For estimation and sales-phase use. Use it to predict what a speed change does before you touch the blower. Not a substitute for the fan manufacturer's data, the motor nameplate or a licensed engineer's review.

Maintained by BuildSolver · Updated October 16, 2026

Calculator · Fan laws

Fan law calculator

Speed up a blower and airflow rises with speed, static pressure with the square and power with the cube. A fan moving 6,500 CFM at 1,000 RPM, 4 in. w.c. and 6.9 BHP moves 6,000 CFM at 923 RPM and needs 3.41 in. w.c. and 5.43 BHP. This page applies all three laws to a known point, finds the speed for a target airflow, and converts pressure and power between standard air and the air at your altitude and temperature. The static pressure calculator has a fan law mode for pressure alone. This one covers speed, airflow and power too, and adds the density correction.

Choose a new speed, a target airflow, or a change of altitude and air temperature.
Enter the airflow and speed you know, and the static pressure and brake horsepower at that point if you want them moved too. For a density change, enter the altitude, the inlet air temperature and the pressure or power to convert.
The result shows the new airflow or speed with the static pressure and brake horsepower, and a warning when the power rises.

One fan in one duct system. Pressure and power are optional, and a blank field skips that result. The form opens on a fan curve point from HVAC School. Replace it with your own readings.

What to calculate

Speed and airflow changes assume the same air density. The density choice changes pressure and power only, since airflow and speed stay the same.

Airflow at the known point, CFM

The airflow when you took the other readings, from a measurement or from the fan's table.

Speed at the known point, RPM

Read it with a tachometer, or take it from the fan's table for that point.

New speed, RPM

The new speed has to be between 0.25 and 2.00 times the known speed.

Static pressure at the known point, in. w.c. (optional)

The fan or system static pressure at the known airflow. In the density choice, this is the pressure you want converted. Blank skips the pressure result.

Brake horsepower at the known point, BHP (optional)

The shaft power at the known point, from the fan's table. Do not enter motor nameplate horsepower. Blank skips the power result.

Example result for the default inputs

Same fan, same duct system, same air density. Airflow moves down 7.7%.

Airflow at the new speed

6,000 CFM

Static pressure

3.41 in. w.c.

Brake horsepower

5.43 BHP

Calculation details
Known airflow	6,500 CFM
Known speed	1,000 RPM
Known static pressure	4 in. w.c.
Known brake horsepower	6.9 BHP
Speed and airflow ratio, new / known	0.9230
Pressure ratio, ratio squared	0.8519
Power ratio, ratio cubed	0.7863
New airflow	6,000 CFM
New speed	923 RPM
New static pressure	3.41 in. w.c.
New brake horsepower	5.43 BHP

A point beyond the airflow range in the fan's catalog is an extrapolation. Check the new point against the fan's published curve.

For estimation purposes only. Consult a licensed engineer for final designs.

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How the fan laws are calculated

Speed and airflow changes use the three fan laws for one fan in one fixed duct system at one air density. The density mode is a separate step that changes only pressure and power. Catalog tables are written at standard air, so a speed change from catalog numbers stays in standard air terms, and the density mode converts the answer to site conditions afterward.

Airflow and speed: CFM2 / CFM1 = RPM2 / RPM1Airflow is directly proportional to speed. The calculator reads the ratio from whichever pair you enter and applies it to the other quantities.
Static pressure: SP2 = SP1 x (CFM2 / CFM1)^2Pressure changes with the square of airflow or speed. This is the same relationship the static pressure calculator uses for a fixed duct system, and the function is shared.
Brake horsepower: BHP2 = BHP1 x (CFM2 / CFM1)^3Power changes with the cube of airflow or speed. Brake horsepower is the shaft power the fan needs. The motor's electrical input is higher because of motor losses, and a brake horsepower above the motor's nameplate rating means the motor is overloaded.
Speed for a target airflow: RPM2 = RPM1 x (CFM2 / CFM1)Shown when you enter the known speed in the airflow mode.
Density ratio: ratio = p(altitude) / p(sea level) x (70 + 459.67) / (T + 459.67)The altitude term is the ASHRAE standard atmosphere, p = 14.696 x (1 - 6.8754e-6 x altitude)^5.2559 psia. The temperature term is the ideal gas law on absolute temperature, with T in °F and 459.67 added to reach Rankine. Standard air is 70°F at sea level, where the ratio is exactly 1. Dry air only.
Density correction: P_site = P_catalog x ratio; BHP_site = BHP_catalog x ratio; CFM and RPM unchangedAerovent writes this as a factor, 0.075 / density, which is 1 / ratio. Going from site values to catalog values divides by the ratio instead of multiplying. At the default 5,280 ft and 70°F the ratio is 0.823, so a catalog pressure of 1.00 in. w.c. is 0.82 in. w.c. on site.

Worked examples from published fan data

The default point is a fan curve example from HVAC School: 6,500 CFM at 1,000 RPM, 4 in. w.c. and 6.9 BHP. At 6,000 CFM the source gives about 923 RPM, 3.4 in. w.c. and 5.4 BHP, and the calculator returns 923 RPM, 3.41 in. w.c. and 5.43 BHP. Eight cases below run through the same function the form uses. Inputs for the residential blower row and the Denver row are chosen for the example, with no product data behind them.

Fan law and density correction results for eight example cases
Case	Change	Airflow	Speed	Static pressure	Power
HVAC School, speed for a lower airflow	Airflow 1,000 to 900 CFM	1,000 to 900 CFM	1,100 to 990 RPM	Not entered	Not entered
HVAC School, filter drop at higher airflow	Airflow 1,000 to 1,200 CFM	1,000 to 1,200 CFM	Not entered	0.90 to 1.30 in. w.c.	Not entered
HVAC School, motor power at higher airflow	Airflow 1,000 to 1,200 CFM	1,000 to 1,200 CFM	Not entered	Not entered	0.206 to 0.356 BHP
HVAC School, fan curve point	Airflow 6,500 to 6,000 CFM	6,500 to 6,000 CFM	1,000 to 923 RPM	4.00 to 3.41 in. w.c.	6.9 to 5.43 BHP
Residential blower, example inputs	Speed 800 to 700 RPM	1,200 to 1,050 CFM	800 to 700 RPM	0.50 to 0.38 in. w.c.	0.25 to 0.167 BHP
Aerovent Example 3, site to catalog pressure	Site to catalog at 5,500 ft, 70°F, ratio 0.817	Unchanged	Unchanged	2.50 to 3.06 in. w.c.	Not entered
Aerovent Example 3, catalog to site power	Catalog to site at 5,500 ft, 70°F, ratio 0.817	Unchanged	Unchanged	Not entered	5.28 to 4.31 BHP
Denver, example inputs	Catalog to site at 5,280 ft, 70°F, ratio 0.823	Unchanged	Unchanged	1.00 to 0.82 in. w.c.	2 to 1.65 BHP

In the filter row, a higher airflow raises the drop across a component: HVAC School rounds 1.296 to about 1.3 in. w.c. Both Aerovent example 3 rows differ from the printed figures in the last digit. Aerovent rounds its altitude factor to 1.22 before using it, and this calculator carries the unrounded 1.2246, so site 2.5 in. w.c. becomes 3.06 in. w.c. at standard air against 3.05 printed.

If you read RPM with a tachometer, enter that reading. A speed tap on a blower motor does not always change the speed by the amount the tap number suggests, so the residential row is a worked example and not a rule for any motor.

Standards and data sources

HVAC School, The 3 Fan Laws and Fan Curve Charts (Tim De Stasio, May 17, 2021) - the three fan laws: airflow proportional to speed, static pressure with the square of airflow or speed, and horsepower with the cube. Its stated conditions are constant air density and constant system friction. It also notes the laws do not apply where automatic dampers adjust themselves.
Aerovent (Twin City Fan Companies), Fan Engineering FE-1600: Temperature & Altitude Effects on Fans (2018) - the density correction. Fan pressure and horsepower vary directly with the ratio of the inlet air density to the standard density, 0.075 lb/ft3 at 70°F and sea level, while fan airflow is not affected by density.
CaptiveAire, Designing Air Flow Systems - the system constant, S.P. = K x CFM squared, the basis for the pressure relationship in a fixed duct system.
ASHRAE Handbook, Fundamentals, Chapter 1, Equation 3 - the standard atmosphere pressure at altitude, the altitude term of the density ratio.

Limitations of this calculator

The fan laws hold for the same fan in the same duct system. A dirtier filter, a damper position change, a closed register or a new duct run changes the system, and a reading taken before the change no longer describes the system after it.
A blower that adjusts its own speed to hold airflow, such as a constant-airflow ECM, does not follow the speed relationships when the system changes. Take every reading from the same system in the same state, and read the speed from the equipment. Where automatic dampers move on their own, the fan laws do not apply either, as HVAC School notes.
The calculator moves along the system curve. It does not model the fan curve, stall regions, or whether the fan can reach the new point at all. The fan's published table and curve decide that.
A point outside the airflow range in the fan's catalog is an extrapolation. The calculator refuses speed or airflow changes below 0.25 or above 2.00 times the known value.
The density correction covers dry air, -40°F to 250°F and 0 to 10,000 ft. Speed and airflow changes assume constant density, so run the density mode as a separate step. Moisture and gases other than air change the density. Aerovent says to get the actual density in those cases.
The density mode changes pressure and power only. The mass of air the fan moves changes with density, so anything that depends on air mass, such as coil or burner capacity, is not covered here.
Pressure readings come from your manometer. See the static pressure calculator for where to take them and how supply and return readings add.
Brake horsepower is the fan's shaft power from its table. It is not the motor nameplate rating and not the electrical input. Check the motor nameplate, the amps and the wheel's maximum safe speed before running a fan faster.
This page does not size equipment or ducts and makes no claim of code compliance.

Common fan law mistakes

Expecting power to rise in step with airflow. Power goes with the cube. 10% more airflow needs 1.331 times the power, which is 33% more. At 1.20 times the airflow the power is 1.728 times the original. A motor that is fine at the old speed can overload at the new one.
Assuming a lower speed saves power in proportion. Slowing a fan to half speed does not halve the power. It cuts it to 12.5% of the original, while airflow falls to 50%. Verify the airflow is still enough for the job before counting the saving.
Applying the laws after the system changed. The pressure relationship describes one fixed duct system. If you swapped the filter, closed dampers or added a run between the two readings, the known point belongs to a different system.
Skipping the wheel's maximum speed. More speed is more stress on the wheel and bearings. The fan's catalog lists a maximum safe speed, and the calculator cannot see it.
Using motor horsepower as brake horsepower. The cube law applies to the power the fan takes at its shaft. Motor nameplate horsepower is a rating, not a reading. Take brake horsepower from the fan's table for the known point.
Forgetting density at altitude. A fan selected from a catalog at standard air makes less pressure on site at altitude. At 5,280 ft and 70°F the ratio is 0.823, so a catalog pressure of 1.00 in. w.c. is 0.82 in. w.c. at the same speed. Convert the site requirement to a catalog value before looking up a fan.

Common questions about the fan laws

What are the three fan laws?

For one fan in one duct system at one air density, airflow is proportional to speed, static pressure changes with the square of airflow, and brake horsepower changes with the cube. A 20% increase in airflow means 1.44 times the static pressure and 1.728 times the power.

How do I find the new CFM when I change blower speed?

Multiply the known airflow by the new speed divided by the old speed. A fan moving 1,000 CFM at 1,100 RPM moves 900 CFM at 990 RPM. Enter the known airflow, the known speed and the new speed in the first mode.

How do I find the RPM for a target airflow?

Use the second mode. Enter the airflow and speed from the known point and the airflow you want, and the calculator multiplies the known speed by the airflow ratio. Pressure and power at the new point come out of the same ratio.

Why does a small increase in airflow raise the power so much?

Power follows the cube of the airflow ratio. A 10% increase in airflow raises power 33%, since 1.1 cubed is 1.331. Check the motor nameplate and the amps before you raise the speed.

Do the fan laws work if I change the filter or close a damper?

Not across the change. The laws describe one fixed duct system, so the known point and the new point must be on the same system. After a filter or damper change, take a new reading and use that as the known point.

How does altitude change fan performance?

At the same speed the fan moves the same CFM, but the air is thinner, so pressure and power fall in proportion to the density. At 5,280 ft and 70°F the density is 0.0618 lb/ft3, a ratio of 0.823 against standard air, so a catalog pressure of 1.00 in. w.c. becomes 0.82 in. w.c.

How does air temperature change fan performance?

Hot air is thinner, so a fan at the same speed makes less pressure and takes less power. At 120°F and sea level the catalog factor is 1.09, so a pressure that must be 1.00 in. w.c. on site is 1.09 in. w.c. at standard air. Cold air does the reverse and can overload a motor.

What is brake horsepower and where do I find it?

Brake horsepower is the power the fan takes at its shaft. It is listed in the manufacturer's fan table or curve for each airflow and static pressure. It is not the motor nameplate rating, and it is lower than the electrical input to the motor.

How is this different from the static pressure calculator?

The static pressure calculator has a fan law mode for pressure only. This page covers the full set for a blower speed change, which adds airflow, speed and power, and it adds the density correction for altitude and temperature.

Is there a limit on how many times I can run this?

No. This calculator runs in your browser with no daily limit and no signup.

Used on the same job

Check the duct side after a speed change

A speed change moves the airflow, and the duct system takes the new airflow at 1.44 times the pressure when the airflow is 20% higher. Read the new total external static pressure with the static pressure calculator and compare it with the equipment rating, or ask the assistant to work through the airflow and duct check with you.

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