Fan Sizes, Speeds, and PWM Control

Fan Sizes, Speeds, and PWM Control

Computer fans come in standard sizes, with 120mm and 140mm being the most common. Smaller fans like 80mm and 92mm are used in compact cases and older systems. Larger fans like 200mm are used in some cases for quiet airflow. The size determines how much air the fan can move at a given speed. Larger fans can move more air while running slower and quieter.

There are two main performance metrics for fans: airflow and static pressure. Airflow, measured in CFM or cubic feet per minute, tells you how much air the fan can move in free space. Static pressure, measured in mmH2O, tells you how well the fan can push air through resistance like radiator fins or dust filters. Radiator fans need high static pressure. Case fans need high airflow.

Fan speed is measured in RPM, or revolutions per minute. A typical 120mm fan runs at 800 to 2000 RPM. Higher RPM means more airflow but also more noise. Fan noise is measured in decibels, and it increases exponentially with speed. A fan at 2000 RPM is much more than twice as loud as the same fan at 1000 RPM. This is why larger, slower fans are preferred for quiet builds.

PWM, or Pulse Width Modulation, is the standard method for controlling fan speed. A PWM fan has four pins: power, ground, tachometer for speed monitoring, and a PWM signal wire. The motherboard sends a pulsed signal on the PWM wire, and the fan adjusts its speed based on the duty cycle of that signal. A 50 percent duty cycle means the fan runs at about half speed. PWM allows for precise, continuous speed control.

Three-pin fans use voltage control instead of PWM. The motherboard varies the voltage to change the fan speed. Voltage control is less precise than PWM and does not work well at very low speeds. Most modern motherboards support both types, but PWM is preferred. When buying fans, look for PWM models with a wide speed range, ideally down to 0 RPM for silent operation at idle.