Fan Performance

Fan Laws Calculator

Calculate how a speed change, or a change in impeller diameter, affects airflow, static pressure and power. Enter your current fan data and get the new performance instantly, with the full working shown.

Enter your current fan data and a new speed. All three fan laws apply at once: Airflow ∝ N · Pressure ∝ N² · Power ∝ N³

Current Speed N₁ (RPM)

New Speed N₂ (RPM)

Airflow Q₁

Pressure P₁

Power W₁

Enter both speeds plus at least one of airflow, pressure or power.

Know the airflow or pressure you need, but not the speed? Enter current conditions and the target. The calculator solves for the required RPM.

Current Speed N₁ (RPM)

Solve From

Current Airflow Q₁

Target Airflow Q₂

Fill in the current speed and both values (current and target).

Required Speed N₂

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RPM

Speed Ratio N₂/N₁

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Comparing impeller sizes at the same speed? The diameter laws scale performance: Airflow ∝ D³ · Pressure ∝ D² · Power ∝ D⁵
Only valid for geometrically similar fans at the same speed and air density. It does not apply to comparing unrelated fan models by nominal diameter alone.

Current Diameter D₁

New Diameter D₂

Airflow Q₁

Pressure P₁

Power W₁

Enter both diameters plus at least one of airflow, pressure or power.

Fan laws scale corresponding points on the fan curve for the same fan (or geometrically similar impellers) at constant air density. The actual operating point also depends on the system resistance curve, installation conditions, and fan efficiency. Results are theoretical estimates. Always verify against the manufacturer's tested fan curve before finalising a specification.

Need help selecting the right EC fan model for your duty point? Use the EC Fan Size Selector, or send your specifications to the Proventofan engineering team.

When You Need to Recalculate Fan Performance

Fan laws let you predict how airflow, pressure and power change when you adjust fan speed or swap to a different impeller diameter, without re-testing the fan. That covers a handful of situations engineers run into regularly: checking what happens to power draw when a VFD or EC fan slows down for part-load operation, working out what speed is needed to hit a lower target airflow after a duct or damper change, or scaling performance data from one frame size to a geometrically similar one.

All three situations run through the same tool. Enter your known values, get the result and the full working, and use it to sanity-check a proposal or a retrofit calculation before committing to it on paper.

Choosing the Right Calculator Mode

Speed Change. You know the current speed, new speed, and at least one current performance value (airflow, pressure or power). The calculator applies all three fan laws and returns whichever new values you have data for.
Find New Speed. You know the current speed and airflow (or pressure), and you have a target airflow or pressure you need to hit. The calculator works backward to tell you what RPM gets you there.
Diameter Change. You are comparing two geometrically similar impellers running at the same speed, and want to know how a size change scales airflow, pressure and power.

Each mode shows the formula and the numbers it used, so you can check the working rather than just trusting the output.

Reading the Results Correctly

The power result is usually the one that matters most for a retrofit or VFD sizing decision, because it drops faster than speed does. Check the "Working" line under your result to see the exact ratio and exponent the calculator used for each output.

For the full explanation of why power falls faster than speed, with worked energy-saving examples, see Fan Laws Explained: How Speed Changes Affect Airflow, Pressure, and Power.

What This Calculation Does Not Account For

The output is a scaled operating point on the fan's performance curve, not a guaranteed field result. System resistance, installation effects, and fan efficiency at the new operating point all sit outside the calculation, so treat the number as an estimate for checking a proposal rather than a final specification.

For the complete list of assumptions behind fan laws and when they stop applying, see Fan Laws Explained.

FAQ

Fan Laws Calculator Questions Answered

You don't need to know the formulas, just the values you have (current speed, airflow, pressure, power, or diameter) and what you're solving for. Each result shows the formula and the working, so you can see exactly how the answer was calculated. For a full explanation of what fan laws are and why they work, see Fan Laws Explained on this site.

The arithmetic is exact once you accept the underlying assumptions of fan laws, which are noted below the calculator. Treat the output as a reliable planning estimate rather than a guaranteed field result, and confirm it against the manufacturer's tested fan curve before finalising a specification.

Yes. Enter the current and new RPM regardless of how the speed change is controlled, whether that's an EC fan's built-in controller, an external VFD, or any other method. The calculator scales the physics of the fan and impeller, not the control method. See Fan Laws Explained for the reasoning behind why this holds true across control types.

Because power follows the cube of the speed ratio. Reducing speed to 80% of its original value cuts power to roughly 51% (0.8 cubed), not 80%. This cube relationship is the main reason variable-speed control saves so much energy in part-load applications such as HVAC systems, where fans rarely need to run at full speed.

Only if the two fans are geometrically similar, meaning the same basic design scaled up or down, running at the same rotational speed and air density. Fan laws cannot be used to compare two unrelated fan models that happen to share a nominal diameter, since differences in blade design, motor efficiency and housing will produce different results that the scaling relationship does not capture.

Speed law holds the impeller fixed and varies RPM. Diameter law holds RPM fixed and varies the impeller. Diameter has a much steeper effect on power (to the fifth power, versus the cube for speed), which is why diameter changes are rarely used as a way to fine-tune an installed fan's output.