Duct Size Calculator
Calculate HVAC duct dimensions from airflow (CFM). Supports round and rectangular ducts with velocity validation.
Use Tool →HVAC Calculators
HVAC, electrical, and measurement tools: insulation R-value compliance, heating and cooling load calculations, equipment sizing, room airflow, duct sizing, system planning, voltage drop/wire sizing, and feet-inches to decimal feet conversion
These tools are built for HVAC engineers, mechanical contractors, and building designers who need to size rectangular and round ductwork accurately. Proper duct sizing directly affects energy efficiency, noise levels, and occupant comfort. Undersized ducts increase velocity and static pressure, causing noise complaints and higher fan energy consumption. Oversized ducts waste material, increase installation cost, and may reduce air velocity below levels needed for effective air distribution.
The calculations are based on well-established fluid dynamics equations. The Darcy-Weisbach equation determines friction loss in straight duct runs, while the Colebrook equation iteratively solves for the friction factor based on Reynolds number and duct roughness. For converting between rectangular and round duct sizes, the Huebscher formula calculates the equivalent circular diameter that produces the same friction loss at the same airflow. All standard duct dimensions follow SMACNA recommended sizes.
The six tools form a complete residential HVAC design pipeline. The Insulation Calculator checks wall, ceiling, and floor assemblies against IECC 2021 climate zone requirements. The Heat Load Calculator uses those envelope values along with room dimensions, window areas, and local climate data to estimate heating and cooling BTU loads per ACCA Manual J. The HVAC Sizing Calculator then matches those loads to equipment capacity using ACCA Manual S guidelines, accounting for altitude derating, sensible heat ratios, and recommended oversizing limits. The Room Airflow Calculator converts room loads to CFM requirements and recommends register sizes. Finally, the Duct Calculator and Duct System Planner size individual runs and full branch layouts using ASHRAE and SMACNA standards. Each tool can be used independently, but together they provide a verified path from building envelope to finished duct design.
Duct System Planner
Free HVAC duct size calculator and system planner. Auto-size ducts by CFM from plenum to registers, calculate friction loss and total pressure drop per ASHRAE and ACCA standards.
Use Tool →Heat Load Calculator
Calculate residential heating and cooling loads room by room. Simplified Manual J methodology with ASHRAE design temperatures.
Use Tool →HVAC Cost Comparison Calculator
Compare annual heating and cooling costs across six HVAC system types. Uses degree-day analysis with real energy prices to find the most affordable and greenest option.
Use Tool →HVAC Repair vs Replace Calculator
Should you repair or replace your HVAC system? Uses the 50% Rule, Rule of 5,000, and ASHRAE lifespan data to give you a clear, data-driven recommendation.
Use Tool →HVAC Sizing Calculator
Calculate the right AC, furnace, or heat pump size for your home. Uses ACCA Manual S sizing limits with standard equipment capacities.
Use Tool →Insulation R-Value Calculator
Calculate insulation R-value for wall assemblies, attics, and floors. Check IECC 2021 code compliance by climate zone.
Use Tool →Mini Split Sizing Calculator
Calculate the right ductless mini split size for each room based on area, insulation, climate zone, and room type.
Use Tool →Psychrometric Calculator
Calculate every moist-air property — dew point, wet bulb, relative humidity, humidity ratio, enthalpy — from any two, with an interactive psychrometric chart.
Use Tool →Refrigerant Charge Calculator
Calculate superheat, subcooling, and look up pressure-temperature values for 10 common refrigerants. Diagnose charge issues for TXV and fixed orifice systems.
Use Tool →Room Airflow Calculator
Calculate CFM airflow requirements and register sizes for each room. Uses ASHRAE 62.2 ventilation rates and load-based airflow calculations.
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