per foot for an island hood, where cross drafts pull smoke away from it. Its floor is 40 cfm per foot on a wall and 50 over an island. Vent the hood outside: HVI notes that recirculating hoods provide no actual ventilation.

Large hoods often need makeup air under local code: a way for outdoor air to come in to replace what the hood pulls out. Without it a big hood can starve itself, rattle doors and, in a house with a gas water heater or furnace, pull exhaust back down a flue. Check with the building department before quoting a large hood.

The math tells you what a room or system should move. A measurement tells you what it does. The common ways, from quick to precise:

• Flow hood at the register or grille. The hood captures the air from one opening and reads cfm directly. Use it to balance rooms and to measure a bath fan at its grille, then compare against the target from this calculator.
• Anemometer and area. Measure the average air speed across a grille in feet per minute and multiply by its free area in square feet. Rough, because grilles do not flow evenly, but useful without a hood.
• Static pressure and the blower table. Measure total external static pressure across the air handler and read the airflow off the maker's fan table at that pressure and speed tap. This is the quickest system airflow check on a service call.
• Temperature rise. On a furnace, the temperature rise across the heat exchanger together with the firing rate gives the airflow. It should land inside the rise range on the label.

When the measured system airflow comes in well under 400 cfm per ton, the usual causes are a dirty filter or coil, undersized returns, or a blower set to the wrong speed. Each one costs capacity and efficiency. Put the reading and the fix on the estimate, so the homeowner sees why the work matters.

Most mix-ups with CFM come from using one kind of airflow to size another. Keep them separate:

Heating and cooling airflow recirculates house air; it does not ventilate. Exhaust fans remove air from one room while they run. Whole-house ventilation trades indoor air for outdoor air steadily, often through a heat or energy recovery ventilator. A tight new house needs all three, and each is sized on its own terms.

There is no single right ACH for a room. For the whole house, HVI recommends continuous ventilation of about 0.35 air changes per hour, which it simplifies to 5 cfm per 100 sq ft of floor. For a 2,000 sq ft house, that is 100 cfm running all the time.

Air changes are useful when a job comes with a target, such as clearing a room after painting, sizing an exhaust for a workshop, or meeting a spec for a commercial space. Put the target in and the calculator gives the cfm. Going the other way, measure the airflow at a fan or register with a flow hood, put in the room size, and the calculator tells you the air changes you are getting.

Once you know the fan or the airflow, add it to an estimate in Koira: the fan, the duct run, the roof or wall cap and your labor, each as its own line.