Two free, browser-based calculators for HVAC and air-filtration engineering. Both implement the Darcy-Weisbach friction equation with the Swamee-Jain explicit formula for the friction factor, the same methods used in ASHRAE Fundamentals and VDI 3802. No login, no install, and the formulas are visible on every page for auditability.
Most HVAC and air-filtration calculations come down to two questions: what size should the duct be? and what is the resulting pressure loss? These are related, but they answer different design-stage questions, and Emifree provides a dedicated calculator for each.
The Duct Sizing Calculator is for the equipment-selection phase: you have a target airflow (m3/h or CFM) and a friction-rate budget (Pa/m or in.wg per 100 ft), and you need to know what cross-section to specify. The Air Pressure Loss Calculator is for the system-design phase: you already have a duct geometry, and you need to know the total static pressure the fan has to overcome, including K-factor losses across elbows, T-junctions, and reducers, plus oil-mist and dust-correction multipliers when the airstream is loaded.
Duct Sizing Calculator
Size round or rectangular HVAC ducts from airflow, friction rate, or velocity. Darcy-Weisbach + Swamee-Jain + ASHRAE equivalent diameter. Imperial and metric.
When to use it: Use this when you need to pick a duct cross-section for a given airflow and a target friction rate per 100 ft (or per meter). Outputs both round and rectangular equivalents so you can compare against existing shaft sizes.
Open toolAir Pressure Loss Calculator
Compute total air pressure loss (Pa) for a duct run. Friction loss for straight tubes + K-factor losses for elbows, T-junctions, reducers. Darcy-Weisbach + ASHRAE. Includes oil-mist and dust correction.
When to use it: Use this when you already have a duct geometry and need to know the static pressure the fan has to overcome, including losses across elbows, T-junctions, and reducers. Supports oil-mist and dust-laden airstreams.
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