Design a complete supply duct network, not just one duct. Enter the segments, airflow at the outlets, lengths and fittings, and the calculator sizes every segment by equal friction or velocity reduction, finds the index run, gives the fan external static pressure, shows which branches need balancing dampers, and builds a sheet metal BOQ.
Preliminary design. Fitting loss coefficients are typical values; use the ASHRAE Duct Fitting Database or manufacturer data for final design, and confirm the fan against its curve. Sheet metal quantities cover straight duct with an allowance, not individually developed fittings.
| Fitting | Qty |
|---|
A single-duct sizer answers one question: what size is this duct? A real job needs more. Every segment of the network has to be sized, the losses of all the fittings added, the longest path found, the fan pressure set from it, and the other branches balanced against it. Then somebody has to price the sheet metal. This calculator does all of that in one pass from a simple table of segments.
For one duct at a time, the Online Ductulator and the Duct Calculator are quicker. Use this tool when you have a run with branches.
Air density and viscosity follow the temperature and altitude you enter, which matters for hot air and for sites well above sea level.
Equal friction sizes every segment for the same pressure loss per metre, typically 0.8–1.2 Pa/m (0.1–0.15 in. wg per 100 ft) for low-velocity commercial systems. It is the most common manual method because it balances itself reasonably well and keeps velocities falling naturally toward the outlets. This tool also caps the velocity, so a large main is never sized into a noisy range.
Velocity reduction picks a velocity for the mains and a lower one for the branches and sizes to those. It gives direct control over noise but leaves larger pressure differences between branches, so more balancing is needed. Static regain, the third classic method, suits long high-velocity mains and is not covered here.
A full comparison of the first two methods, with examples, is in the guide on equal friction vs velocity method.
| Fitting | C (typical) |
|---|---|
| Elbow 90°, smooth radius (r/W = 1.0) | 0.22 |
| Elbow 90°, mitred with turning vanes | 0.25 |
| Elbow 90°, mitred without vanes | 1.20 |
| Elbow 45°, smooth radius | 0.15 |
| Tee / take-off, straight-through path | 0.15 |
| Branch take-off, 45° shoe (branch path) | 0.40 |
| Branch take-off, 90° tee (branch path) | 1.00 |
| Transition / reducer, gradual | 0.10 |
| Volume control damper, fully open | 0.20 |
| Fire damper, open (blades out of airstream) | 0.15 |
Loss coefficients depend on geometry and velocity ratio. Values here are representative of common fittings; enter a different value in the "Other ΣC" box of a segment when you have a project-specific coefficient.
The fan has to push air down the hardest path. Every other path has less resistance, so without a damper it would take more than its share of air and starve the far end. The balancing check shows, for every outlet path, how much pressure is left over compared with the index run. A branch with a large excess needs a volume control damper set partly closed, or a smaller duct, to bring it into line.
The example that loads with the page: an office floor fed by one AHU. The main runs S1 → S2 → S4 → S6, with branches S3 and S5 taken off with 45° shoes. Each outlet segment carries 800 L/s, so the fan handles 2,400 L/s. Equal friction at 1.0 Pa/m, commercial velocity limits (8 m/s mains, 6 m/s branches), rectangular duct up to 400 mm deep, galvanized steel, 25 Pa across the outlets and a 10% margin.
The BOQ gives surface area per segment, the SMACNA gauge from the longest side, and weight in galvanized steel with the allowance you set. The same net surface area is the insulation and cladding quantity. For straight-duct take-offs from a drawing, the Duct Weight Calculator handles any list of sizes, and the Fan Static Pressure Calculator and AHU Fan Shaft Power Calculator take the result on to fan selection.
Lay out the network, assign airflow to each outlet, and add it up back to the fan. Size each segment by a method such as equal friction, add friction and fitting losses, find the path with the highest loss (the index run), set the fan external static pressure from it, and balance the other branches with dampers.
For low-velocity commercial supply ducts, 0.8 to 1.2 Pa/m (about 0.1 to 0.15 in. wg per 100 ft) is common. Lower rates give bigger, quieter ducts and a smaller fan; higher rates save space and sheet metal but need more fan pressure.
It is the path from the fan to an outlet with the highest total pressure loss. It is not always the longest path, because fittings and outlet pressure drops count too. The fan's external static pressure is set by the index run.
Add the friction and fitting losses along the index run, the outlet pressure drop at its end, and any other external losses such as a return duct or external filter. Then add a safety margin, commonly 10 to 15 percent.
Friction charts and equations are written for round duct. The Huebscher equivalent diameter gives the round duct with the same friction loss at the same airflow, so a rectangular duct can be sized through it. The actual velocity in the rectangular duct is still calculated from its real area.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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