CFM per Ton Calculator
Enter the tons and CFM per ton to get the total airflow. Split it across rooms if you need to.
How to use ↓Result
Duct Chart · CFM per Ton Calculator · ductchart.com
Estimates only: they can differ from your chart or the manufacturer's data by 5% or more (flex by 2 times or more), so verify accuracy for your own application against the design documents, manufacturer data and local code.
How to use
- Enter the system size in tons.
- Leave CFM per ton at 400 or pick 350 or 450. You can type your own.
- To split the air, open Split across rooms and add a percentage or load for each room.
- Press Calculate, then use the Size link to size each branch.
Tons to CFM
Total airflow is the system size in tons times the CFM per ton. A 3 ton system at 400 CFM per ton moves 1,200 CFM. 350, 400 and 450 CFM per ton are a common rule of thumb, and 400 is the usual starting point, which is why it is the default here. It is a starting estimate, not design airflow: ACCA Manual D ties airflow to the sensible heat ratio and the equipment maker’s data. The CFM per ton guide explains the reasoning.
Tons are the nominal size on the nameplate. The airflow the blower actually delivers depends on the speed tap and the static pressure it is working against, so check the manufacturer’s blower table for the real figure.
To split the air across rooms, enter a percentage for each room, or the room loads in Btu/h. With loads, each room gets the total airflow times its load divided by the total load, which is the Manual D method. Percentages that do not add to 100 are shared in proportion, and the calculator tells you.
Each row has a Size link that opens the duct size calculator with that CFM filled in.
Example: a 3 ton system at 400 CFM per ton moves 1,200 CFM. With room loads of 12,000, 8,000 and 4,000 Btu/h, the rooms get 600, 400 and 200 CFM. Each of those goes straight into the duct size calculator.
FAQ
Is 400 CFM per ton right for my system?
It is the common figure and the default here. Common rule of thumb: 350 in humid climates and 450 or more in dry ones. Manual D ties airflow to the sensible heat ratio, so it is a starting estimate only. The equipment manufacturer’s blower table and design documents come first.
Do I split by percent or by load?
Use loads if you have a room by room load calculation. Use percentages if you are working from the design airflow for each room.
Why do the room CFM not add up exactly?
Each room is rounded to a whole CFM for display. The calculation itself is not rounded.