HVAC Duct Calculator — CFM, Duct Sizing & Airflow Tool

Calculate HVAC duct CFM, determine required duct dimensions, and analyze airflow for round and rectangular ductwork. Free online duct calculator with step-by-step formula breakdown, copy & share support, and educational explanations.

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HVAC Duct Calculator

Calculate CFM from duct dimensions and air velocity, or determine the required duct size for a target airflow.

inches
FPM (feet per minute)
Enter values and click Calculate to see the result.

HVAC Duct CFM Formula Explained

The duct CFM formula calculates airflow by multiplying the air velocity by the duct's cross-sectional area. This is the fundamental equation used by HVAC engineers for duct sizing and system design.

CFM = Air Velocity (FPM) × Duct Cross-Sectional Area (sq ft)

Round Duct Formula

CFM = V × π × (d / 24)2
where d = duct diameter in inches, V = velocity in FPM

Rectangular Duct Formula

CFM = V × (W × H) / 144
where W = width in inches, H = height in inches, V = velocity in FPM

Variable Definitions

  • CFM — Cubic Feet per Minute, the volumetric airflow rate through the duct
  • FPM — Feet per Minute, the linear speed of air traveling through the duct
  • d — Round duct diameter measured in inches
  • W, H — Rectangular duct width and height measured in inches
  • 144 — Conversion factor from square inches to square feet (12×12)

How to Calculate HVAC Duct CFM and Duct Size

Follow these steps to calculate duct airflow or determine the required duct dimensions for your HVAC system:

  1. Identify your known values — For CFM calculation, you need duct dimensions and air velocity. For duct sizing, you need target CFM and desired velocity.
  2. Calculate the cross-sectional area — For round ducts: Area = π × (d/2)2 in square inches. For rectangular: Area = W × H in square inches.
  3. Convert area to square feet — Divide the area in square inches by 144 to get square feet.
  4. Apply the CFM formula — Multiply velocity (FPM) by area (sq ft) to get CFM. For duct sizing, rearrange: Area (sq ft) = CFM / Velocity (FPM).
  5. Verify against standards — Check that air velocity falls within recommended ranges (400-500 FPM for residential supply, 300-400 FPM for returns).

For example, a 12-inch round duct with air moving at 500 FPM: Area = π × (12/24)2 = 0.785 sq ft, CFM = 500 × 0.785 = 392.7 CFM.

HVAC Duct Calculator Examples

Example 1: Calculate CFM for a Round Duct

A 10-inch diameter round duct with air velocity of 600 FPM. What is the CFM?

Area = π × (10/24)2 = 0.545 sq ft
CFM = 600 × 0.545 = 327.2 CFM

Example 2: Calculate CFM for a Rectangular Duct

A 14×8 inch rectangular duct with air velocity of 450 FPM.

Area = (14 × 8) / 144 = 0.778 sq ft
CFM = 450 × 0.778 = 350 CFM

Example 3: Size a Round Duct for Target CFM

You need 600 CFM at 500 FPM. What round duct diameter is required?

Area = 600 / 500 = 1.2 sq ft
d = 24 × √(1.2 / π) = 14.8 inches
Use a 15-inch round duct

Example 4: Size a Rectangular Duct for Target CFM

You need 800 CFM at 500 FPM, constrained to 12-inch width.

Area = 800 / 500 = 1.6 sq ft = 230.4 sq in
Height = 230.4 / 12 = 19.2 inches
Use 12×20 inch rectangular duct

Real-World HVAC Duct Sizing Applications

  • Residential HVAC Design: Sizing supply and return ducts for proper air distribution throughout a home, ensuring each room receives adequate conditioned air.
  • Commercial Ductwork Layout: Calculating trunk and branch duct sizes for office buildings, retail spaces, and industrial facilities.
  • Duct Replacement Projects: Determining correct replacement duct sizes when upgrading HVAC equipment or modifying existing ductwork.
  • Energy Efficiency Audits: Verifying that existing ductwork is properly sized to minimize static pressure losses and reduce energy consumption.
  • Noise Control: Selecting duct dimensions that keep air velocity within recommended ranges to prevent excessive airflow noise.
  • Building Code Compliance: Ensuring duct sizing meets local mechanical code requirements for ventilation and air distribution.
  • Kitchen Exhaust Systems: Sizing ductwork for range hoods and commercial kitchen ventilation based on CFM requirements.

People Also Ask

CFM is calculated by multiplying the air velocity in FPM by the duct cross-sectional area in square feet. For round ducts, use CFM = V × π × (d/24)² where d is the diameter in inches. For rectangular ducts, use CFM = V × (W × H) / 144 where W and H are width and height in inches.
The duct sizing formula rearranges the CFM equation: Required Area (sq ft) = CFM / Velocity (FPM). For round ducts, the diameter in inches is d = 24 × √(Area / π). For rectangular ducts, the required cross-sectional area in square inches is Area = (CFM / V) × 144, and you can solve for one dimension if the other is constrained.
Residential supply ducts should maintain 400-500 FPM, while return ducts operate best at 300-400 FPM. Branch runs typically use 400-600 FPM. Staying within these ranges balances airflow efficiency with noise control. Higher velocities increase noise and static pressure, while lower velocities require larger, more expensive ductwork.
The industry standard is 400 CFM per ton of cooling capacity. A 3-ton air conditioner requires approximately 1,200 CFM of airflow. In humid climates, 350 CFM per ton may be used for better dehumidification, while dry climates might use 450 CFM per ton for increased sensible cooling.
Undersized ductwork causes excessive air velocity, leading to increased noise, higher static pressure, reduced airflow to conditioned spaces, decreased system efficiency, higher energy bills, and potential compressor damage from inadequate airflow across the evaporator coil. It can also create hot and cold spots throughout the building.

Frequently Asked Questions

CFM stands for Cubic Feet per Minute and measures the volume of air flowing through a duct. It is the primary metric for sizing HVAC ductwork and ensuring proper air distribution. Typical residential systems deliver 400 CFM per ton of cooling capacity. Accurate CFM calculation is essential for system efficiency and comfort.
Yes. Use the shape toggle buttons to switch between round duct and rectangular duct modes. The calculator automatically adjusts the input fields and formulas based on your selection. Round ducts use diameter, while rectangular ducts use width and height.
CFM measures the volume of air moved per minute, while FPM measures the linear speed of the air. Think of it like water in a pipe: CFM is the total flow rate (gallons per minute), and FPM is how fast the water is moving (feet per minute). Both are needed for proper duct design.
Use the equal area method: the rectangular duct should have the same cross-sectional area as the round duct. Formula: W × H = π × (d/2)². For best performance, keep the aspect ratio (width/height) below 4:1. A 12-inch round duct is approximately equivalent to a 10×12 inch rectangular duct.
No. All inputs must be positive numbers greater than zero. Duct dimensions, air velocity, and CFM values represent physical quantities that cannot be negative or zero in practical applications. The calculator includes validation to prevent invalid inputs.
Static pressure is the resistance to airflow within a duct system, measured in inches of water column. Higher static pressure means the blower works harder to move air. Proper duct sizing keeps static pressure within the manufacturer's recommended range, typically 0.5 inches or less for residential systems.

HVAC Ductwork Glossary

CFM

Cubic Feet per Minute — the volumetric airflow rate through a duct, the primary metric for sizing HVAC ductwork and equipment.

FPM

Feet per Minute — the linear velocity of air traveling through a duct. Recommended ranges vary by application and duct type.

Static Pressure

The resistance to airflow within a duct system, measured in inches of water column (in. WC). Excessive static pressure reduces system efficiency.

Aspect Ratio

The ratio of a rectangular duct's width to its height. Lower aspect ratios (closer to 1:1) are more efficient. Ratios above 4:1 should be avoided.

Equivalent Diameter

The diameter of a round duct that has the same friction loss as a given rectangular duct, used for comparing different duct shapes.

Friction Loss

The pressure drop per 100 feet of duct due to air rubbing against duct walls. Expressed in inches of water column per 100 feet.

Plenum

A large air distribution box attached to the supply or return side of an air handler that connects to multiple duct runs.

Register

A grille with a built-in damper that covers a duct opening where conditioned air enters a room, allowing airflow adjustment.

Editorial Review & Methodology

This HVAC duct calculator was built and reviewed by the NumbrWiz Editorial Team. The duct CFM formula and sizing methodology are based on standard HVAC engineering principles found in ASHRAE handbooks, ACCA Manual D for residential duct design, and SMACNA duct construction standards.

  • Formula verification: Cross-checked against ASHRAE Fundamentals Handbook and ACCA Manual D guidelines.
  • Velocity recommendations: Based on industry-standard ranges for residential and light commercial applications.
  • Edge case testing: Tested with standard duct sizes, extreme aspect ratios, and typical residential/commercial CFM ranges.

Transparency note: All calculations run client-side in your browser. No data is ever collected, stored, or transmitted. Results are for educational and planning purposes; always consult a licensed HVAC professional for final system design and verify critical calculations independently.

Page last reviewed: May 2026 · NumbrWiz Editorial Team