Quick answer: A ductulator is a circular slide-rule (or online tool) that links four things — airflow, friction rate, velocity and duct diameter. Set any two and it gives the others. To size a duct: set the friction rate (~0.8–1.0 Pa/m), line it up against the airflow, and read off the diameter and velocity. Use the online ductulator for an instant answer.
What is a ductulator?
Every HVAC engineer has held the cardboard wheel. A ductulator solves the duct-friction equation graphically so you don't have to. Its four scales — airflow (CFM or L/s), friction loss (Pa/m or in.wg/100ft), air velocity, and round duct diameter — are related, so fixing two fixes the rest.
How to use a ductulator, step by step
- Set the friction rate — rotate the wheel to your design value, usually 0.8–1.0 Pa/m.
- Find your airflow on the airflow scale.
- Read the round diameter lined up against it — that's your duct size.
- Read the velocity at the same point and check it against the noise limit for the space.
- Convert to rectangular if needed, using the equivalent-diameter scale (keep the aspect ratio ≤ 4:1).
Round-to-rectangular conversion
A rectangular duct that behaves like a given round one has the same equivalent diameter, not the same area. The ductulator's inner scale does this for you; keep the long side no more than about four times the short side, or friction and cost climb.
Common ductulator mistakes
- Ignoring the friction rate — reading duct size without first setting the friction scale gives a meaningless answer.
- Chasing velocity — a smaller, faster duct looks efficient but roars; always check the velocity limit.
- Bad rectangular conversion — matching area instead of equivalent diameter undersizes the duct.
Worked example
Airflow = 800 CFM, friction rate = 0.9 Pa/m Ductulator → round diameter ≈ 315 mm (12.5") Velocity ≈ 4.6 m/s ✓ Rectangular equivalent ≈ 350 × 250 mm
FAQ
What friction rate does a ductulator use? Whatever you set — 0.8–1.0 Pa/m is the usual comfort-HVAC range.
Is an online ductulator as accurate as the wheel? Yes — both solve the same friction equation; the online version just removes reading error. See also how to size ductwork.
What's a good velocity? 3–5 m/s for quiet spaces, up to 7.5 m/s on commercial mains.