Duct Size Calculator

Duct Size Calculator is a practical HVAC tool for technicians, contractors, engineers, installers, estimators, and students who need quick duct sizing, airflow, pressure loss, grille, diffuser, and HVAC system calculations in one place.

Choose the calculation method that fits the job: airflow, target velocity, friction rate, or known duct size. The app supports round, rectangular, flat oval, and flexible ducts, then shows calculated dimensions, recommended standard sizes, and useful smaller or larger alternatives.

Results include air velocity, cross-sectional area, hydraulic diameter, equivalent diameter, Reynolds number, friction loss, velocity pressure, airflow capacity, and estimated pressure drop. Clear status indicators help identify high velocity, excessive resistance, possible noise, or an undersized duct. Rectangular duct calculations include aspect-ratio checks and practical width-and-height combinations, while flexible duct calculations can account for stretching and compression.

A responsive 2D duct visualization displays duct dimensions and airflow direction for round, rectangular, and oval sections.

Room airflow and ACH calculations can estimate required CFM, m³/h, or L/s from room dimensions, air changes per hour, occupancy, and outdoor-air requirements. Supply and return airflow can also be distributed across multiple rooms.

Grille, register, louver, and diffuser sizing tools help calculate face velocity, free-area velocity, net free area, airflow per terminal, number of outlets, neck size, velocity pressure, and basic noise risk.

The duct system designer allows you to create supply and return networks with main trunks, branches, and room connections. Airflow is tracked through each section so you can review duct size, velocity, friction, remaining trunk airflow, and supply-return balance.

Fittings and HVAC components such as elbows, tees, wyes, takeoffs, transitions, reducers, dampers, filters, coils, plenums, grilles, and flexible connectors can be added to the system. Pressure loss can be estimated using K-factor, equivalent length, or a custom value. The system summary shows straight duct loss, fitting loss, effective length, static pressure use, remaining pressure, and the critical path.

Additional tools include material and labor cost estimation, a duct cleaning quote calculator, and a 12-point inspection checklist. Project and calculation history makes it easy to organize previous work, while results can be shared as text or images or saved as supported PDF reports.

The app supports US Imperial, Metric SI, and mixed units, includes an HVAC unit converter, and works offline for included calculations. Input validation and calculation warnings help reduce common input errors. Light and dark themes are available, and all included tools are accessible for free.

Important: This app is intended for preliminary calculations, estimating, education, and field reference. Final HVAC design, equipment selection, fabrication, and installation should be checked against applicable codes, project requirements, manufacturer data, and reviewed by a qualified professional.