Airflow and duct calculators
Airflow, duct sizing, air changes and air-side capacity tools for designers, technicians and curious homeowners.
CFM Calculator
Calculate the airflow (CFM) needed to carry a heating or cooling load at a given temperature difference, or to achieve a target number of air changes in a room.
Answers: How many CFM do I need for a room or a heat load?
CFM per Ton Calculator
Check whether a system’s airflow suits its cooling capacity, or find the target blower airflow for a given tonnage.
Answers: How many CFM per ton should my AC have?
Duct Size Calculator
Size round and rectangular ducts for a given airflow by the equal-friction or velocity method, with friction loss and velocity at the nearest standard size.
Answers: What size duct do I need for a given CFM?
Air Changes per Hour Calculator
Convert between airflow (CFM) and air changes per hour (ACH) for any room or building.
Answers: Calculate air changes per hour from CFM and room size, or the CFM for a target ACH.
Cooling Capacity Calculator
Measure delivered cooling capacity — sensible, latent and total BTU/h — from airflow and the dry-bulb and wet-bulb temperatures across the evaporator coil.
Answers: Calculate actual AC cooling capacity from airflow and air temperatures.
HVAC Temperature Split Calculator
Calculate the return-to-supply temperature difference (delta T) across an AC coil or furnace and see what it suggests about airflow and performance.
Answers: What should the temperature difference between return and supply air be?
Airflow connects everything
Airflow links the load to the equipment and the equipment to the ducts. The sensible heat equation, Q = 1.08 × CFM × ΔT, appears in several of these tools; the duct calculator turns airflow into a size using the same friction physics as an ASHRAE duct chart; and the capacity and temperature-split tools work backwards from measurements to what a system is actually doing.