Path Load → Airflow → Duct Size → Comfort Check

Duct Sizing Calculator

Enter your CFM airflow result to get the recommended round or rectangular duct size. Uses the equal-friction method based on ASHRAE fundamentals.

Duct Sizer

Equal-friction method (ACCA Manual D)

Enter CFM above to size your duct

Educational estimate only. Results use a simplified equal-friction approach and are not a substitute for full ACCA Manual D duct design. Local building codes and manufacturer specs govern final installation. Consult a licensed HVAC engineer for commercial or permitted work.

Next: Comfort Check

Use your dry bulb temp and humidity to verify comfort conditions and check ASHRAE 55 status.

Continue to Comfort Check →

How Duct Sizing Actually Works

Duct sizing starts with a number you already have: the CFM your system needs to move. From there, the calculator applies the equal-friction method — the industry-standard approach where every run of duct is sized so air loses pressure at roughly the same rate per 100 feet, typically around 0.08–0.10 in. of water column. That keeps airflow balanced across the whole house instead of starving the far bedroom while the room next to the air handler gets blasted.

The two numbers that matter most are CFM (how much air) and velocity (how fast it's moving through the pipe). Get the diameter too small and velocity climbs — which means noise, pressure drop, and a blower working harder than it should. Get it too large and velocity drops too low to carry the air effectively.

Round vs. Rectangular Ducts

Round duct is more efficient — for the same cross-sectional area, it has less internal surface friction than rectangular duct, so it moves air with a smaller pressure drop. Rectangular duct exists mainly because it fits better in tight joist bays and soffits where a round pipe simply won't fit. The calculator above gives you both: a round diameter and an equivalent rectangular width × height if your installation calls for it.

Maximum Duct Velocity by Role (FPM)

Return runs get the lowest velocity ceiling (they're usually larger and quieter matters more), main trunks the highest.

Bar chart 700 Return 900 Supply 1200 Main Trunk

Feet per minute (FPM) velocity limits used by the calculator above to flag an undersized duct run.

Signs Your Ducts Are Too Small

That last one is common after an equipment replacement — a bigger unit was installed on ductwork sized for the smaller, older system, and airflow becomes the bottleneck no matter how good the new equipment is.

Frequently Asked Questions

What is the equal friction method for duct sizing?
The equal friction method sizes all duct branches so they have the same pressure drop per 100 feet of duct (typically 0.08–0.12 in. WC/100 ft). This keeps airflow balanced across the system without needing extensive damper adjustment. It's the most common manual method for residential duct design — and what this calculator uses.
What friction rate should I use for duct sizing?
0.10 in. WC per 100 feet is the most common residential design value. Use 0.08 for systems with longer runs (over 100 feet total equivalent length) or low-static equipment. Use 0.12 only for shorter, simpler systems. ACCA Manual D is the authoritative reference for residential duct design.
What is the maximum velocity for supply ducts?
ACCA recommends a maximum of 900 FPM for residential supply ducts, 700 FPM for return ducts, and 1,200 FPM for main trunk ducts. Exceeding these velocities causes noise (often described as a 'whooshing' sound at registers) and increases static pressure throughout the system.
How do I convert round duct to rectangular duct?
Use the hydraulic diameter formula: De = 1.3 × (a×b)^0.625 ÷ (a+b)^0.25, where a and b are the rectangular duct dimensions. A rectangular duct is equivalent to a round duct when they produce the same De. This calculator does the conversion automatically when you select Rectangular mode.
What duct size do I need for 400 CFM?
At the standard 0.10 in. WC/100 ft friction rate, 400 CFM requires approximately a 10-inch round duct. Velocity in a 10-inch duct at 400 CFM is about 733 FPM — within the 900 FPM supply limit. Use the calculator above to verify for your specific friction rate and duct type.