Ductwork Calculator

How to Size Ductwork for HVAC Systems?

Ductwork sizing depends on two factors: the airflow volume (measured in cubic feet per minute, or CFM) and the maximum acceptable air velocity (feet per minute, or FPM). Divide the CFM by the velocity to get the required duct cross-sectional area in square feet. Convert to square inches and find the corresponding round duct diameter or rectangular duct dimensions. The calculator above takes your CFM requirement and velocity limit, then outputs the correct duct size for round or rectangular configurations along with estimated friction loss for the duct run.

What CFM Does Each Room Need?

The CFM requirement for each room is calculated by dividing the room BTU load by the system BTU-per-CFM ratio (typically 25-30 BTU per CFM for cooling). A room needing 6,000 BTU of cooling requires approximately 200-240 CFM. As a rough guide, bedrooms need 100-200 CFM, living rooms and kitchens need 200-400 CFM, and bathrooms need 50-100 CFM. The total system CFM (all rooms combined) determines the main trunk duct size. Each branch duct feeding an individual room is sized for that room individual CFM. Manual J and Manual D calculations from ACCA provide the most accurate room-by-room CFM values.

What Air Velocity Should I Use?

Residential ductwork targets 600 FPM maximum in main trunk ducts and 400-500 FPM in branch ducts. Higher velocities generate audible noise. Air rushing through undersized ducts creates a whooshing or whistling sound that occupants find annoying. Commercial systems tolerate 900 FPM in main ducts because ambient noise levels in offices and retail spaces are higher. Industrial ductwork can run at 1,200 FPM or more. The calculator lets you select residential (600 FPM), commercial (900 FPM), or industrial (1,200 FPM) velocity limits and sizes the duct accordingly. Lower velocity targets produce larger but quieter ducts.

Round vs Rectangular Ductwork

Round ducts move air more efficiently because the circular shape minimizes friction against the duct walls. A round duct carries the same CFM as a rectangular duct with 25-30% more material. Round ducts are easier to seal at joints, reducing air leakage. However, rectangular ducts fit into tighter spaces. A 6x14-inch rectangular duct carries similar airflow to a 10-inch round duct but fits between floor joists where a 10-inch round duct would not. In practice, main trunk lines are often rectangular to conserve headroom in basements and attics, while branch runs to individual rooms use round flexible ducts for ease of routing.

What Is Friction Loss in Ductwork?

Friction loss measures the pressure drop as air moves through the duct, expressed in inches of water gauge per 100 feet of duct. Residential systems typically target 0.08-0.10 inches per 100 feet. Higher friction loss means the blower must work harder, consuming more energy and reducing system efficiency. Friction increases with air velocity, duct surface roughness, and duct length. Fittings (elbows, tees, transitions) add equivalent length to the run: a 90-degree elbow adds roughly 10-15 feet of equivalent straight duct. The calculator estimates total friction loss based on duct size, velocity, and run length to help you verify that the system stays within acceptable limits.

How to Reduce Duct Noise?

Size ducts to keep velocity below 600 FPM in residential branch runs. Use turning vanes inside 90-degree elbows to smooth airflow and reduce turbulence. Insulate metal ducts with fiberglass duct wrap or use internally lined ducts. Transition gradually between duct sizes rather than making abrupt changes. Install flexible duct connections between metal ductwork and registers to absorb vibration from the air handler. Avoid sharp 90-degree turns where possible; two 45-degree elbows create less turbulence than a single 90. Register and grille selection also affects noise: larger registers with more free area reduce velocity at the room entry point.

Ductwork Materials and Insulation

Galvanized sheet metal is the most durable and efficient duct material. It comes in round, rectangular, and oval profiles. Joints are sealed with mastic or metal tape (never cloth duct tape, which degrades over time). Flexible duct (insulated flex) connects metal trunk lines to ceiling or floor registers. Limit flex duct runs to 25 feet and keep them as straight as possible, since the ribbed interior creates more friction than smooth metal. Duct board (rigid fiberglass) is used to fabricate rectangular ducts on-site. All ducts in unconditioned spaces (attics, crawl spaces) must be insulated to a minimum of R-6 (R-8 in colder climates) to prevent condensation and energy loss.

Common Ductwork Sizing Mistakes

Undersizing trunk ducts is the most frequent error. If the main duct cannot deliver enough CFM to feed all branches, rooms at the end of the run starve for airflow. Oversizing return ducts is less common but wastes material and space. Ignoring equivalent length from fittings leads to systems that look right on paper but underperform because every elbow, tee, and transition adds friction the blower must overcome. Using excessive flexible duct lengths (more than 25 feet) chokes airflow and degrades efficiency. Failing to seal joints leaks conditioned air into unconditioned spaces, wasting 20-30% of the system capacity in poorly sealed duct systems.

Frequently asked questions