Enter test points, build a curve, and predict pressure at flow easily. Apply fan laws to compare speeds, power demand, and efficiency fast online.
| Scenario | Base RPM | Points (Flow, Pressure) | Target RPM | Typical use |
|---|---|---|---|---|
| Commissioning check | 1200 | (2000, 1.10), (3000, 0.85), (4000, 0.55) | 1500 | Compare expected static pressure after VFD change. |
| Design refinement | 900 | (1200, 0.62), (2000, 0.48), (2600, 0.30) | 1050 | Estimate duct pressure at upgraded airflow. |
| Energy review | 1400 | (2600, 1.35), (3200, 1.05), (3900, 0.70) | 1100 | Assess savings from reducing fan speed. |
Use your own measurements for best accuracy.
These relationships assume similar air density and fan geometry.
For VFD work, use stable duct conditions during testing.
Air systems rarely operate at catalog duty points once ductwork, filters, coils, and dampers are installed. A fan curve links airflow and static pressure, letting you predict how a change in system resistance shifts delivered air. On site, this supports balancing, troubleshooting comfort complaints, and verifying design intent.
Common data sources include traverse readings, fan inlet grids, or calibrated airflow stations. Static pressure is often measured in inches of water column or pascals using pitot ports and a manometer. For many building systems, operating static pressures land roughly between 0.5 and 4.0 in. w.g., depending on duct length, fittings, and filtration.
This calculator fits a quadratic relationship P(Q)=aQ²+bQ+c from three measured points at one speed. While manufacturers may publish higher-order polynomials, a quadratic is practical for commissioning because it captures the typical downward trend of pressure with increasing flow across the mid operating range.
When a VFD changes speed, airflow scales approximately with speed ratio (Q₂=Q₁·N₂/N₁) and static pressure scales with the square (P₂=P₁·(N₂/N₁)²). Power demand rises quickly, scaling with the cube (W₂=W₁·(N₂/N₁)³). A 10% speed increase can mean about 33% more power.
The fan does not choose the duty point alone. The system curve typically behaves like ΔP≈k·Q², representing friction and dynamic losses. The operating point is where the fan curve intersects the system curve. Closing a damper increases k, pushing the intersection toward lower flow and higher pressure.
Fan laws assume similar air density. If temperature or altitude changes significantly, pressure capability and required power shift. A quick rule is that pressure scales with density, and brake power scales with density as well. Use consistent test conditions when comparing two measurements.
Collect three stable points at a fixed speed, spaced across the expected operating range. Fit the curve, then test a target speed and compare measured pressure to predicted pressure at the same flow. Large deviations often indicate measurement error, unexpected system resistance, or fan performance issues.
Exported tables help crews communicate expected airflow and pressure at multiple speeds, supporting O&M manuals and turnover packages. Include fan identification, measurement locations, instrument calibration date, and notes about damper positions. Clear records reduce callbacks and speed up acceptance testing.
You need three points to fit a quadratic curve. With two points, you can only fit a straight line, which may misrepresent pressure behavior. If you only have two points, take a third measurement at a different damper position or speed.
That usually indicates inconsistent measurements, unstable fan speed, or incorrect pressure taps. Recheck instrument zero, confirm ports are in straight duct sections, and ensure the fan speed is constant during each reading before refitting.
Yes, as long as your three points reflect the fan’s stable operating region. Avoid points near stall or surge. Centrifugal fans often fit well in the mid-range; axial fans can be sensitive near instability and may need careful point selection.
The calculator solves for airflow where the fitted curve equals your target static pressure at the chosen speed. If the system curve dominates, the actual operating flow may differ. Use this as a fan capability estimate, not a final balance value.
Fan power scales roughly with the cube of speed, so small speed increases can create large power increases. This is why modest VFD reductions often produce meaningful energy savings while maintaining acceptable airflow in variable-volume systems.
If you don’t have nameplate data, use a typical range of 0.85–0.95 for common motors. For better estimates, use measured input power and manufacturer efficiency at the operating load. Keep efficiency between 0 and 1.
Fifteen to twenty rows usually balance detail and readability. Use more rows for smooth reporting or spreadsheet analysis, and fewer rows for quick checks. The CSV export is convenient for plotting the curve in your preferred tools.
Better curves mean quieter systems, lower energy, safer workplaces.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.