CFM Calculator: Ton to CFM & CFM to Tons

Multiply tons by the selected airflow rate. These supply-air estimates help plan blower and duct capacity; they are not outdoor-air requirements or proof of delivered cooling.

Pick a tonnage to see the 400-CFM/ton supply-air reference scale linearly and watch the airflow animation change with it. The presets compare total system airflow as air moves from the air handler through the duct toward the conditioned space. Typical design rates are about 350–400 CFM/ton, with the approved setting determined by the matched system, operating mode, and climate. Use the main calculator to compare airflow settings and convert between CFM and tons, including the effect of lower airflow for humidity control.

• 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 1 CFM per sq ft without a load check — a fixed area ratio cannot represent rooms with different glazing, occupancy or supply temperatures
• 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 compressor need the correct airflow to deliver their published performance. The blower's actual volume affects total capacity, sensible heat ratio and coil temperature, and it is sensitive to the resistance of filters, coils, ducts and terminals. ACCA Manual S uses the building loads and manufacturer performance at the proposed airflow and design conditions. A nominal 3-ton label does not tell you whether the installed system meets its sensible or latent load. Check the proposed flow before selecting equipment, then measure it after installation. A high thermostat temperature or persistent humidity problem may involve airflow, refrigerant operation or building load; airflow alone cannot diagnose the cause.

The 400-CFM/ton rule is a common planning reference, not a unit conversion like 12,000 BTU/h per ton. Reducing flow within an approved range can lower coil temperature and change moisture removal, which is why 350 CFM/ton may suit some humid-climate designs. Variable-speed and staged systems have manufacturer-defined airflow profiles, and high efficiency does not mandate a particular CFM/ton. The lower-flow setting must still satisfy sensible capacity and operating limits. Ask for the airflow used in the selection calculation, its associated sensible and latent capacities, and the blower setting that will provide it at the measured system resistance.

The methods answer related but distinct questions. Tonnage × CFM/ton gives a whole-system airflow reference. Volume × ACH / 60 converts a selected circulation, outdoor-air or exhaust rate into CFM; the source of that ACH and the air path must be stated. Residential outdoor-air ventilation and commercial ventilation use separate requirements and cannot be satisfied simply by recirculating supply air. The sensible-load equation gives heat-transfer airflow for a specified room-to-supply temperature difference. Its 1.08 coefficient represents 60 minutes per hour × air density × specific heat at standard conditions, so elevation and air conditions can change it. Enter sensible load only, excluding the latent moisture load.

Room airflow needs to be reconciled with the selected equipment's design flow rather than assigning every room the same CFM per square foot. Use the room loads and supply temperature, then account for duct heat gains, leakage and the load-calculation method when comparing room sums with block and equipment loads. Room-by-room peaks can occur at different times, so their summed peaks are not automatically identical to the coincident whole-house peak. Return paths, transfer grilles or jumper ducts must accommodate supply flow when doors are closed. Restricted return paths can alter room pressure and increase infiltration. Commissioning should verify both total blower flow and distribution at the terminals under the intended operating conditions.

• Confirm airflow in manufacturer expanded performance data and blower tables; use AHRI information to identify the certified match
• 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, with lower branch targets where needed for noise control. These are planning ranges; flex needs product-specific resistance data. Push velocity too high and registers can 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. A constant-airflow ECM blower can hold its target over a limited pressure range, often using more power as resistance rises; other ECM control types behave differently. No motor removes the need for correctly sized ducts. Efficiency ratings such as SEER2, EER2, and HSPF2 describe tested equipment performance; they do not guarantee performance with restrictive or leaking installed ducts. Compare measured flow and external static pressure with the manufacturer tables before adjusting the blower. Then check register delivery and room comfort in the operating modes the household will actually use.

Choose the rate used by your equipment design. Supply airflow alone cannot tell you the actual refrigeration capacity: entering-air humidity, indoor and outdoor temperatures, and the matched coil affect performance. Use manufacturer expanded performance data for equipment selection and a simplified Manual S check.

The sensible equation addresses temperature change, not moisture removal. ACH presets do not replace an outdoor-air design. ASHRAE Handbook - Fundamentals explains air heat transfer; ACCA Manual D addresses residential duct design. Use the duct size calculator after estimating airflow, and confirm blower delivery against the manufacturer's static-pressure data.

The tonnage tab starts at 3 tons and 400 CFM/ton: 3 × 400 = 1,200 CFM. Switch only the rate to 350 and the result becomes 1,050 CFM. The reverse tab starts at 1,200 CFM: it represents 3 airflow-equivalent tons at 400, or approximately 3.43 at 350. Neither reverse answer is a measurement of refrigeration output.

The ACH tab starts with a 20 × 15 × 8-ft living room: volume is 2,400 cubic ft and the illustrative 6-ACH preset gives 2,400 × 6 / 60 = 240 CFM. The sensible-load tab starts at 20,000 BTU/h and 20°F: 20,000 / (1.08 × 20) = 925.93, displayed as 926 CFM. These defaults describe different tasks, so their results are not supposed to match.

Changing ΔT in the equation shows sensitivity, not an instruction to reduce blower flow until a target number appears. The coil must operate inside its approved range and deliver the required total and latent capacity. Use the room-by-room load estimate to organize room demands, then reconcile them with equipment performance.

The 350 and 400 CFM/ton rates and room ACH presets are planning assumptions. They do not set outdoor-air requirements, establish an equipment minimum, or replace manufacturer commissioning instructions. The sensible equation assumes standard air; it excludes moisture heat transfer.

ACCA Manual S addresses equipment selection using loads and manufacturer performance. ASHRAE Handbook: duct design explains airflow resistance and sizing methods. Use the duct size calculator for a straight-duct screening check and a full Manual D design for the distribution system.