Ductwork Calculator Cfm

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• Duct sizing (XLSX)
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• Trunk worksheet (PDF/DOCX)
• Installation checklist (PDF/DOCX)

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From room airflow to duct sizes

1. Find each room’s airflow (CFM) — ideally from a room-by-room heat gain/loss calculation, or roughly from volume and air changes per hour.
2. Choose a design friction rate from the blower’s available static pressure, or a velocity limit for quiet operation.
3. Size each branch for its room’s CFM.
4. Add up the branches for the total airflow and size the main trunk.
5. Reduce the trunk as branches leave it, keeping velocity within limits.
6. Check the longest run and fittings against available static pressure.

Air changes per hour

CFM = room volume (ft³) × air changes per hour ÷ 60. For a 12 × 16 ft bedroom with 9 ft ceilings (1,728 ft³), four air changes an hour is 1,728 × 4 ÷ 60 ≈ 115 CFM. Air change targets are a rough guide for comparing rooms; heating and cooling loads, which depend on windows, insulation and orientation, are the proper basis for supply airflow.

Worked example

Four rooms need 110, 90, 300 and 150 CFM — 650 CFM in total, about 1.6 tons of cooling at 400 CFM per ton. At a 0.08 friction rate, the branches size to 7 in, 6 in, 10 in, 8 in. The trunk starts at 14 in for 650 CFM and steps down after the living room branch to 10 in, then after the kitchen to 8 in. Switching to velocity sizing with 600 fpm branches gives 6 in, 6 in, 10 in, 7 in.

Friction rate vs velocity sizing

Many residential designs land between about 0.06 and 0.10 in. w.c. per 100 ft; common velocity limits are around 600 fpm for branches and 700–900 fpm for trunks.

Returns

Every cubic foot of supply air has to come back to the air handler. Undersized returns are one of the most common causes of noisy, underperforming systems. Size return ducts and grilles for the full system airflow at a low friction rate or velocity, and give closed-off bedrooms a return path — a dedicated return, a jumper duct or a transfer grille.

Rectangular ducts and equivalent sizes

Trunks are often rectangular so they fit between joists or above a ceiling. A rectangular duct carries the same air at the same friction as a round duct of its equivalent diameter, given by De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. For example, an 8 × 14 in duct is roughly equivalent to an 11.4 in round duct, and a 10 × 20 in duct to about 15.2 in. Keep the aspect ratio (long side ÷ short side) below about 4:1 where you can — flatter ducts need more metal and have more friction for the same airflow.

Choosing a friction rate

The friction rate comes from the static pressure the blower can deliver, minus the losses in the coil, filter, registers and grilles, divided by the total effective length of the longest run including fittings, per 100 ft. A blower with 0.5 in. w.c. available after the coil and filter, 0.1 in. used by the supply register and return grille, and a 300 ft effective length gives (0.5 − 0.1) × 100 ÷ 300 ≈ 0.13 in./100 ft. A lower design rate gives larger, quieter ducts. Fittings matter: one sharp elbow can count as 30 ft or more of straight duct.

Registers and grilles

Pick supply registers for the room’s CFM and the throw you need to reach the far wall, and keep face velocity modest to avoid whistling — manufacturers publish charts for each size. A 4 × 10 in register often suits about 60–100 CFM and a 4 × 12 in around 80–130 CFM, but check the specific model. Return grilles should be generous; a common guide is at least 2 in² of free area per CFM.

Common ductwork problems

• Crushed or kinked flex duct.
• Too many elbows on long runs.
• Leaky joints — seal with mastic or approved tape.
• Uninsulated ducts in hot or cold attics.
• Supply registers blocked by furniture.
• Dampers closed and forgotten.

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