CFM Per Ton: The 400 Rule (and When to Use 350 or 450)

A ton of cooling is 12,000 BTU/h. The 400 CFM per ton target falls out of the standard-air sensible heat equation used throughout ASHRAE and ACCA design material:

BTU/h (sensible) = 1.08 × CFM × ΔT (°F)

The 1.08 constant is three numbers multiplied together: 0.075 lb/ft³ (density of standard air) × 60 min/h × 0.24 BTU/lb·°F (specific heat of air). That also tells you its limits — it's only accurate for dry air around 70°F at sea level.

Run it at 400 CFM with a typical 20°F drop across the evaporator coil: 1.08 × 400 CFM × 20°F = 8,640 BTU/h of sensible capacity per ton. The rest of the ton — roughly 3,400 BTU/h — comes out as latent capacity, meaning moisture condensing on the coil. That's about a 70/30 sensible-to-latent split, which is close to what most residential equipment is rated to deliver.

The rule in one line: tonnage × 400 = target CFM.

System size	350 CFM/ton (humid)	400 CFM/ton (baseline)	450 CFM/ton (dry)
1.5 tons	525 CFM	600 CFM	675 CFM
2 tons	700 CFM	800 CFM	900 CFM
2.5 tons	875 CFM	1,000 CFM	1,125 CFM
3 tons	1,050 CFM	1,200 CFM	1,350 CFM
3.5 tons	1,225 CFM	1,400 CFM	1,575 CFM
4 tons	1,400 CFM	1,600 CFM	1,800 CFM
5 tons	1,750 CFM	2,000 CFM	2,250 CFM

These are design targets, not gospel. The blower table in the installation manual is the actual spec — the rule of thumb tells you whether the number you measured is even in the right neighborhood.

Airflow across the coil sets the trade between sensible and latent capacity. Slow the air down and the coil runs colder, so more of the capacity goes into condensing water out of the air. Speed it up and the coil runs warmer, so more of the capacity shows up as temperature drop.

ACCA's own blog runs the numbers on a 2.5-ton unit. At 875 CFM (350 CFM/ton) it delivers about 29,500 BTU/h total, with 52% sensible — 15,340 BTU/h sensible and 14,160 BTU/h latent. Bump the same unit to 1,125 CFM (450 CFM/ton) and total capacity rises slightly to about 30,300 BTU/h, but the sensible share jumps to 61% — roughly 18,500 BTU/h. Same box, same charge: about 3,000 BTU/h moved from moisture removal to temperature drop just by changing blower speed.

So the climate logic is:

• 350 CFM/ton — hot-humid (Gulf Coast, Southeast). The latent load is the fight. A house at 75°F and 65% RH feels worse than one at 78°F and 50% RH.
• 400 CFM/ton — mixed climates. The balanced default.
• 450 CFM/ton — hot-dry (Phoenix, Las Vegas, high desert). There's almost no moisture to remove, so extra sensible capacity is free performance.

One hard limit: don't freelance below the manufacturer's minimum airflow, which is typically around 350 CFM/ton. Below that, the coil surface can drop under 32°F saturation and start icing. Check the data plate or installation manual before pulling blower speed down.

Every superheat and subcooling target on a charging chart assumes the system is moving design airflow. Check the charge with low airflow and the chart lies to you.

Low airflow starves the evaporator: less heat reaches the refrigerant, so suction pressure drops and superheat drops. On the gauges that reads like an overcharge — HVAC School's charging guide points out that a dirty coil or weak blower produces low superheat and low suction pressure, exactly the signature techs recover refrigerant to "fix." Now the system has two problems: bad airflow and a short charge.

That same guide sets the prerequisite: verify roughly 350–425 CFM per 12,000 BTU/h across the indoor coil before setting a charge. Practically, before gauges go on:

1. Filter — clean, correct size, not collapsed into the rack
2. Evaporator coil — look at it with a light, not just through the panel gap
3. Blower wheel — a wheel packed with dust can lose a big chunk of its rated CFM
4. Registers and dampers — open, not crushed flex or a closed-off bonus room
5. Static pressure — measure it; a 0.9 in. w.c. system on a blower rated for 0.5 in. w.c. isn't moving nameplate air

If those check out and airflow is in the 350–450 CFM/ton band, your superheat and subcooling readings actually mean something.

Two field methods, in order of preference.

Method 1 — static pressure plus the blower table. Measure total external static pressure (TESP) with a manometer: one probe in the supply plenum, one in the return drop, both outside the cabinet. Take the reading to the blower performance table in the installation manual, find your speed tap or CFM setting, and read the delivered CFM at your measured static. This is the closest thing to a spec-grade number without a flow hood or a TrueFlow grid, because it uses the manufacturer's own fan data.

Method 2 — temperature rise (needs a gas furnace or electric heat in the airstream). Per the TruTech Tools method sheet, for a gas furnace:

CFM = (input BTU/h × steady-state efficiency) ÷ (1.08 × temperature rise °F)

Example: an 80,000 BTU/h input furnace at 80% steady-state efficiency puts out 64,000 BTU/h. Measured rise of 50°F: 64,000 ÷ (1.08 × 50) = about 1,185 CFM.

With electric strip heat it's simpler — each kW delivers 3,412 BTU/h (standard conversion) at effectively 100% efficiency, so CFM = (kW × 3,412) ÷ (1.08 × rise).

Caveats from the TruTech write-up: let the furnace run about 10 minutes first, keep the supply probe out of line of sight of the heat exchanger so radiant heat doesn't skew it, close any bypass humidifier, and treat the result as a ballpark for matching the manufacturer's target rise — not capacity verification. And remember the 1.08 factor assumes sea-level standard air at 0.075 lb/ft³; at elevation, thinner air carries less heat per cubic foot, so correct for density or lean on the blower table instead.