Fan Affinity Laws Calculator
Scale a fan performance point for changes in rotational speed and geometrically similar diameter using ideal fan affinity laws.
YOUR CALCULATION
Result
—
Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Form speed and diameter ratios, then apply the fan similarity exponents to the entered baseline flow, pressure and power.
Q₂/Q₁=(N₂/N₁)(D₂/D₁)³; Δp₂/Δp₁=(N₂/N₁)²(D₂/D₁)²; P₂/P₁=(N₂/N₁)³(D₂/D₁)⁵
Worked example
At unchanged diameter and 75% speed, predicted flow is 75%, pressure is 56.25% and shaft power is 42.1875% of baseline.
Assumptions and input details
Similarity-law estimate for geometrically similar fans with similar flow, constant fluid density and comparable efficiency. A real fan/system operating point comes from the intersection of fan and system curves; motor, drive, Reynolds, compressibility, stall and static-pressure effects can invalidate simple scaling. Check manufacturer curves and ratings.
References: AMCA: derivation of the fan laws · US DOE: variable-speed fan power relationship
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
SI prefixes are explicit: for example, milli means one thousandth, micro one millionth and kilo one thousand. Changing a unit changes the interpretation of the entered number.
One revolution is 2π radians or 360 degrees. One minute is 60 seconds, so 1 rpm = 2π/60 rad/s. Rotational frequency in hertz counts revolutions per second.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
Convert entered replacement airflow to air changes per hour and model the fraction of original room air remaining under perfect mixing.
Convert volumetric flow and average speed in a full circular pipe. Calculate mass flow and the time to fill an entered volume.
Compare input energy with useful output over the same time. Find average input/output power, energy loss and conversion efficiency.
Reserved advertising space.