# Reading the results Source: https://meptools.com/tools/duct-pressure-drop-calculator/docs/results/ 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. ![Duct Pressure Drop Calculator after Calculate, showing a populated result panel](./_assets/duct-getting-started-flow.png) ## 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](/tools/duct-pressure-drop-calculator/docs/fittings#manual-sigma-k) 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](/tools/duct-pressure-drop-calculator/docs/method): ![The result's evidence blocks after a Calculate](./_assets/duct-evidence-blocks.png) | 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](/tools/duct-pressure-drop-calculator/docs/exporting-and-saving#pdf-calculation-record). | ## 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](/tools/duct-pressure-drop-calculator/docs/exporting-and-saving#saving-projects). | | **Share** | Copies a link that reproduces the current inputs. See [Exporting and saving](/tools/duct-pressure-drop-calculator/docs/exporting-and-saving#share-link). | | **Print** | 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](/tools/duct-pressure-drop-calculator/docs/exporting-and-saving#pdf-calculation-record). | **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.