What the Duct Pressure Drop Calculator does, what it assumes, and what it does not certify.
Duct Pressure Drop Calculator (MEC-01) is a formula-only HVAC calculator for preliminary
single-run duct pressure-drop checks. Everything it reports is computed in the browser from
the numbers you type — there is no duct library, no fan database, and no project model
behind it.
Nothing is computed until you ask for it: fill in the input cards, then press Calculate.
See Getting started for the full
flow, including what happens when you edit an input after a result is on screen.
What it calculates
From an airflow, a duct length, a duct size and an air state, the tool reports:
Air velocity in the duct, and the Reynolds number and flow regime
(laminar / transitional / turbulent).
Straight-duct friction loss using the Darcy-Weisbach equation with a Swamee-Jain
friction factor, reported both as a total (Pa) and as a friction rate (Pa/m).
Fitting losses, from either a single manual Σ K value or a row-by-row
fitting schedule.
Total pressure drop in Pa and mmH₂O, split between the straight run and the fittings.
A velocity advisory against an optional main/branch band, plus warnings for
transitional flow, high Mach number, high aspect ratio and implausible roughness.
Rectangular ducts are handled with the Huebscher circular-equivalent diameter: the
friction calculation runs on the equivalent circular duct, while the velocity shown to you
is the real duct velocity. The method page
sets out every equation as implemented, including which velocity feeds which term.
What it is not
The tool covers one duct run at a time. It does not size ducts, balance a system, sum a
network, select a fan, or check pressure class, acoustics or code compliance. It has no
knowledge of your project standards.
The default K values are simplified typical figures published for preliminary checking
(library mvp_default_v1); they are not reproduced from a named standard, and any value can
be overridden per row. See Fittings.
Treat the output — including the PDF report — as a transparent engineering check, not an
approved design or a construction document. Final 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.