# Duct Pressure Drop Calculator FAQ

Source: https://meptools.com/tools/duct-pressure-drop-calculator/docs/faq/

## Why is my pressure drop 1000× too big?

Almost always the airflow unit. The unit selector sits inside the airflow field and defaults
to `m3/s`, so a value typed as litres per second is read as cubic metres per second. Switch
the selector to `L/s`.

## Why does the tool show a velocity different from the one my friction calculation uses?

Because for a rectangular duct they genuinely are two different velocities. The friction loss
is solved in the Huebscher **equivalent circular duct**, whose velocity is always higher than
the real duct's. The metric labelled **Velocity** is the real duct velocity — the one to judge
against acoustic and distribution criteria. Open **Calculation steps** to see the equivalent
diameter and equivalent velocity used inside the friction term, and see
[Method](/tools/duct-pressure-drop-calculator/docs/method#rectangular-ducts-and-the-huebscher-equivalent-diameter).

## Where do the default K values come from?

They are simplified typical figures assembled for preliminary checking (library
`mvp_default_v1`). They are **not** reproduced from a named standard and are not
manufacturer-specific. Override any of them per row — the PDF report records which rows used
the library default and which used your override. See
[Fittings](/tools/duct-pressure-drop-calculator/docs/fittings).

## Can I leave the duct length blank to get fittings-only losses?

No. Duct length is required and must be greater than zero. A zero-length run reports a
fittings-only figure that reads exactly like a duct-run result, so the tool asks for a real
length rather than producing a number that invites that mistake. To isolate the fitting loss
on a real run, read the **Fittings** figure in the split bar.

## Why can't I change the duct role while using a fitting schedule?

The **Duct role (advisory)** selector is only rendered in **Manual Σ K** mode. Switch back to
Manual, set or clear the role, then switch to Schedule — the role you chose stays active in
the gauge.

## Does the advisory band change my result?

No. The band drives the gauge verdict and the band warnings only. Pressure drop is identical
whichever role you select, including **Not selected**.

## I pressed Calculate and nothing happened — what's missing?

Calculate is blocked when a required input fails validation. Each failing field is marked
invalid with its own message right underneath it, and the action rail reports how many fields
still need a value. Fix every highlighted field and press **Calculate** again — the full rule
list is on [Inputs](/tools/duct-pressure-drop-calculator/docs/inputs).

![Duct Pressure Drop Calculator with a blocked Calculate attempt, showing an inline field error](./_assets/duct-inputs-blocked-validation.png)

## What does the transitional-flow warning mean in practice?

Reynolds between 2300 and 4000 has no reliable friction correlation — neither the laminar nor
the turbulent form applies. The tool applies the turbulent form and flags it. For ventilation
ductwork this range is very unusual, so treat it as a prompt to check your inputs rather than
as a result to refine.

## Is my calculation stored anywhere?

Only if you save it to a project, which requires an account. Otherwise everything is computed
in your browser and lives only in the page — a reload with no share link and no open project
returns you to defaults. See [Privacy](/docs/privacy-ads).

## Can I use the PDF report for a submittal?

No. It is a calculation record for engineering checking. Duct sizing, pressure-loss
assessment, fan selection, acoustic review, code compliance and construction documentation
remain the responsibility of the qualified engineer and the applicable project standards.

## Does it work on mobile?

Yes — MEC-01 runs fully on mobile, not as a read-only viewer.

## Does it handle a whole duct network?

No. It calculates one run of constant cross-section at a time. Summing an index run across
several segments means running each segment and adding the totals yourself.