Online Ductulator

How This Ductulator Sizes Ducts

Enter airflow in Imperial (CFM) or Metric (L/s) and choose either a target velocity or an allowable friction loss per 100 ft (or Pa/m). The calculator converts units in real time, so a 400 CFM / 0.06 in. w.g. design instantly becomes 189 L/s / 0.5 Pa/m when you flip the toggle. Velocity-based sizing divides airflow by area to keep branch runs within typical 600–900 FPM ranges, while friction-based sizing applies the Equal Friction method to hold pressure drop steady across the system.

Round duct diameters use the Darcy–Weisbach shortcut common in ASHRAE Manual D. Rectangular outputs are derived from the Huebscher equivalent diameter correlation: De = 1.30 × [(a × b)^0.625] / [(a + b)^0.25], then back-calculated into width × height pairs for the aspect ratio you selected. If both velocity and friction are blank, the tool prompts you to pick a governing variable so you never get a misleading result.

Use the aspect ratio selector when space forces rectangular ductwork; the tool automatically adjusts for the extra corner turbulence so friction loss stays realistic. Staying within 4:1 ratios keeps performance closest to the equivalent round duct.

Quick Duct Sizing Reference

Start with the velocity column that matches your space (600 FPM for quiet rooms, 700 FPM for living spaces, 800 FPM for utility runs) and read across to find the minimum round diameter. Rectangular pairs are equivalent areas rounded to carpenter-friendly dimensions.

Values assume galvanized steel ducts at sea level. Add one inch to the diameter (or bump one rectangular size) when using compressed flex duct or when friction allowances are below 0.06 in. w.g./100 ft.

How to Use This Ductulator

1. Pick a unit system with the toggle up top before entering values; all fields and labels update instantly.
2. Enter airflow (required), then supply either velocity or friction loss—leaving both blank prompts you to choose one so results stay valid.
3. Select round or rectangular duct. If rectangular, set the aspect ratio that matches the space you have; the calculator auto-adjusts friction for higher ratios.
4. Press Calculate to view the recommended diameter or width × height pair plus the resulting velocity and friction so you can sanity check the numbers.
5. Use the warnings below each result to catch edge cases (e.g., velocity above 900 FPM, friction beyond 0.1 in. w.g./100 ft) before you finalize a layout.

Need a full walkthrough? Read the sizing HVAC ducts online guide for a scenario-based example.

Why Engineers Use This Tool

This page bundles every duct workflow in one place so you can move from concept to documentation without bouncing between apps.

• Multi-calculator suite: sizing, velocity, friction loss, and round↔rectangular conversion all share the same inputs and unit toggle.
• Dual-unit parity: metric and imperial data stay synced, so a single dataset supports submittals in any region.
• Built-in QA: guardrails surface when velocities exceed comfort ranges or when friction overshoots Manual D recommendations.
• Share-ready outputs: formatted result boxes copy cleanly into emails, RFIs, or drawings with both numeric answers and design context.