Getting started — MEP Tools
Run your first duct pressure-drop calculation in about a minute, with a worked example.
Open Duct Pressure Drop Calculator from the tools catalog. No sign-in is needed to calculate — you only need an account to save projects or export a PDF report.
The page is a single screen: five input cards (airflow & duct run, geometry, material & air properties, fittings, and a duct schematic that sketches the run as you type), followed by the result once you calculate one. Nothing computes on its own — enter your inputs, then press Calculate. Once a result is on screen, editing any input marks it stale and swaps the button to Recalculate, so you are never looking at a result that no longer matches the form.
The fast path: Load a worked example
Before your first calculation, the start zone offers Load a worked example. It fills in a
400 × 300 mm rectangular galvanised-steel main carrying 0.5 m³/s over 15 m, with a manual Σ K
of 2.4 and the Main duct advisory role, and runs it immediately. Use it to see a populated
result before typing anything of your own. The button is hidden once you arrive via a share
link or a restored draft, and it disappears after your first Calculate.
A worked example (typed by hand)
These are different numbers from the one-click example above — a second, independently verified case if you want to enter values yourself. Enter the following into the four input cards, then press Calculate:
| Card | Field | Value |
|---|---|---|
| 01 Airflow & duct run | Airflow (Q) | 0.75 m3/s |
| 01 Airflow & duct run | Duct length (L) | 30 m |
| 02 Duct geometry | Shape | Rectangular |
| 02 Duct geometry | Width (w) | 500 mm |
| 02 Duct geometry | Height (h) | 300 mm |
| 03 Material & air properties | Roughness preset | Galvanised steel (0.15 mm) |
| 03 Material & air properties | Air temperature / pressure | 20 °C / 101.325 kPa (defaults) |
| 04 Fittings | Mode | Manual Σ K |
| 04 Fittings | Total fitting coefficient (Σ K) | 1.5 |
The result panel should now read:
| Output | Value |
|---|---|
| Total pressure drop | 46.2 Pa (≈ 4.71 mmH₂O) |
| Friction rate | 0.787 Pa/m |
| Straight duct | 23.6 Pa |
| Fittings | 22.6 Pa |
| Velocity | 5 m/s (the gauge marker reads 5.00) |
| Reynolds | 124,494 (turbulent) |
| Friction factor | 0.01872 |
If your numbers differ, check the airflow unit selector first — it sits inside the airflow
field and defaults to m3/s, so a value meant as L/s is 1000× too large.
Add an advisory band
In Manual Σ K mode, set Duct role (advisory) to Main duct - 5-8 m/s. The velocity
gauge now draws the 5–8 m/s band and reports Within advisory band for the 5 m/s above.
The band is advisory only: it never changes the calculated pressure drop, it only produces a
verdict and a warning when you fall outside it.
Then
- Swap Σ K for a real fitting schedule — pick fittings from the library, set quantities, override any K you disagree with.
- Read the result panel in detail, including the warnings and the six evidence blocks (Pressure split, Fittings & valves, Calculation steps, Assumptions & warnings, Formulas & references, Report).
- Copy, share or export the calculation.