Size supply, return, and branch ducts using equal friction or velocity methods. Supports round, rectangular, and flat oval ducts per SMACNA standards.
Calculate required airflow per room from heating and cooling loads and supply air temperature difference. The foundation input for every duct sizing calculation.
Calculate total external static pressure and available static pressure for duct sizing from air handler ESP minus component losses.
Calculate pressure drop per 100 feet and total pressure drop for round, rectangular, and oval ducts using the Darcy-Weisbach method.
Calculate air velocity in ducts from CFM and duct size, or find the duct size needed for a target velocity. Instantly flags noise-causing velocities.
Build your duct run fitting by fitting. Get total effective length of all fittings plus straight duct for accurate friction rate design.
Calculate CFM25 leakage rate, energy cost, and ENERGY STAR compliance status from duct blaster test results or estimated leakage percentage.
Find the round duct diameter that matches a rectangular duct for equal friction loss, or convert round to rectangular with a height constraint.
Calculate cross-sectional area, perimeter, hydraulic diameter, and surface area for round, rectangular, and flat oval ducts.
Calculate the design friction rate in one step from fan ESP, component losses, and longest duct run length.
Calculate supply-to-return air balance to diagnose pressure imbalances causing doors to move and rooms to stay uncomfortable.
Calculate heat loss from ducts in unconditioned spaces at any insulation level, plus annual energy cost and payback period for upgrades.
Calculate heat loss and gain from duct conduction and air leakage in unconditioned spaces like attics, crawlspaces, and garages.
Compare design CFM to measured CFM for each room to identify which registers need damper adjustment during commissioning.
Find the right supply register or return grille size for any room. Calculate face velocity, free area, and estimated throw distance.
Calculate pressure drop through multiple duct fittings using loss coefficients for precise commercial duct system design.
Convert between air velocity and velocity pressure for Pitot tube duct traverse measurements and TAB commissioning work.
Calculate friction loss in flex duct accounting for compression. Shows how a sagging or kinked run destroys airflow to that room.
The complete duct design tool. Enter system parameters and room loads to generate a full duct schedule with branch sizes and velocity checks.
Balance airflow across multiple zones and branches to verify the full system delivers design CFM to every room simultaneously.
Size return air grilles and ducts to match supply airflow and maintain proper system balance and filter face velocity.
Size individual branch takeoffs from a trunk line based on branch CFM and target friction rate, with round and rectangular options.
Size the main trunk duct feeding multiple branches from total system CFM, reducing size progressively as branches split off.
Size supply and return plenums from air handler CFM to keep plenum velocity low enough to avoid noise and excess pressure drop.
Size variable air volume boxes for commercial zones from minimum and maximum CFM requirements and inlet duct static pressure.
Calculate total exposed surface area of a duct run for insulation material takeoff and duct heat loss calculations.
Estimate sheet metal, fasteners, and sealant quantities for a duct system from total surface area and gauge requirements.
Calculate the correct airflow per ton of cooling capacity for the equipment type and check against the standard 350-450 CFM/ton range.
Find the correct sheet metal gauge for any duct size and pressure class per SMACNA standards, preventing vibration and noise.
Calculate new CFM, static pressure, and power when fan speed or impeller size changes. Quantifies VFD energy savings.
About Duct Design Calculations
Proper duct design is critical to HVAC system performance. Undersized ducts create excessive static pressure, reducing airflow and causing noise. Oversized ducts waste material and reduce air velocity, potentially causing comfort and humidity problems. The tools in this category follow ASHRAE Fundamentals and SMACNA standards.
Friction Rate Method - The Standard for Residential Duct Design
The friction rate (equal friction) method is the industry standard for residential duct system design, used alongside room-by-room load calculations. It defines the procedure for sizing ducts using the equal friction method - selecting duct sizes so that the friction rate (pressure drop per 100 feet) is consistent throughout the system. Properly designed duct systems deliver the right CFM to each room as calculated from room-by-room loads.
Key Duct Design Concepts
- Available Static Pressure: The pressure available for duct friction after accounting for equipment, filter, and component losses. Use the Available Static Pressure Calculator.
- Total Effective Length (TEL): The sum of actual duct length plus equivalent lengths of all fittings. Used to calculate the friction rate. Use the Equivalent Length Calculator.
- Friction Rate: Available static pressure ÷ Total effective length × 100. All ducts are then sized to this friction rate. Use the Duct Sizing Calculator.
- Duct Leakage: Leaky ducts can waste 20–30% of conditioned air. Test with a duct blaster and compare to ENERGY STAR limits using the Duct Leakage Calculator.
Trunk and Branch Sizing
Larger residential and light commercial systems typically split into a main trunk duct feeding multiple branch takeoffs. Size the main trunk with the Trunk Duct Sizer from total system CFM, then size each individual branch run with the Branch Duct Sizer based on that branch's CFM share. Don't forget the return side: the Return Air Calculator keeps return grilles and ducts sized to match supply airflow.
Commercial Systems: Plenums and VAV
Commercial systems add a few extra considerations beyond residential duct sizing. The Plenum Sizing Calculator keeps supply and return plenum velocity low enough to avoid noise, while the VAV Box Calculator sizes variable air volume boxes for individually controlled zones. Use the CFM per Ton Calculator to confirm airflow matches the equipment's rated capacity before finalizing duct sizes.
Related Tools You May Need
Duct design works hand-in-hand with load calculations. Start with the Heating & Cooling Load Estimator to get room loads, then use the CFM Calculator to determine required airflow per room, then size ducts with the Duct Sizing Calculator. Verify the system with the Static Pressure Calculator. For material takeoff, the Duct Surface Area Calculator and Duct Material Estimator estimate sheet metal and fastener quantities for the finished design.
Frequently Asked Questions
The friction rate (equal friction) method is the industry standard for residential duct system design. It sizes ducts so that the pressure drop per 100 feet of duct stays consistent throughout the system, ensuring every room gets its required airflow. Use the Duct Sizing Calculator to apply this method to your project.
Leaky ducts can waste 20 to 30 percent of conditioned air. ENERGY STAR sets duct leakage limits based on conditioned floor area. Test with a duct blaster and use the Duct Leakage Calculator to compare your system against these limits and estimate the energy cost of the leakage.
Total effective length (TEL) is the sum of actual duct length plus the equivalent lengths of all fittings such as elbows, tees, and transitions. TEL is used together with available static pressure to calculate the design friction rate for sizing every duct in the system. Use the Equivalent Length Calculator to build up your TEL fitting by fitting.