A 20-inch fan generally moves more air than an 18-inch fan, but the real difference comes down to motor power, blade pitch, and CFM, not just diameter.
Choosing between an 18 inch fan vs 20 inch fan comes down to one honest trade: the smaller model wins on footprint and fit, while the larger one typically wins on airflow and coverage. The catch is that blade diameter alone tells you almost nothing about performance. An 18-inch fan with a strong motor can push more air than a weak 20-inch model, so the spec sheet matters more than the number stenciled on the guard.
How Much Difference Does 2 Inches Make?
The extra blade span on a 20-inch fan creates a larger swept area, which lets it move more cubic feet of air per minute at the same motor speed. Published examples back this up. That is more than double the airflow from a two-inch increase.
But the gap narrows fast when motors differ.
Airflow, Power, and Noise: The Specs That Matter
Comparing fans by diameter only is the most common mistake buyers make. The numbers that decide performance are CFM (cubic feet per minute), RPM, wattage, and sound level. Here is how real 18-inch and 20-inch models stack up across published specifications.
| Fan Model | Blade Size | Airflow / Power |
|---|---|---|
| Patterson Truck Cooling Fan | 18 inch | 3,800 CFM, 1/2 HP, 1,725 RPM |
| McMaster High-Output Fan | 18 inch | 1,800 CFM |
| Hurricane Pro High Velocity | 20 inch | 4,500 CFM, 140 W |
| McMaster Industrial Fan | 20 inch | 4,100 CFM, 1,075 RPM |
| Canary Exhaust Fan | 20 inch | 3,440 CFM exhaust airflow |
| Schaefer Versa-Kool VK20 | 20 inch | 1,725 max RPM, circulation use |
Notice the spread. A 20-inch Hurricane Pro hits 4,500 CFM, but a Schaefer VK20 of the same diameter focuses on circulation at lower volumes. The 18-inch Patterson actually outflows both of those 20-inch units. Voltage and plug type matter too — these fans come in 120V AC, 240V, and even 115/208-230V variants, so match the model to your outlet and mounting setup before ordering.
Which Size Fits Your Space?
Physical footprint is the other half of the decision, and retail “size” labels rarely tell the whole story. A label reading “20-inch” usually refers to blade diameter, not the full housing. The Schaefer VK20 measures 23 x 23 x 18 inches as a packaged unit, while the Canary 20-inch exhaust fan is 17 inches wide and 25 inches tall. Guard design, motor housing, and mounting brackets shift the real dimensions substantially.
An 18-inch fan is the better pick for tight quarters, wall mounting between studs, or spaces where you do not need gale-force output. A 20-inch fan earns its larger footprint when you want whole-garage circulation, greenhouse air movement, or high-velocity cooling from across the room. For heavier mounted units, check the manufacturer’s documentation on wall structure and clearance — these fans are heavy, and a proper guard installation is a safety issue, not an accessory.
If you are comparing specific models for your garage or workshop, our curated list of tested 18-inch fan picks breaks down real-world performance across leading brands.
FAQs
Is a 20-inch fan always louder than an 18-inch fan?
Not necessarily. Sound level depends on motor design, blade pitch, and RPM, not diameter alone. Check the decibel rating on the specific spec sheet rather than assuming the bigger blade means more noise.
Can a 20-inch fan replace an 18-inch window or wall mount?
Only if the mounting frame and opening fit. A 20-inch fan’s housing is often wider and deeper than an 18-inch model from the same brand, so the cutout or bracket must accommodate the larger frame. Measure your existing opening and check the fan’s full dimensions, not just the blade, before ordering.
What does CFM mean when comparing fans?
CFM stands for cubic feet per minute, measuring how much air the fan moves. Higher CFM means more airflow and wider coverage. It is the single most useful spec for comparing fans of different sizes, because it accounts for motor strength and blade design that diameter alone ignores.
References & Sources
- McMaster-Carr. “Fans by Overall Height — 18 Inch.” Lists airflow and voltage specs for 18-inch fan models.
