CFM Calculator
Airflow is the third variable in every capacity equation. Enter two of BTU, temperature rise and CFM, and get the third — at your elevation.
CFM calculator
Airflow at different temperature rises
| Temperature rise | CFM required | CFM per ton | Typical for |
|---|
Where the 1.08 comes from
The sensible heat equation for air is CFM times 60 minutes times 0.075 pounds per cubic foot times 0.24 BTU per pound-degree, which collapses to 1.08. Every one of those three numbers is an assumption: 0.075 lb/ft³ is air density at sea level and 70°F, and 0.24 is the specific heat of dry air. Change the elevation and you change the density, and the constant changes with it.
At 5,000 feet the density is about 0.062 lb/ft³, which puts the constant near 0.90 instead of 1.08 — a 17 percent difference. The practical consequence is that the same blower moving the same CFM delivers 17 percent less heat at altitude, so a system sized at sea level and installed in Denver behaves like a smaller system. This calculator applies the density correction, and shows you the factor it used so you can check it against a blower table.
400 CFM per ton is a convention, not a target
Nominal airflow for residential cooling is 400 CFM per ton, and it is nominal because it is the midpoint of a range that manufacturers publish as a table. A 3-ton air handler may be approved for 1,050 to 1,350 CFM depending on the speed tap, and the same cabinet at 1,050 CFM behaves differently from the same cabinet at 1,350.
The trade picks within that range by climate. In a humid climate, dropping to 350 CFM per ton makes the coil colder, pulls more moisture out of the air, and gives a lower sensible capacity with better comfort — the system runs longer and the house feels drier. In a dry climate, 450 CFM per ton is common, because there is little latent load to chase and the extra airflow lets the coil deliver more sensible capacity at a smaller temperature split. Neither is wrong; both are decisions about what the system is optimising.
What the temperature rise tells you about a running system
Measure the air temperature in the return plenum and again six to ten feet down the supply trunk, and subtract. A cooling system in normal condition shows 16 to 22°F, and a gas furnace shows a rise specified on its rating plate, typically 35 to 65°F. Those two numbers are the cheapest diagnostic in HVAC because they are computed from the same equation this page uses, run backwards.
- Split too low on cooling — too much airflow, a dirty coil, low refrigerant charge, or a duct system leaking supply air into the attic before it reaches the register.
- Split too high on cooling — not enough airflow. Filter, return duct, closed dampers, or a blower on the wrong speed tap. Left alone it ends in a frozen coil.
- Furnace rise above the rating plate — under-airflow, which overheats the heat exchanger. This one is a safety issue, not just an efficiency one.
Why low airflow is worse than it looks
Capacity is airflow times temperature difference, so a system at 70 percent of design airflow can still hold the house at setpoint on a mild day — right up until the coil temperature drops below freezing. Ice on the coil then blocks the very airflow that was already short, and the system spirals: less air, colder coil, more ice. A restricted filter, a collapsed flex duct, or a return that was never sized for the blower will all do this, and none of them show up in a load calculation.
High airflow fails more quietly. The coil stays warm, moisture passes through to the rooms, and the house feels clammy at the thermostat setting while the equipment works harder against a higher static pressure. On a variable-speed system the blower ramps up to compensate, which raises noise and power draw without improving comfort.
Ventilation airflow is a different calculation
The CFM a system moves for heating and cooling has nothing to do with the CFM a tight house needs for fresh air. ASHRAE 62.2 sets the whole-house rate at 0.01 CFM per square foot of floor area plus 7.5 CFM per occupant, with the occupant count taken as bedrooms plus one. A 2,000 square foot three-bedroom house therefore needs 20 plus 30, or 50 CFM continuous — well under one ton's worth of airflow, but enough that a sealed, spray-foamed house without mechanical ventilation will hold cooking smells, moisture and CO&sub2;.
That number is why an ERV or a small dedicated make-up air unit appears in tight new construction and not in a 1970s house that already leaks 0.8 air changes an hour. Sealing a house without adding ventilation trades a heating bill for indoor air quality, which is a bad trade.
What this calculator does not do
It does not tell you what your blower will actually deliver, because that depends on the static pressure of your specific duct system — the blower table is a curve, and the duct system is the load line it operates against. It also does not account for the difference between total and sensible capacity, which matters in humid climates: a 3-ton unit might deliver 36,000 BTU per hour total but only 25,000 sensible, and it is the sensible number that the temperature rise equation uses. If you are checking a running system, use the measured rise and the rated sensible capacity; if you are designing one, use the duct calculator next to this page.
Frequently asked questions
How many CFM do I need for a 3-ton unit?
Nominal is 1,200 CFM at 400 per ton, but the approved range on most 3-ton air handlers runs from about 1,050 to 1,350 CFM depending on the speed tap. Humid climates usually run toward the low end for better moisture removal, dry climates toward the high end.
What is a normal temperature split for an air conditioner?
Between 16 and 22°F between the return and the supply, measured at the equipment rather than at a distant register. Under 16 suggests too much airflow, low charge or a dirty coil; over 22 suggests restricted airflow.
Does altitude really change CFM calculations?
It changes the constant, not the CFM. At 5,000 feet air density is about 17 percent lower, so the same CFM carries 17 percent less heat. A system sized at sea level and installed at altitude effectively loses capacity unless the blower is sped up or the equipment is derated.
Is 400 CFM per ton always right?
No, it is a convention. Manufacturers publish approved ranges, and the right point inside that range depends on latent load: lower airflow for humid climates, higher for dry ones. What matters is staying inside the approved range and matching the charge to the airflow you chose.
How much fresh air does a house need?
ASHRAE 62.2 gives 0.01 CFM per square foot plus 7.5 CFM per occupant, with occupants counted as bedrooms plus one. A 2,000 square foot three-bedroom house works out at about 50 CFM continuous.
Why does my furnace have a temperature rise on the rating plate?
Because the heat exchanger has a design airflow window. Too little air and it overheats, which can crack the exchanger; too much and the flue gases condense in the wrong place. The plate gives the acceptable rise, and measuring it is the fastest check that the blower is set correctly.
Can I just measure CFM at a register?
A balancing hood gives a reliable reading at a single register, and summing the registers approximates total system airflow. The sum is usually lower than the blower's rated CFM because of duct leakage, which is exactly the number worth knowing.
Disclaimer: This tool converts between airflow, heat and temperature change for planning purposes. It does not replace a blower performance table, a static pressure measurement, or a Manual D duct design.
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