Round metal
11 in round
- Cross-section
- 95.0 sq in
- Velocity
- 606 fpm
- Equivalent diameter
- 11.00 in
- Friction rate
- 0.054 in. w.c./100 ft
- Loss over entered length
- 0.107 in. w.c.
Room airflow first. Engineering second.
Why does this room barely get any air?
Start with what you can see: room size, vents, distance, floor, and the symptom. If you already know the airflow and pressure budget, open the professional tab for the full math.
- Runs in your browser
- No sign-up
- Assumptions stay visible
Ductulator
Use the side that matches what you know.
The first tab is a screening check for a homeowner. The second tab sizes metal duct by velocity and friction rate. Neither tab can replace measurements inside your system.
No duct terms required
Describe the room, not the equipment.
This screen compares a rough room airflow target with one common setup: one 6-inch branch behind each vent. If your branches are a different size, the percentage is not yours.
Screening result
A typical 6-inch branch is not the obvious problem.
Weak air can come from branch size, a damper, a leak, the filter, or the return path.
- Room airflow screen
- 200 CFM
- Each vent would carry
- 100 CFM
- Round branch screen
- 6 in each
- 6-inch assumption
- No area gap shown
What is usually checked next
Velocity and friction rate
Size a metal duct from a pressure budget.
Enter the airflow for one duct section, the static pressure left for the duct path, and the longest total effective path. The calculator checks friction and velocity, then rounds up to buildable whole-inch dimensions.
Available friction rate 0.100 in. w.c. per 100 ft Size governed by velocity
Rectangular metal
14 x 7 in
- Cross-section
- 98 sq in
- Velocity
- 588 fpm
- Equivalent diameter
- 10.66 in
- Friction rate
- 0.063 in. w.c./100 ft
- Loss over entered length
- 0.125 in. w.c.
Math shown
What the calculator solves
- Available friction rate
FR = available static x 100 / total effective length- Air velocity
V = CFM x 144 / area in square inches- Rectangular equal-friction diameter
De = 1.30(ab)^0.625 / (a + b)^0.25- Straight-duct pressure loss
delta P = f(L/D)(rho V^2/2), using Darcy-Weisbach and a Swamee-Jain friction factor.
Assumptions: standard air near 68 F, galvanized sheet metal, whole-inch dimensions, no leakage, and no flex-duct compression. The entered path must already include fitting equivalent lengths. The shown loss applies one section's friction rate across the entered length. A real path changes airflow and size from section to section.
This is an estimate. Final sizing to ACCA Manual D must be completed by a licensed contractor because it requires measured static pressure, total equivalent length, and fitting losses.
Why the answer stays conditional
Weak airflow is not one problem.
Room demand comes first.
Duct size starts with the airflow that the room actually needs. Floor area is only a screen. Glass, sun, insulation, ceiling height, and leakage belong in a room-by-room load calculation.
Pressure is a system number.
A clean 6-inch branch can still feel weak when the filter, coil, return, damper, or another branch uses the pressure budget. The blower table and measured static pressure tell that story.
Fittings count as length.
A 40-foot tape-measure run may have far more effective length once elbows, boots, takeoffs, and transitions are counted. Long or compressed flex duct adds another uncertainty.
Common questions
Before anyone starts cutting ductwork.
Can this prove that my duct is undersized?
No. The room tab can flag a common 6-inch branch assumption. It cannot see your actual branch size, delivered airflow, static pressure, leakage, fittings, or return path. Those measurements decide the repair.
Why can an upstairs room stay hot even with enough branch area?
Roof and attic heat gain, duct leakage, a poor return path, and system balance can all create the same symptom. Adding supply air without checking the return can raise pressure and make the whole system louder.
What does total effective length mean?
It is the longest supply-and-return path after each fitting is converted to an equivalent length of straight duct. It is normally much longer than the distance you can measure with a tape.
Why does the professional tab show two acceptable sizes?
Round and rectangular ducts can carry the same airflow with similar friction while having different area and velocity. The equivalent diameter connects their friction behavior. Space, noise, fittings, and construction limits decide the shape.
Need the airflow measured?
Measure first. Scope the fix second.
We check delivered air, total external static pressure, the filter, and the return path. The repair scope and estimate come after the diagnosis, when there are real readings to work from.