What this calculator does
A fan does not deliver one flow at one pressure — the ductwork pushes back harder as flow rises. This tool draws that resistance curve so you can see how much the system costs to push past its design point, and it tells you exactly what is still missing before an operating point can be found.
How to use it
- Enter the design airflow and the pressure the system loses at that airflow — build that point up with the Fan Static Pressure tool.
- Enter the highest airflow you want to plot; it must extend past the design point.
- The curve is anchored on your design point, so that point has to be right for the rest to be right.
- An operating point needs the manufacturer’s fan curve; this tool will not invent one.
Governing formula
ΔP(Q) = ΔP_d × (Q / Q_d)² Where:
-
ΔP(Q)= Pressure required at flow Q, Pa -
ΔP_d= Design pressure loss, Pa -
Q_d= Design airflow, L/s
Derived Equations:
K = ΔP_d / Q_d² Worked example — 400 Pa at 100 L/s
The figures below were produced by running this page's own engine with the example inputs (Design airflow (L/s), Design pressure loss (Pa), Highest airflow to plot (L/s)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| System resistance K | 0.04 Pa/(L/s)² |
|---|---|
| Flow for 2× design pressure | 141.42 L/s |
| Flow for half design pressure | 70.71 L/s |
| Fitted curve exponent | 2 |
| To find an operating point you need | A fan operating point needs the manufacturer’s fan curve (flow against total pressure at a given speed). This tool plots the system the fan works against; supply the fan curve from the manufacturer’s selection data to find the intersection. |
- The resistance is K = 400 / 100² = 0.04 Pa/(L/s)².
- At 200 L/s the curve needs 1,600 Pa — four times the design pressure.
- That is the point of the quadratic curve: extra airflow is paid for quadratically.
Assumptions & limitations
Engineering calculation. System resistance curve from your design point, following ΔP ∝ Q². It plots the system the fan works against, not the fan.
- System curve only. No manufacturer fan curve, efficiency, motor power or operating point is claimed; the intersection needs fan data this tool does not hold.
- Resistance follows ΔP ∝ Q², which is a turbulent-flow result. It is a poor fit at low Reynolds number, and this tool has no viscosity with which to test that.
- K is back-calculated from the design point you supplied. It is not derived from duct dimensions, fittings or roughness data — use hv-fan-static to build that point up.
- No duct leakage, filter loading, device resistance over time, or altitude air-density correction is included.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ASHRAE Handbook — Fundamentals, air system design and the system curve.
- CIBSE Guide B — fan system resistance and the system curve.