Ductulator
The Ductulator is an interactive tool for sizing ductwork based on airflow and design parameters.
Duct Sizing Basics
Section titled “Duct Sizing Basics”Key Parameters
Section titled “Key Parameters”Airflow (CFM or L/s)
Section titled “Airflow (CFM or L/s)”The volume of air flowing through the duct.
Velocity (FPM or m/s)
Section titled “Velocity (FPM or m/s)”The speed of air in the duct. Higher velocity means:
- Smaller duct size
- More noise
- Higher pressure drop
Friction Rate
Section titled “Friction Rate”Pressure drop per unit length (in. wg/100 ft or Pa/m).
Duct Size
Section titled “Duct Size”Resulting dimensions:
- Round: diameter
- Rectangular: width × height
Using the Ductulator
Section titled “Using the Ductulator”Inputs
Section titled “Inputs”Enter your design constraints:
- Flow Rate — the volume of air moving through the duct (CFM or L/s). Leave it empty and the tool assumes the flow that the current duct carries at your max velocity — about 236 CFM for the 6 in round starting size at the 1200 FPM default — so a full result is shown before you type anything. Clear the max velocity as well and there is no flow to report: Pressure Drop and Velocity stay empty
- Max Velocity — the highest air velocity to allow, measured on the clear area inside the liner (FPM or m/s; defaults to 1200 FPM)
- Max Pressure Drop — the highest friction rate to allow per unit length (in. wg/100 ft or Pa/m; defaults to 0.1)
- Max Height — the highest outside duct height to allow. It also decides the shape of the result — see below
- Liner Thickness — internal acoustic liner. It reduces the clear airflow area, so the tool sizes a larger duct to hold your velocity and friction limits
- Shape — Round or Rectangle. This control converts the current result instead of steering the sizing — see Overriding the result
- Sizing Increment — the step the tool uses when generating candidate sizes (defaults to 2 in). A step of 0 falls back to 2 in, and a step under 0.06 in is treated as 0.06 in
How sizing works
Section titled “How sizing works”The Ductulator starts at a 6 in round duct and grows it by your sizing increment. It stops at the first candidate that stays within both the max velocity and the max pressure drop. There is no separate “velocity” or “equal-friction” mode — the tool satisfies every constraint at once. The friction rate is that of turbulent flow in galvanized steel; there is no material selection. Because the sweep only grows the duct, the result is never smaller than 6 in.
Max Height decides whether the result is round or rectangular:
- While the next round candidate still fits under the cap, the result stays round.
- When the next round candidate would exceed the cap, the tool switches to a square duct at the current height and then widens it. 2000 CFM at the default limits gives a 20 in round duct; add a 10 in max height and the result is 10 × 30 in.
- A max height below 6 in is smaller than the starting candidate, so no sweep runs. The result stays a 6 in round duct, and its reported velocity and friction rate exceed your limits.
When no candidate can satisfy your constraints — a max velocity or max pressure drop of 0, for example — the sweep gives up at a very large duct and reports the last size it tried. Always read the reported Velocity and Pressure Drop back against your limits. A result above them means that no duct satisfies the constraints, not that the tool chose that duct.
Overriding the result
Section titled “Overriding the result”To override the result, type a dimension directly:
- Round — set the Diameter; clear it to fall back to the calculated value
- Rectangle — set the Height and/or Width. Clearing either field resets both dimensions back to their calculated values — the two overrides are not independent
- Height shows the calculated value as its default. Width shows one only when the calculated result is itself rectangular, which needs a max height cap
The Shape control is an override too. Selecting a shape the result does not already have holds the size you see:
- Round to Rectangle converts the result to a square at its diameter — a calculated 20 in round duct becomes 20 × 20 in.
- Rectangle to Round takes the height of the rectangle as the diameter.
Automatic sizing stops while any override is in place, so the reported velocity and friction rate are those of the size you hold. Clear Diameter, Height, or Width to return to the calculated size.
Results
Section titled “Results”The results update as you commit each input:
- A scaled cross-section preview of the duct and its liner. The dimension labels give the clear inside-liner size, so a 22 in duct with a 1 in liner is labeled 20 in. A rectangle flatter than about 3.3:1 shows no labels
- Pressure Drop — friction rate at the current flow and size
- Velocity — air velocity through the clear area at the current flow
- Area — clear airflow area of the duct
Design Guidelines
Section titled “Design Guidelines”Velocity Limits
Section titled “Velocity Limits”| Application | Max Velocity |
|---|---|
| Main ducts | 2000 FPM |
| Branch ducts | 1200 FPM |
| Noise-sensitive | 800 FPM |
| Residential | 600 FPM |
Friction Limits
Section titled “Friction Limits”| Application | Friction Rate |
|---|---|
| Low velocity | 0.05 in/100 ft |
| Medium velocity | 0.10 in/100 ft |
| High velocity | 0.20 in/100 ft |
Aspect Ratio
Section titled “Aspect Ratio”For rectangular ducts:
- Keep aspect ratio < 4:1
- 1:1 to 2:1 is optimal
- Higher ratios increase friction
Practical Tips
Section titled “Practical Tips”Round vs. Rectangular
Section titled “Round vs. Rectangular”Round ducts have:
- Lower friction for same airflow
- Less material for same capacity
- Better sealing
- May not fit in tight spaces
Equivalent Diameter
Section titled “Equivalent Diameter”Compare rectangular to round:
- Same airflow and friction
- Use for mixed systems
- Verify with friction chart
Real-World Adjustments
Section titled “Real-World Adjustments”Consider:
- Actual available sizes
- Fitting losses
- Installation constraints
- Insulation thickness