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CFM, water lift and air watts, in plain words
Updated
Three numbers describe how a shop vac cleans. CFM is how much air it moves. Water lift, also called sealed pressure, is how hard it pulls. Air watts combine the two. Most listings skip all three and print peak HP instead. Of the seven vacs here, three state at least one of them.
What each number means
- CFM (cubic feet per minute). Shop-Vac's FAQ calls it the maximum airflow the vac can achieve at the largest opening, and says it is best for picking up lighter materials such as wood chips and dust.
- Water lift or sealed pressure (inches). Shop-Vac describes it as how far up a glass tube the motor will lift a column of water. Its FAQ says numbers above 75 are at the high end for utility vacuums. Its product pages add that maximum suction happens at zero airflow, and suction drops as airflow rises.
- Air watts. Shop-Vac calls this the amount of power that is converted into airflow at the inlet. It is a function of suction and airflow together.
So the two headline numbers trade off. A vac cannot deliver its top CFM and its top water lift at the same moment. Light, bulky debris wants airflow. Water and heavy grit want lift.
Which listings state them
| Model | CFM | Water lift | Air watts | Real watts |
|---|---|---|---|---|
| CRAFTSMAN 16 gal | Not on the listing | Not on the listing | Not on the listing | 1,440 W |
| Vacmaster 12 gal | 140 | 60 in | 360 | 1,416 W |
| Vacmaster Beast 5 gal | Not on the listing | 82 in | 320 | 1,320 W |
| DEWALT 6 gal | Not on the listing | Not on the listing | Not on the listing | 864 W |
| Stanley 6 gal | Not on the listing | Not on the listing | Not on the listing | 1,200 W |
| Vacmaster 5 gal steel | 80 | 66 in | 135 | 960 W |
| Armor All 2.5 gal | Not on the listing | Not on the listing | Not on the listing | 720 W |
- Vacmaster 12 gal60 in
- Vacmaster Beast 5 gal82 in
- Vacmaster 5 gal steel66 in
The Vacmaster Beast 5 gallon states 82 inches, past the 75 that Shop-Vac calls the high end. The Vacmaster 12 gallon states 60 inches but 140 CFM through a 2-1/2 inch hose. One leans toward pull, the other toward airflow.
How the numbers are measured matters
Vacmaster's maker page for the 12 gallon model says its CFM, water lift and air watts were tested to an ASTM standard at the end of the included hose. Shop-Vac's product page says its figures were ASTM tested with a 1.50 inch hose. A number taken at the motor with no hose would be higher, and listings rarely say which they are quoting. Compare numbers within one brand with more trust than across brands.
Real use is lower again. Shop-Vac notes that the hose type, size and length, the tool and the filter all set the load, and that a dirty filter can drastically cut suction and airflow. Clean the filter before you blame the motor.
When a listing has none of the three
Use what is there. The amps give you the power draw. Run them through the peak HP calculator. The hose diameter tells you what debris it is built for. Owner feedback tells you whether people find the suction strong. It is less than you deserve from a listing, which is the point of this site.
Run any listing through the peak HP calculator, match a vac to your job with the job checker, or compare all seven picks on the roundup page.