Maintained by the BuildSolver engineering team. Last updated September 15, 2026.
Calculator · Airflow
HVAC CFM calculator for design airflow
Pick whichever pair of numbers you already have for this job. It could be cooling capacity in tons, a sensible load in BTU/h, or a furnace output with its temperature rise. The calculator runs the same sensible heat equation three ways and returns one design CFM, updated as you type.
- Pick the mode that matches your numbers.
- Type the load or capacity value, then the CFM per ton, supply air temperature difference, or temperature rise for that mode.
- The result updates as you type and lists the derived totals plus any warning for a value outside the typical range.
Example result for the default inputs
CFM = 3 tons x 400 CFM per ton.
Design airflow
1,200 CFM
| Cooling capacity | 3 tons |
|---|---|
| CFM per ton | 400 |
| Total cooling capacity | 36,000 BTU/h |
For estimation purposes only. Consult a licensed engineer for final designs.
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How design CFM is calculated
Three ways to land on the same design CFM, depending on what you already know. All three use the sensible heat equation Qs = 1.08 x CFM x dT, with 1.08 valid for standard air at sea level.
- Sensible heat constant
1.08 = 60 min/h x 0.075 lb/ft3 x 0.24 BTU/(lb x °F)Minutes per hour, the standard density of air at sea level, and the specific heat of dry air. ASHRAE Handbook, Fundamentals, Chapter 1.- From cooling tons
CFM = tons x CFM per tonStart at 400 CFM per ton, drop toward 350 in humid climates, and go up to 450 only where latent load is low. These are common trade figures. The coil's expanded performance data sets the final number for a specific unit.- From sensible load
CFM = Qs / (1.08 x supply dT)Take Qs from the sensible line of your Manual J report. A cooling supply air dT of roughly 16 to 22°F is a common field range. Check the coil's expanded performance data for the actual value.- From furnace output
CFM = output / (1.08 x temperature rise)Output and the allowed temperature rise come off the furnace nameplate or the manufacturer's installation instructions, typically inside a 30 to 70°F range.
Worked example with a 3-ton replacement in a humid climate
A Gulf Coast replacement calls for a 3-ton condenser. At the nominal 400 CFM per ton, that is 1,200 CFM of design airflow. Humid climates often run 350 CFM per ton for better moisture removal. At that setting the same 3 tons comes to 1,050 CFM. Enter 3 tons in the calculator above and switch CFM per ton between 400 and 350 to get both numbers.
3 tons · 400 CFM per ton
1,200 CFM
3 tons · 350 CFM per ton
1,050 CFM
Dropping CFM per ton from 400 to 350 cuts total airflow by 150 CFM on this system. Check that figure against the trunk and branch sizes already run for the house before locking in the lower airflow.
Standards and data sources
- ASHRAE Handbook, Fundamentals, Chapter 1 - the moist air sensible heat equation and the air density and specific heat values behind the 1.08 constant.
- ACCA Manual S, equipment selection - equipment selection from the manufacturer's expanded performance data, where the airflow a unit actually runs sets its sensible and latent capacity.
- ACCA Manual D, duct design - the next step that turns a design CFM into trunk and branch duct sizes.
- Furnace manufacturer installation instructions - the allowed temperature rise range and rated output for a specific furnace model.
Limitations of this calculator
- Sea-level air is assumed. At 5,000 ft atmospheric pressure is about 0.83 of sea level (ASHRAE Handbook, Fundamentals, Chapter 1), and duct air at the same temperature is less dense by the same ratio, so the 1.08 constant drops to about 0.9 there. This calculator does not apply an altitude correction.
- CFM per ton, supply dT and temperature rise are values you enter or read off a nameplate. The calculator does not check them against a specific unit's blower table.
- The sensible load and furnace output modes need a real number from a Manual J calculation or a nameplate. If you don't have a Manual J number yet, run the free Manual J calculator first and bring the sensible load back here.
- This page returns one design CFM. It does not size ducts, registers or a blower speed tap to hit that airflow. Manual D handles that step.
- Latent capacity and sensible heat ratio are outside this calculation. A humid climate can call for a lower CFM per ton at the same tonnage. Run both values here and pick the one the coil's SHR data supports.
- Whole-house Manual J and the formula calculators on this site are free. Room-by-room airflow allocation and full Manual S equipment selection are part of the Pro plan.
Common CFM sizing mistakes
- One CFM per ton number on every job. Adjust 400 for the climate and the coil on this job. Lower airflow per ton leaves the coil colder and pulls more moisture, which matters in humid coastal climates. Dry climates with light latent load can run more.
- Supply dT picked from a rule of thumb without checking the coil data. A dT far outside the common 16 to 22°F range usually means the coil's rated airflow and its sensible heat ratio do not match the load you are trying to hit. Check both before trusting the CFM.
- Furnace rise pulled from a generic table without reading the nameplate. Every furnace model has its own allowed temperature rise printed on the rating plate. Use the generic 30 to 70°F range only as a sanity check after you read the plate.
- Total CFM mixed up with CFM per ton. A 3-ton system at 400 CFM per ton moves 1,200 CFM total. Plug 400 CFM into a duct sizing run for a 3-ton system and every trunk and branch comes out a third of the airflow it needs.
- No check for coil freeze at low airflow. A dirty filter, a closed damper or an undersized return can drop actual airflow well below the design CFM per ton, even when the equipment was sized correctly on paper.
Common questions about design CFM
What CFM per ton should I use for cooling?
Start at 400, the nominal trade figure. A Gulf Coast house usually runs closer to 350 so the coil pulls more moisture, and a house in a dry western climate can run up to 450. The engineering data for the specific coil has the final say.
Where does the 1.08 constant come from?
1.08 is 60 minutes per hour times 0.075 lb/ft3, the standard density of air at sea level, times 0.24 BTU per lb per °F, the specific heat of dry air. ASHRAE Handbook, Fundamentals, Chapter 1 gives the full sensible heat equation.
Which mode should I pick if I only have a Manual J printout?
Use the sensible load mode. Enter the sensible cooling BTU/h from the report and a supply air dT in the 16 to 22°F range, and the calculator returns the CFM the equipment needs to move.
What temperature rise should I enter for a furnace?
Use the rise range printed on the furnace rating plate or in the manufacturer's installation instructions. On most furnaces it falls somewhere inside 30 to 70°F. The calculator flags entries outside that range as a reminder to check the plate.
Does this calculator correct for altitude?
No. It assumes sea-level air. At 5,000 ft atmospheric pressure is about 0.83 of sea level (ASHRAE Handbook, Fundamentals, Chapter 1), and air at the same temperature is less dense by the same ratio, so the 1.08 constant drops to about 0.9 there. A job at elevation needs a constant corrected for local air density.
Why does the calculator warn me at some CFM per ton values?
Many coil manufacturers list a minimum near 300 CFM per ton in their installation instructions, and below that the coil can ice, especially at part load. Above 450, moisture removal weakens unless the climate is dry. The calculator still returns a CFM at those values, so check the coil's expanded performance data for the actual minimum and maximum airflow on that unit.
Is design CFM the same as duct size?
No. Design CFM is the airflow target. Turning that into trunk and branch duct sizes is a separate step under ACCA Manual D, using friction rate and available static pressure.
How many calculations can I run?
This calculator runs in the browser with no daily limit. In the BuildSolver chat you can take the design CFM into a first-pass duct size and run the whole-house load for free. Room-by-room Manual J and Manual S equipment selection are part of the Pro plan.
Used on the same job
Take the design CFM into chat
Give the assistant the design CFM from this page, the longest run and the duct material. It returns first-pass trunk and branch sizes from the same code as the free duct sizing calculator and runs the whole-house load for the same house. Room-by-room Manual D is part of the Pro plan.
Open the chat