Supply airflow comes from the sensible cooling load, not the total load: CFM = sensible load ÷ (1.08 × ΔT), where ΔT is room temperature minus supply air temperature. At sea level, an 18,000 Btu/h sensible load with 75°F room and 55°F supply air needs about 833 CFM.
Below is where the formula comes from, when the 1.08 changes, and why "400 CFM per ton" gets commercial spaces wrong.
The formula
Sensible heat carried by an air stream: q = 1.08 × CFM × ΔT
Rearranged for airflow: CFM = q ÷ (1.08 × ΔT)
- q is the space sensible cooling load in Btu/h, at the peak hour.
- ΔT is room dry-bulb minus supply air dry-bulb, in °F.
- 1.08 comes from standard air: 0.075 lb/ft³ × 60 min/h × 0.24 Btu/lb·°F. The ASHRAE Handbook—Fundamentals nonresidential load chapter uses 1.10, which uses the specific heat of moist air (about 0.244) instead of dry air. The difference is under 2%.
Why sensible, not total
The supply air temperature handles sensible heat. Latent heat (moisture from people, outdoor air, cooking) is removed at the coil, by how far below dew point the coil runs. If you size airflow on total load you oversize the air. If you size it on tons with a rule of thumb, you can undersize it. Size airflow on sensible load, then check the coil can handle the latent load at that airflow.
Step by step
- Get the space sensible load at its peak hour. Room by room, from a load calculation (ASHRAE RTS or Heat Balance). Don't use a block load for room airflow.
- Pick the supply air temperature. 55°F is the common starting point for VAV and constant-volume systems with a chilled water or DX coil. Warmer supply air (60–65°F) is common with chilled beams, some underfloor systems, and DOAS-plus-terminal designs.
- Set ΔT. Room setpoint minus supply temperature. 75°F room and 55°F supply gives 20°F.
- Compute CFM. CFM = q ÷ (1.08 × ΔT).
- Check the minimums. Final supply is the largest of the load-driven airflow, any code or design minimum (air changes per hour or CFM per ft²), and the airflow needed to deliver outdoor air per ASHRAE 62.1 on systems that mix outdoor air into the supply. Then check VAV box minimums.
Worked example
An interior office zone with an 18,000 Btu/h peak sensible load and a 2,400 Btu/h latent load, 75°F room, 55°F supply, sea level. The loads are illustrative.
- Load-driven supply: 18,000 ÷ (1.08 × 20) = 833 CFM.
- With ASHRAE's 1.10 factor: 18,000 ÷ (1.10 × 20) = 818 CFM.
- If the supply air were 60°F (ΔT 15°F): 18,000 ÷ (1.08 × 15) = 1,111 CFM. Warmer supply air means more air for the same load, and bigger ducts.
- Rule of thumb for comparison: the total load of 20,400 Btu/h is 1.7 tons. At 400 CFM per ton that's 680 CFM, about 18% short of what the sensible load needs.
Why 400 CFM per ton misleads in commercial work
400 CFM per ton is a rule of thumb from DX equipment at typical sensible heat ratios. It quietly assumes a supply temperature split and a sensible heat ratio. Offices, classrooms, and labs with high internal sensible gains need more air per ton. Spaces with high latent loads (gyms, kitchens, auditoriums with lots of people) need less air per ton, and too much air there leaves the space humid. Use it as a sanity check on equipment, not to size room airflow.
Altitude correction
The 1.08 assumes sea-level air density. Thinner air carries less heat per CFM, so you need more of it. Multiply 1.08 by the ratio of site air density to standard air density. At 5,000 ft the standard-atmosphere pressure ratio is about 0.83, so at the same temperature the factor becomes about 0.90, and the 18,000 Btu/h example needs about 1,000 CFM instead of 833.
Latent check
Latent heat in an air stream: q = 0.68 × CFM × Δgrains, where Δgrains is room humidity ratio minus supply humidity ratio in grains per pound. Rearranged: Δgrains = q ÷ (0.68 × CFM).
For the example, 2,400 ÷ (0.68 × 833) ≈ 4.2 grains. The supply air has to be about 4 grains/lb drier than the room. At 75°F and 50% RH (about 65 grains), that means supply air at about 60 grains. Saturated 55°F air holds about 64 grains, so it isn't dry enough. The coil has to leave the air at 55°F with a dew point near 53°F (about 94% RH). Check the coil leaving condition on a psychrometric chart before you call it done.
How HVAKR does it
HVAKR runs cooling loads with ASHRAE RTS for every hour of every month (288 month-hour points) and finds each space's sensible peak. In Air Balancing, the supply default for each space is the largest of three numbers: the load-determined supply (the sensible load at the unit's supply air temperature), the code-driven minimum (the larger of the space type's minimum air changes and minimum CFM per ft²), and the ventilation requirement on a coupled terminal. Supply Basis shows the values behind it, so you can see which one controls. Change the supply air temperature or a window and the load-determined supply recalculates. Re-run Size Ducts to resize the ductwork.
FAQ
How do you calculate CFM from a cooling load?
Divide the space sensible cooling load in Btu/h by 1.08 times the difference between room and supply air temperature: CFM = sensible Btu/h ÷ (1.08 × ΔT). An 18,000 Btu/h sensible load at a 20°F ΔT needs about 833 CFM at sea level.
Where does the 1.08 in 1.08 × CFM × ΔT come from?
It's standard air density (0.075 lb/ft³) times 60 minutes per hour times the specific heat of air (0.24 Btu/lb·°F). ASHRAE's nonresidential load chapter uses 1.10, based on moist air.
Should I use sensible or total load to calculate CFM?
Sensible. Supply airflow and supply temperature handle the sensible load. The latent load is handled by the coil's dew point, which you check after setting airflow.
Is 400 CFM per ton correct?
It's a rule of thumb, not a design method. Commercial spaces with high sensible loads often need more than 400 CFM per ton, and high-latent spaces need less. Size room airflow from the sensible load.
How does altitude affect CFM?
Thinner air carries less heat per cubic foot, so you need more CFM for the same load. Multiply the 1.08 factor by the site-to-standard air density ratio, about 0.83 at 5,000 ft.
What supply air temperature should I use?
55°F is the common starting point for conventional VAV and constant-volume systems. Chilled beams, underfloor air, and some DOAS designs use warmer supply air. The colder the supply air, the less airflow you need.
Sources
- ASHRAE Handbook—Fundamentals: Chapter 1, Psychrometrics, and Chapter 18, Nonresidential Cooling and Heating Load Calculations.
- HVAKR docs: Air Balancing and Cooling Loads.
- PsychroLib, used for the grain values: github.com/psychrometrics/psychrolib
