Duct Sizing Calculator
Sizing HVAC ducts for a room? Enter the BTU capacity, design friction rate, and duct shape to determine the correct duct diameter, equivalent rectangular dimensions, and air velocity for efficient airflow.
Estimate only — not professional advice. Always verify results independently before purchasing materials or beginning work. Terms of Use
What is Duct Sizing?
A duct sizing calculator determines the correct diameter (for round ducts) or width-by-height dimensions (for rectangular ducts) needed to deliver a specified volume of conditioned air (measured in CFM — cubic feet per minute) to a room. Properly sized ducts maintain comfortable air velocities, minimize noise, reduce energy waste, and ensure even temperature distribution throughout the conditioned space. The fundamental relationship is between airflow volume (CFM), duct cross-sectional area, and air velocity. The CFM requirement for a room is derived from the cooling or heating load (in BTU) divided by approximately 30 — this is the industry standard rule of thumb for residential cooling systems. A 12,000 BTU cooling capacity (one ton of cooling) requires approximately 400 CFM of airflow at the standard 20-degree temperature differential across the coil. Duct sizing follows ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) guidelines that balance three competing requirements. First, air velocity must stay within acceptable ranges — typically 600 to 900 feet per minute (FPM) in main trunks and 400 to 700 FPM in branch ducts for residential systems. Velocities above 900 FPM cause audible whooshing and whistling noise. Velocities below 400 FPM waste duct material and space without providing benefit. Second, the friction rate (pressure loss per unit length) must stay within the available static pressure budget of the air handler. A standard residential system operates at 0.5 inches of water column total external static pressure. With typical supply and return runs totaling 100 to 200 equivalent feet (including fittings), the available friction rate is approximately 0.05 to 0.10 inches per 100 feet of duct length. The default value of 0.08 represents a common middle-ground design condition. Third, the total system must be balanced — each room receives its proportional share of system airflow. This requires that branch duct sizes be coordinated so that pressure losses are roughly equal to all outlets. Rooms farther from the air handler need proportionally larger ducts or balancing dampers. Round ducts are preferred for branch runs because they have the least surface area per unit of airflow capacity, resulting in lower friction losses and material cost. Rectangular ducts are used where vertical clearance is limited (such as between floor joists) or for main trunks that must fit in tight spaces. The equivalent rectangular size provides the same airflow capacity at the same friction rate as the round duct. Flexible duct (flex) is common in residential construction due to low material cost and easy installation, but it must be fully extended (not bunched or kinked) and limited to 25-foot runs. A compressed or sagging flex duct can have 3 to 5 times the friction loss of the same diameter rigid duct, drastically reducing airflow and system efficiency.
How to Calculate
- Identify the BTU capacity assigned to that room from the HVAC load calculation (or use 20 BTU per sq ft as a rough estimate for cooling)
- Leave the friction rate at 0.08 unless your HVAC designer specifies otherwise (a higher friction rate yields a smaller duct; a lower one yields a larger duct)
- Select round or rectangular duct type based on installation constraints
- Review the CFM requirement and duct diameter
- Verify the air velocity is within 400-900 FPM for quiet residential operation
- For rectangular ducts, use the equivalent size shown
Formula
CFM Required = BTU Capacity / 30 Duct Diameter uses the ASHRAE equal-friction equation, which solves for the round duct diameter that produces the chosen friction rate at the required CFM: Diameter (raw, inches) = ( 0.109136 x CFM^1.9 / FrictionRate ) ^ (1 / 5.02) The raw diameter is then rounded UP to the next standard round duct size from [4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20] inches. Air Velocity (FPM) = CFM / Duct Area (sq ft) Where Duct Area = pi x (Diameter/2/12)^2 (diameter in inches converted to feet) Because the chosen size is rounded up, the actual velocity is at or slightly below the design target, keeping the run within the quiet 400-900 FPM range. Rectangular equivalents provide equal airflow at equal friction loss.
Example Calculation
12,000 BTU, 0.08 in/100ft friction rate, round duct: CFM Required = 12,000 / 30 = 400 CFM Raw Diameter = (0.109136 x 400^1.9 / 0.08) ^ (1/5.02) = 10.27 inches Duct Diameter = 12 inches (next standard size up from 10.27) Duct Area = pi x (12/2/12)^2 = pi x 0.0625 = 0.7854 sq ft Air Velocity = 400 / 0.7854 = 509 FPM Rectangular Equivalent = 14 x 10 in At a tighter 0.10 in/100ft friction rate the same 400 CFM yields a 10-inch round duct (raw 9.83 in) running at about 733 FPM — right in the target band.
Frequently Asked Questions
How many CFM per ton of air conditioning?
The standard is 400 CFM per ton of cooling capacity. One ton equals 12,000 BTU per hour. This assumes the standard 20-degree Fahrenheit temperature differential across the evaporator coil. In humid climates, some contractors design for 350 CFM per ton to improve dehumidification at the expense of slightly higher supply air temperature.
What happens if ductwork is too small?
Undersized ducts create high air velocity, which causes noise (whooshing or whistling), increased static pressure on the blower motor (reducing its lifespan and efficiency), reduced total airflow to the room (causing hot/cold spots), and potential vibration that transmits through the building structure. The system works harder to deliver less air — increasing energy costs by 20 to 40 percent.
Is round or rectangular duct better?
Round duct is aerodynamically superior — it has less surface area per unit of airflow, lower friction losses, and fewer leak points. However, rectangular duct fits in tighter spaces (between joists, above drop ceilings). For branch runs to individual rooms, round duct or flex duct is standard. For main trunk lines with space constraints, rectangular duct is appropriate.
How do I size return air ducts?
Return air ducts should be sized 10 to 25 percent larger than the supply duct for the same room, because return air typically runs at lower velocity (300-500 FPM) for quiet operation. A common rule of thumb is one square inch of return grille free area for every BTU of system capacity. For a 3-ton system (36,000 BTU), that is 36,000 square inches of return duct cross-section — usually achieved with a 20x20 or 24x24 inch main return trunk.
What is the maximum length for flex duct?
Limit flexible duct runs to 25 feet maximum, and ideally under 15 feet. Flex duct must be pulled taut with no sags exceeding 0.5 inches per foot of span between supports. Supports must be spaced no more than 5 feet apart. Every bend increases friction significantly — avoid turns greater than 45 degrees, and never kink the duct. A sagging, kinked flex run can deliver only 30 to 50 percent of its rated airflow.