Duct Sizing · HVAC Pro Tool

Duct Sizing
Calculator

Equal friction rate duct sizing for HVAC professionals. Enter CFM and friction rate, get round duct diameter plus rectangular equivalents.

Airflow (CFM)

~400 CFM per ton of cooling, or per-room load

Friction Rate (in. wc / 100 ft)

Duct Material

Roughness factor multiplier

Max Aspect Ratio

Rectangular duct width-to-height ratio limit

Recommended Round Diameter

— inches

Air Velocity—

Round duct area—

Rectangular equiv (low profile)—

Rectangular equiv (square-ish)—

Pressure loss / 100 ft—

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Duct Sizing Methodology

The equal friction rate method is the standard residential duct sizing approach. The principle: design every duct in the system to have the same pressure drop per foot, so air distributes evenly through the registers without dampers.

Sizing Steps

Calculate total external static pressure (ESP) of your blower (typically 0.5" wc for residential)
Subtract pressure losses from components (filter, coil, registers — typically 0.25" wc total)
Divide remaining pressure by total duct length (use longest run) and multiply by 100 to get friction rate
Size each duct section to this friction rate using its actual CFM

Velocity Guidelines

Supply trunks: 700-900 FPM (residential), 1000-1500 FPM (commercial)
Supply branches: 600-800 FPM
Return trunks: 500-700 FPM (lower = quieter)
Return branches: 400-600 FPM
Maximum residential: 1,200 FPM (becomes audible above)

Why Flex Duct Is Tricky

Flexible duct has roughly 15% more friction than smooth metal when properly installed — and significantly more when kinked, compressed, or sagging between supports. If you must use flex, oversize by one size and ensure it's pulled taut with proper saddles every 4 feet.

Frequently Asked Questions

What is friction rate?

Friction rate (inches of water column per 100 feet) is the pressure loss per 100 ft of duct. Most residential systems target 0.08-0.10 in. wc/100 ft for supply ducts. Commercial: 0.10-0.15. Higher friction rates = smaller ducts but more fan power.

How do I find CFM for each room?

CFM per room = (room's cooling load in BTU/hr) ÷ (supply temp differential × 1.08). Typical residential: 1 CFM per 1-1.5 sq ft of floor area is a fast estimate. For accurate sizing, do a Manual D calculation room-by-room.

Why does this give round duct sizes?

Round duct is the most efficient shape (lowest friction per unit airflow). The calculator gives the round equivalent first, then converts to rectangular equivalents if your space requires flat duct. Round flex or hard-pipe is always preferred where it fits.

What about return ducts?

Return ducts are typically sized one size larger than supply. The same calculator works — just enter your return CFM (usually 90% of supply) and use a slightly lower friction rate (0.06-0.08) for quieter operation.

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