Reading Results

What the numbers mean and how to use them.

Running a Calculation

Click Calculate after building your system. Results update instantly. The diagram highlights the critical path in a distinct color.

Per-Segment Results

Each segment shows:

Velocity. Air speed in fpm (or m/s). Useful for checking whether you're within acceptable velocity ranges for noise and erosion.
Velocity pressure (Pv). Kinetic energy of the air, in in. w.g. Used as the reference for fitting loss calculations.
Friction loss. Pressure drop from wall friction over the straight-run length.
Fitting losses. Each fitting's loss listed individually, with its Co value shown.
Segment total. Friction plus all fitting losses for the segment.

Units shown here are the default English view (fpm, in. w.g.); the header toggle switches results to SI (m/s, Pa).

Per-Path Results

Each path shows the sum of all segment losses from fan to terminal. Paths are listed from highest to lowest total loss.

Critical Path

The critical path has the highest modeled loss at the assigned design airflows. It is an input to fan selection after you check the pressure basis and any losses outside the model. The calculator does not solve a fan-curve operating point or balance actual branch flows.

The critical path is not always the longest path. A short run packed with elbows and tees can easily outweigh a long straight run. This is exactly why you need to calculate instead of guess.

Modeled loss summary

The raw result sums modeled losses: friction, fittings, and entered published pressure drops on the critical path. With inlet and outlet ductwork attached, the critical inlet and outlet losses are added together.

Adjusted loss = raw loss × (1 + safety factor % / 100)

This result is not automatically fan static pressure or equipment external static pressure. Check the system boundary, terminal static/total rating basis, and fan outlet velocity pressure before selecting equipment. See fan pressure and modeled duct losses.

Safety Factor

The safety factor is a user-selected allowance for uncertainty. Enter known equipment losses explicitly, including the selected filter loading condition, rather than hiding them in the allowance. Record why the factor is appropriate; the default is not a design recommendation.

A larger safety factor is not always better. Oversizing the fan wastes energy, increases noise, and forces the system to damper down, which adds back the resistance you were trying to avoid. Size the fan to the job.

Balancing

Paths shorter than the critical path will have excess pressure available. In a balanced system, that excess is absorbed by dampers or by intentionally undersizing branches. DuctStatic shows the pressure difference between each path and the critical path, so you know exactly how much balancing is needed at each branch.

Sound Results

With a fan that has a sound source and at least one diffuser or louver, the Sound (Prelim) tab shows a per-terminal table:

Terminal. The diffuser or louver.
CFM. Airflow through the terminal.
Target NC. The design criterion you set on the terminal.
Terminal NC*. The diffuser's own self-noise, marked rated or est (with neck velocity for estimates). "—" means there's no CFM set, so it can't be estimated.
Predicted NC. Duct-borne fan noise combined with terminal self-noise after a single room-absorption correction.
Controlling band. The octave band driving the NC rating (usually a low band).
Result. PASS or FAIL against the target. PASS requires every band at or below the target.

Click a terminal row to highlight it in the diagram. Self-noise is on by default and auto-estimated from each terminal's CFM and neck size; to use a manufacturer rating instead, open the diffuser's properties and set Self-noise → Rated NC. For the full model, see the Calculation Method page.

Sound results are a preliminary estimate for screening, not a certified acoustical design. They point the general direction for good duct design, not the exact answer, and don't replace an acoustical engineer's judgement. On a quiet fan with a heavily-attenuated run, the diffuser's own self-noise often sets the room NC, so including it (as this model does) gives a more realistic reading than fan noise alone.