Reading the results — MEP Tools
What every number, gauge and warning in the result panel means, and when to distrust it.
Nothing here appears until you press Calculate, and it stays on screen — unchanged — until you press it again. Editing an input after a run marks the result stale and swaps the button to Recalculate; the numbers you are looking at are always the ones from the run whose timestamp and run number are shown, never a blend with whatever you have typed since. Everything is calculated in your browser.
The headline
The hero figure is the total pressure drop in Pa, rounded to 1 decimal. Beneath it sit two restatements of the same result:
- ≈ x mmH₂O — the same total converted at 9.80665 Pa per mm of water column.
- x Pa/m friction rate — the straight-run loss per metre. This is the number to compare against a project's sizing criterion (a typical low-velocity target is around 1 Pa/m). It covers the straight duct only and is independent of duct length.
The split bar below divides the total between Straight duct and Fittings. A run dominated by the fitting share is usually a signal to check your Σ K rather than the duct size.
The four metrics
| Metric | Meaning |
|---|---|
| Velocity | The actual duct velocity, Q ÷ (w × h) or Q ÷ (πD²/4). This is the number to review against acoustic and distribution criteria — not the equivalent velocity used inside the friction term. |
| Reynolds | Computed from the actual velocity and the hydraulic diameter. |
| Flow regime | laminar below Re 2300, transitional below 4000, turbulent at or above 4000 — classified on the Reynolds number shown. |
| Friction factor | The Darcy friction factor f that produced the friction rate. |
The velocity advisory gauge
The gauge plots the actual velocity on a fixed 0–12 m/s scale. If a duct role is selected, its band is drawn on the track and a verdict appears: Within advisory band or Outside advisory band. With no role selected the gauge still plots the velocity but offers no verdict.
Two things worth knowing: a velocity above 12 m/s pins the marker at the right-hand end rather than rescaling, and the band is advisory only — it never moves the pressure drop.
Warnings
Active warnings appear at the top of the Assumptions & warnings evidence block (see below). They never block a calculation; they mark a result whose assumptions deserve a second look.
| Trigger | What it is telling you |
|---|---|
| Velocity below the selected band | Check duct size, air distribution requirements and balancing assumptions. |
| Velocity above the selected band | Check acoustic impact, fan energy, pressure class and project criteria. |
| Reynolds in the transitional range (2300–4000) | The friction factor is approximate here. Neither the laminar nor the turbulent correlation is reliable in this window. |
| Laminar flow (Re < 2300) | Verify this is expected — ventilation ductwork is essentially never laminar, so this usually means an input is wrong by orders of magnitude. |
| Mach number ≥ 0.1 | High for a low-velocity duct calculation. The tool assumes incompressible flow; check whether compressibility, noise, or specialist analysis is required. |
| Aspect ratio > 4 | Equivalent-diameter methods become less representative for very flat ducts. Verify against your project design method. |
| Relative roughness ε/Dₑ > 0.05 | Unusually high — check the material roughness and the duct dimensions, one of them is probably in the wrong unit. |
The aspect ratio is max(w, h) ÷ min(w, h); circular ducts are treated as 1 and never
trigger that warning.
The six evidence blocks
Below the metrics, the result is broken into six collapsible blocks. Together they let any figure above be reproduced by hand from the method page:
| Block | What it shows |
|---|---|
| Pressure split | Straight duct friction, Fittings (minor loss) and Total, each with its share of the total and the formula term behind it, plus the friction rate, applied Σ K and dynamic pressure q. |
| Fittings & valves | The manual Σ K sentence, or — in Schedule mode — the full fitting table: each row's K, quantity, Σ contribution and pressure drop, flagging any row using an override. |
| Calculation steps | The run walked step by step — symbolic formula, then the same formula with your numbers substituted, then the numeric result — 20 steps for a rectangular duct, 18 for circular. |
| Assumptions & warnings | Any active warnings, then a rounding note, a data-source note, and the two standard disclaimers. |
| Formulas & references | The formula catalogue behind the calculation (Darcy-Weisbach, Swamee-Jain, Huebscher, Sutherland, and the rest), collapsed by default, plus the fitting library version. |
| Report | A preview of the printed sheet / exported PDF, with its own Print / Save as PDF and Export branded PDF buttons — see Exporting and saving. |
Actions
The action rail carries Calculate (or Recalculate once a run is stale), Save, Share, Print and Export PDF:
| Action | Effect |
|---|---|
| Calculate / Recalculate | Runs the calculation from the current inputs. Disabled with a per-field message list if any input fails validation. |
| Save | Saves the project. Prompts to create a free account first if you are not signed in. See Exporting and saving. |
| Share | Copies a link that reproduces the current inputs. See Exporting and saving. |
| Disabled until a result is current. Opens the browser print dialog on the Report block's sheet. | |
| Export PDF | Disabled until a result is current; carries a PRO badge without the export.pdf entitlement. See Exporting and saving. |
Copy results and Copy share link live in the File menu rather than the rail; the File menu also disables Copy results until a result exists (the share link stays available at any time, since it only ever encodes inputs). Both clipboard actions confirm themselves: a toast reads Results copied or Link copied for about a second and a half. If the clipboard write is refused — which browsers do on insecure origins, or when clipboard permission is denied — the toast reads Copy failed instead. Trust it: it changes only after the write actually succeeds, so a success toast means the text really is on your clipboard.
There is no in-tool reset action. To clear every input back to its default, use File → New, which also detaches the current project; there is no undo, so the only recovery is your last save.