Duct Size Calculator
Size HVAC ductwork from airflow (CFM). Get the round duct diameter and rectangular equivalents for supply, return, and trunk lines using the velocity method.
Rule of thumb: ~1 CFM per sq ft | 400 CFM per ton of cooling
Ductwork Sizing Calculator
The velocity method sizes a duct so air moves at a target speed. Lower velocity means a larger, quieter duct with less friction. For a full design, verify total external static pressure against the blower rating and keep runs within the equipment's available static.
Duct Sizing Chart (CFM to Duct Size)
A 6-inch round duct carries about 100 CFM, an 8-inch carries ~200 CFM, a 10-inch ~400 CFM, and a 12-inch ~700 CFM at typical residential friction (0.1 in. WC per 100 ft). Use the chart below to match airflow to round duct size and its rectangular equivalent.
| Round Duct | Max Airflow (CFM) | Rectangular Equivalent | Typical Use |
|---|---|---|---|
| 4" | 25 | 6×4 | Single small register |
| 5" | 50 | 8×4 | Small branch run |
| 6" | 100 | 8×4 | Standard branch / register |
| 7" | 150 | 8×5 | Larger branch |
| 8" | 200 | 10×5 | Room / small trunk |
| 9" | 300 | 10×6 | Trunk section |
| 10" | 400 | 12×6 | Main trunk (small home) |
| 12" | 700 | 14×8 | Main trunk (2–3 ton) |
| 14" | 1,000 | 18×8 | Main trunk (3–4 ton) |
| 16" | 1,400 | 20×10 | Main trunk (4–5 ton) |
| 18" | 1,800 | 24×10 | Large / light commercial |
| 20" | 2,400 | 24×12 | Commercial main |
CFM values are for sheet-metal duct at about 0.1 in. WC / 100 ft friction. Flexible duct carries roughly 15–20% less airflow at the same diameter, so size flex one step larger. To find the airflow a system needs, use ~400 CFM per ton — see our AC size calculator.
How to Calculate Duct Size
Step 1: Find the CFM
Step 2: Size the Duct
Frequently Asked Questions
How do you calculate duct size?
Divide the airflow (CFM) by the target air velocity (FPM) to get the duct area in square feet, then convert to a round diameter: diameter (in) = √(area × 183.3). For example, 400 CFM at 900 FPM needs 0.44 sq ft, or about a 9-inch round duct — most installers round up to a 10-inch (or use 10-inch flex) for margin.
What is the rule of thumb for duct sizing?
Size the system for about 400 CFM per ton of cooling (or 1 CFM per sq ft of floor area), keep supply velocity around 700–900 FPM in residential trunks, and size flexible duct one step larger than sheet metal. A 6-inch branch handles roughly 100 CFM.
How many CFM can a 12x12 duct handle?
A 12×12 inch duct (1 sq ft of area) carries roughly 700–900 CFM at typical residential velocities of 700–900 FPM. It is equivalent to about a 13-inch round duct, suitable for a main trunk on a 2–3 ton system.
What size duct do I need for 4,000 CFM?
For 4,000 CFM, a round duct of about 24–26 inches (or a rectangular duct near 30×14) works at moderate commercial velocities of ~1,200–1,500 FPM. At lower residential velocities you would need an even larger duct, which is why 4,000 CFM systems use commercial-scale ducts.
Is it better to oversize or undersize ductwork?
Slightly oversized ductwork is better than undersized. Oversized duct lowers velocity, noise, and static pressure, helping the blower deliver rated airflow. Undersized duct restricts airflow, raises static pressure, and can cause noise, poor comfort, and premature equipment failure. Avoid extreme oversizing, which can reduce throw and mixing at registers.