CFM / Airflow Calculator

The diagram below pairs cooling capacity with required airflow at the residential standard of 400 CFM per ton. Pick a tonnage to watch the air handler output scale linearly.

• Sizing whole-house CFM but not per-room — leads to under-served rooms
• Ignoring duct losses — 15–25% of air is lost in unsealed ducts; design for delivered CFM
• Using rule of thumb 1 CFM/sqft — wildly inaccurate for high or low loads
• Mismatched blower speed and tonnage — running a 3-ton blower at 1,600 CFM (4 ton speed) destroys efficiency
• Undersized return air paths — supply CFM is meaningless if return can't match it

A cooling coil and a compressor are only half of an air-conditioning system. The other half is the cubic feet per minute of air the blower actually pushes across that coil. Equipment nameplate capacity in BTU/hr is rated by AHRI at a specific airflow, and if the real airflow drifts away from that point the delivered capacity, the sensible heat ratio, and the coil temperature all change with it. The ACCA Manual S process that follows a Manual J load calculation explicitly checks that the selected equipment can hit its rated capacity at the airflow the duct system will provide. That is why a 3-ton unit starved of air can behave like a 2-ton unit, and why a CFM figure is not an afterthought — it is a design input on equal footing with the load itself.

The 400 CFM per ton rule of thumb (1 ton = 12,000 BTU/hr) exists because most single-stage residential coils are rated near that point. Variable-speed and high-efficiency systems are frequently rated and commissioned closer to 350 CFM/ton, which lowers the coil temperature, wrings more moisture out of the air, and suits humid climates and lower sensible heat ratios. Two-stage equipment may drop to roughly 300 CFM/ton on first stage. The takeaway is that "CFM per ton" is a manufacturer specification, not a universal constant — always confirm the value in the equipment's expanded performance data before you set blower taps or program an ECM motor.

This tool offers three routes to a target airflow, and a careful designer uses all of them as cross-checks rather than picking one in isolation. The tonnage method (tons × 400) sets the air handler's total output. The air-changes-per-hour method (volume × ACH ÷ 60) is a ventilation-driven check that matters most in dense or high-occupancy spaces, where ASHRAE 62.1 for commercial buildings and ASHRAE 62.2 for dwellings govern minimum outdoor-air and whole-building ventilation rates. The sensible-load method, CFM = sensible BTU/hr ÷ (1.08 × ΔT), is the one that gives you accurate room-by-room numbers because it is driven by each room's actual heat gain rather than a blanket average. The 1.08 factor bundles air density and specific heat at standard conditions; adjust it at altitude.

When you size the supply registers, the per-room sensible loads from Manual J should sum to the whole-house total, and the register CFM should sum to the air handler output. A sun-exposed living room with large west-facing glazing and a poor U-value assembly will demand far more airflow than a small, shaded, well-insulated bedroom — sizing both to the same register defeats comfort. Return air is the silent partner: supply CFM is meaningless if the return paths, transfer grilles, or jumper ducts cannot move the same volume back to the air handler. Imbalanced supply and return pressurizes or depressurizes rooms, drives infiltration, and can backdraft combustion appliances.

• Confirm rated CFM/ton in the AHRI expanded performance data, not from a rule of thumb
• Keep supply and return balanced — pressure imbalance wastes the airflow you sized
• Hold duct velocity (FPM) in a reasonable band so registers stay quiet
• Re-run the sensible-load method per room, then verify the rooms sum to the total

Once a CFM target exists for each run, the next step is turning that volume into duct dimensions, which is the domain of ACCA Manual D. Duct size follows from the required CFM and an acceptable air velocity in feet per minute (FPM): supply trunks commonly run around 700–900 FPM, branch runs lower, and flexible duct lower still to control friction and noise. Push velocity too high and registers whistle and static pressure climbs; run it too low and ducts become oversized and expensive. You can translate the airflow figures from this page into trunk and branch dimensions with the duct size calculator, and the step-by-step method is walked through in how to size ductwork.

Airflow and capacity are a closed loop: the load sets the tonnage, the tonnage and the manufacturer's CFM/ton set the airflow, and the airflow sets the duct. If you have not yet established the cooling and heating load, start with the system size calculator and work back to this page. An ECM blower helps hold the target CFM across a range of external static pressures, but it cannot rescue an undersized duct system — it merely consumes more watts trying. Efficiency ratings such as SEER2, EER2, and HSPF2 are all measured at rated airflow, so a duct system that cannot deliver the design CFM quietly erases the efficiency you paid for. Getting the cubic feet per minute right at every register is what lets the equipment, the ductwork, and the published ratings finally agree.