Once you know the BTU load on a room, the next question is how much air it takes to deliver that load. That's CFM — cubic feet per minute — and it drives everything else in the duct system. Here is the math, the rule of thumb, and the static-pressure problem nobody wants to talk about.

Sensible heat formula

The standard equation for air-side heat transfer:

CFM = BTU/hr ÷ (1.08 × ΔT)

  • BTU/hr = the heating or cooling load on the room or system
  • 1.08 = a constant that combines air density (0.075 lb/ft³), specific heat of air (0.24 BTU/lb·°F), and 60 minutes per hour
  • ΔT = temperature difference between supply air and return air

Run the live calculation on the duct CFM calculator.

Pick the right ΔT

The temperature differential you use changes the answer significantly:

  • Cooling: 20°F ΔT — supply air at 55°F when return is 75°F. This is the standard residential cooling design point.
  • Furnace heating: 50-70°F ΔT — supply air at 130-140°F when return is 70°F. Gas and propane furnaces have higher supply temps than heat pumps.
  • Heat pump heating: 30-40°F ΔT — heat pumps deliver lower-temperature heat than furnaces, so the airflow is similar to or higher than cooling.

Same room, same load: a furnace needs about 1/3 the CFM that AC needs because the higher ΔT lets each cubic foot of air carry more heat.

The 400 CFM per ton rule of thumb

For residential AC, the industry rule is 400 CFM per ton of cooling capacity. That 's a ratio that comes out automatically when you run a 20°F ΔT through the formula:

12,000 BTU ÷ (1.08 × 20°F) = 555 CFM theoretical, which derates to 400 in practice

The 400-per-ton rule assumes a properly designed return-air side and a typical residential coil. High-static-pressure systems may end up at 350; high-humidity climates may design at 350 to extend dehumidification cycle time.

Why the duct system has to match

CFM is just the airflow target — the duct sizing has to actually deliver it. Manual D (the airflow companion to Manual J) sizes ducts to:

  • Maintain velocity below 800 fpm in trunk lines and 500 fpm in branches (above that, ducts are noisy and deliver poor airflow)
  • Stay below 0.5 in. w.c. external static pressure (above that, the blower can't deliver design airflow)
  • Balance airflow proportionally to room load — the room's CFM is what the calculator gives you, but it has to actually show up at the register

A 2.5-ton system needs ~1,000 CFM total. If the existing ductwork was sized for a 2-ton (800 CFM) system and you upsize the equipment without resizing ducts, you've just capped your delivered capacity at 800 CFM regardless of what the equipment can do.

Common errors

Designing on rated CFM, not delivered CFM. A blower rated for 1,000 CFM at 0.5 in static delivers 750 CFM at 0.8 in static. Real-world residential ducts often run at 0.7-0.9 in static, so equipment specs lie if you don't measure.

Sizing ducts to the registers, not the load. A 6×10 supply register has roughly 60 in² of opening, which moves about 100 CFM at 1.5 in/sec face velocity. The register isn't the bottleneck — the duct feeding it is.

Forgetting the return side. The return-air system has to move the same CFM as the supply, or the blower can't deliver design airflow. Most residential systems are starved for return — one giant 25×25 in central return for a whole house should really be 2-3 returns.

Quick FAQ

How many CFM for a 2-ton AC? About 800 CFM at 400 CFM/ton — that's 24,000 BTU ÷ (1.08 × 20°F).

Can I undersize the CFM and get more dehumidification? Yes — the 350 CFM/ton derate is a deliberate move for humid climates. The lower airflow keeps the coil colder and pulls more water out of the air, at the cost of slower temperature reduction.

Why is my room hot when the system is the right size? Almost always a duct problem, not a system problem. Cracked or disconnected duct in the attic, blocked register, or undersized branch is the usual culprit. Measure the actual CFM at the register with a flow hood — it's almost never what the design calls for.

Estimate only. The duct CFM calculator gives the airflow needed to move a given heat load. Real duct system performance also depends on static pressure, duct sizing per Manual D, and the return-air design — verify with a licensed HVAC contractor for full system design.