What size ductwork does a 2.5 Ton AC need? At the standard 400 CFM per ton, that system moves about 1,000 CFM, which needs a 16 inch round (or 30×8, 24×10, 20×12 rectangular) main supply trunk running at 716 FPM. The return side needs about 16 inch because return air travels slower. The calculator below is pre-set to this system.
HVAC Airflow: Static Pressure, CFM, and Friction Rates
Correct duct sizing is the most ignored aspect of HVAC design, yet it dictates the performance of your entire system. If your ducts are too small, airflow (CFM – Cubic Feet per Minute) is restricted, causing the blower motor to overwork, increasing static pressure, and eventually freezing the evaporator coil. When calculating duct size, you must differentiate between rigid metal ducts (smooth interior, lower friction rate) and flexible ductwork (higher friction rate). The calculator below uses standard Manual D principles to help you find the correct trunk and branch dimensions for optimal, quiet airflow.
🌬️ What Size Duct Do You Need for Your Airflow?
Convert CFM into the right round duct diameter and rectangular equivalent, with the correct air velocity for supply trunks, branches, returns and exhaust. Enter CFM directly, or derive it from your AC tonnage or house size — and it adjusts for flexible vs rigid duct.
🔢1. How Do You Want to Enter Airflow?
Enter CFM directly if you know it, or let the calculator derive it from AC tonnage (400 CFM per ton) or from house floor area.
💨2. Airflow
Range: 50 – 4,000 CFM. A single room register is typically 100–200 CFM.
🧭3. Which Part of the Duct System?
Main trunks run faster (700–900 FPM) because they carry the whole system's air. Branches run slower (400–600 FPM) so air doesn't whistle at the registers.
🔩4. Duct Material
Flexible duct has a ribbed inner liner that creates far more friction than smooth sheet metal, so it must be sized one size larger for the same airflow. Keep flex runs short, pulled taut and free of sharp bends.
2.5 Ton AC Ductwork at a Glance
All figures for 1,000 CFM (2.5 Ton AC, 400 CFM per ton):
| Duct run | Target velocity | Round duct | Actual velocity |
|---|---|---|---|
| Supply trunk | 700–900 FPM | 16 inch | 716 FPM |
| Supply branch | 400–600 FPM | 18 inch | 566 FPM |
| Return trunk | 600–800 FPM | 16 inch | 716 FPM |
| Flexible duct (same run) | 20% lower allowable | 16 inch | 716 FPM |
Sizing the Whole System, Not Just the Trunk
A 2.5 Ton AC moving 1,000 CFM needs three things sized correctly: the supply trunk (16 inch here), the branch runs to each room (typically 100–200 CFM each, so 6–8 inch), and the return (16 inch). Return air is the part most often undersized — if the system can’t get air back, the supply side cannot deliver it either, and you get weak airflow, high static pressure and shortened equipment life. Confirm your tonnage first with the AC Tonnage Calculator.
Frequently Asked Questions
What size duct for a 2.5 Ton AC? About 16 inch round (or 30×8, 24×10, 20×12) for the main supply trunk, carrying 1,000 CFM at 716 FPM.
How many CFM does a 2.5 Ton AC move? Roughly 1,000 CFM, using the standard 400 CFM per ton. Some variable-speed systems run 350–450 CFM per ton — check the equipment data plate.
What size return duct for a 2.5 Ton AC? About 16 inch. Returns are sized at a lower velocity than supply ducts, so they end up larger for the same airflow.
Once the duct is sized, read our AHU installation guide.
Estimates vary by contractor. Compare free quotes from licensed local pros before you commit. Request a quote →
High-efficiency systems may qualify for utility rebates and federal or state incentives. Find current programs at ENERGY STAR & DSIRE.
Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.