Duct Pressure Drop Calculator FAQ — MEP Tools

Common questions about units, equivalent diameter, K values, warnings and exports for MEC-01.

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.

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.

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.

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.

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.