# Introduction

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

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](/tools/duct-pressure-drop-calculator/docs/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](/tools/duct-pressure-drop-calculator/docs/fittings).
- **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](/tools/duct-pressure-drop-calculator/docs/method)
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](/tools/duct-pressure-drop-calculator/docs/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.

## Where to go next

- [Getting started](/tools/duct-pressure-drop-calculator/docs/getting-started) — a worked
  first calculation.
- [Inputs](/tools/duct-pressure-drop-calculator/docs/inputs) — every field, its unit, and its
  default.
- [Reading the results](/tools/duct-pressure-drop-calculator/docs/results) — what each number
  and warning means.
- [Method and assumptions](/tools/duct-pressure-drop-calculator/docs/method) — the equations,
  constants and limits.
- [Exporting and saving](/tools/duct-pressure-drop-calculator/docs/exporting-and-saving) —
  copy, share links, PDF reports and projects.