HVAC Duct Calculator

Residential supply trunks generally run about 700–900 FPM, supply branches around 600 FPM, and return air ducts lower still. Lower velocities are quieter and lose less pressure, while higher velocities let you use smaller, cheaper ducts at the cost of noise and static pressure. Commercial main ducts can run 1,200–1,800 FPM or more.

You match them by equivalent diameter — the round size that carries the same airflow at the same friction rate as the rectangular duct. That's exactly what this tool does: it finds the equivalent diameter for your airflow and velocity, then solves the rectangular width for the height you choose so the two carry the same air.

Friction rate is the pressure the air loses to friction for every 100 feet of duct, measured in inches of water column (in. wg). Residential systems are typically designed around a friction rate of about 0.1 in. wg per 100 ft. A higher friction rate means a smaller, noisier duct that works the blower harder; a lower rate means a larger, quieter duct.

At a typical residential velocity of about 700 FPM, a 6-inch round duct carries roughly 110 CFM, an 8-inch carries about 195 CFM, and a 10-inch carries about 305 CFM. These are approximate — the exact figure depends on the design velocity you choose, which you can set with the application presets.

Round duct is more efficient and quieter for the same airflow because it has the least surface area and the smoothest airflow, so use it where there's room. Rectangular duct fits tight spaces like joist bays and above ceilings where a round pipe won't fit. This tool sizes both so they carry the same airflow.

Return air ducts should be sized for a lower velocity than supply ducts to keep the system quiet, since returns are often closer to living spaces and grilles. Size the return for the same airflow as the supply it serves, but pick a lower design velocity — that yields a larger duct that moves air quietly.

High velocity means the duct is too small for the airflow you're pushing through it. Undersized ducts cause noise, higher static pressure, and a harder-working blower. Drop to a lower design velocity or step up to a larger duct size until the velocity lands in a comfortable range — around 700–900 FPM for residential supply.

Yes, the HVAC duct calculator is completely free with no signup required. Your inputs are saved locally in your own browser so they persist between visits — nothing is uploaded to a server. Clearing your browser data will erase your saved values.