PC Airflow Calculator
Intake fans pull cool air into your PC case, and exhaust fans push hot air out. Enter each fan's CFM to see which side wins, and whether your case runs positive, negative, or balanced pressure.
Enter Your Case Fans
Add a row for every case fan and radiator fan. Leave expected airflow at 100% unless a fan runs below full speed or sits behind a filter or radiator.
What Your Results Mean
- Rated CFM is the airflow printed on each fan's spec sheet.
- Expected CFM is what each fan really delivers once you count its speed and anything blocking it.
- Case pressure names the stronger side: positive when intake is bigger, negative when exhaust is bigger, and balanced when the two are close.
Positive vs Negative Case Pressure
Case pressure compares the air entering your case with the air leaving it. Noctua's pressure guide sums up the trade-off: positive pressure cuts down on dust but can trap warm air, while negative pressure clears heat fast but pulls dust in through gaps.
Intel suggests a balanced setup that leans a little toward intake. That way, most air enters through your filtered fans instead of the open seams around the panels.
How Many Intake and Exhaust Fans You Need
Noctua's fan count advice uses five fans as a baseline for most builds: three front intakes, one rear exhaust, and one top exhaust. A budget build can get by with three, two in the front and one in the back.
More fans let each one spin slower for the same airflow, so the PC runs quieter. Noctua also says to use the biggest fan that fits each slot, since larger blades move more air at lower speeds.
Thinking of filling the bottom slots too? Our sister site tested whether bottom intake fans help in seven cases, including when they make things worse.
How the CFM Totals Are Calculated
Each fan's expected CFM is its rated CFM times its expected airflow percentage. The tool adds every intake fan, adds every exhaust fan, then subtracts exhaust from intake to get net airflow.
The intake-to-exhaust ratio is expected intake divided by expected exhaust, so a case with no exhaust fans has no ratio. When the two totals are within 5% of each other, the result reads balanced pressure.
Take the starting setup: two 50 CFM front intakes and one 50 CFM rear exhaust. That's 100 CFM in and 50 CFM out, for +50 CFM net, a 2.00 ratio, and positive pressure.
How Filters and Radiators Cut Airflow
The CFM on the box is the most air a fan can move at full speed. Dust filters, radiators, and solid front panels all cut that number in a real case. In our 3D model of the Corsair 4000D Airflow, the front dust filter held back 25% of the air coming in.
Fan speed matters too. Noctua's NF-A12x25 drops from 60.1 to 49.7 CFM when its top speed falls from 2,000 to 1,700 RPM, so airflow falls about in step with speed.
Leave expected airflow at 100% to compare ratings only. Lower it when you have a fan curve, a test result, or a careful guess for a known restriction. The tool never adds a hidden correction.
Static pressure is a different spec. It's how hard a single fan can push through a radiator or filter, not the balance of air in your case.
How to Find a Fan's CFM Rating
Find the model number on the fan's label, box, or order receipt, then look up that exact model on the maker's site. Fans that look alike can have very different top speeds and CFM ratings.
| Fan | Size | Top speed | Rated CFM |
|---|---|---|---|
| Arctic P12 PWM | 120 mm | 1,800 RPM | 56.3 |
| Noctua NF-A12x25 PWM | 120 mm | 2,000 RPM | 60.1 |
| Arctic P14 PWM | 140 mm | 1,700 RPM | 72.8 |
| Noctua NF-A14 PWM | 140 mm | 1,500 RPM | 82.5 |
Some makers list airflow in cubic meters per hour (m³/h) instead. Multiply m³/h by 0.589 to get CFM. Can't find your model at all? Use the closest fan in the table as a stand-in.
How to Check Your Airflow in the Real PC
First, make sure every fan blows the way you entered it. Hold a strip of tissue at the outside of each fan: an intake pulls it in, and an exhaust blows it away. On most fans, air comes out of the side with the sticker.
To check case pressure, hold the tissue at a gap or open vent with no fan. If it blows outward, the case runs positive. If it gets pulled in, the case runs negative.
Then run the same game or stress test before and after each change. Record CPU and graphics card temperatures, keep fan speeds repeatable, and change one thing at a time.
A fan curve guide helps you match fan speed to temperature, and our fan curve calculator works out the points for your parts.
Plan Your Whole Fan Layout
To place your parts and compare air paths around them, use the PC airflow simulator.
About to order fans? Here's how to test a fan layout before you buy, so you only pay for the fans that make a difference.