How to Measure Vac Strength: CFM and Water Lift

CFM and water lift are the key to understanding how powerful a vacuum is. These two specifications indicate the vac strength which helps you determine whether they are suitable for the job.

Note that not all vacuum types have a water lift rating, generally it is only wet/dry shop vacs that have this characteristic. That is because a regular household vacuum is usually only suitable for dry conditions and is not heavy duty enough for a handyman.

So, if you are browsing for a new vac and there is no mention of a water lift then chances are that model is meant for daily household chores instead of use in a workshop or professional use.

Already filtered for the best professional shop vacs? This is how you can determine the vac strength and find the right power for the task at hand.

Table of Contents

CFM vs. Water Lift

WHAT IS CFM

CFM stands for Cubic Feet per Minute and is the quantification of the airflow in a vac unit. In other words, it describes how strong the air suction of a vacuum is.

Basically, the higher the CFM value, the greater the suction of the unit. Household vacuums are generally in the 50 – 100 CFM range while the best shop vacs start around 125 CFM.

One thing to note about the CFM rating is that is measured without a hose or any other type of connection. In practice, this means that the maximum CFM when in use is actually lower than the CFM rating in the product description.

Keep this mind when you are deciding which model is the most appropriate and consider getting a unit with extra power if the vac strength is important.

WHAT IS WATER LIFT

Similar to CFM, water lift also measures the suction power of a vacuum but it does not quantify the suction of air. water lift quantifies how much water the vac can lift and is expressed in inches.

This is also referred to as the static pressure or the seal suction test. Seal suction is a strong clue to the difference between CFM and water lift – when ‘lifting water’ there is no air which creates a ‘sealed suction’.

The actual test goes as follows:

A vacuum is completely sealed off and its hose placed inside a tube of water. How high the hose can pull the water inside the tube is quantified as its water lift.

Household vacuums will have a water lift of about 35 – 50 with the more powerful units going as high as 85 inches. For professional shop vacs look for a water lift rating of 85 -125 inches.

Differences Between CFM & Water Lift

WHY ARE THESE DIFFERENCES IMPORTANT

Now, from the above explanations it might seem as though CFM and water lift are very similar specifications but you cannot use them interchangeably. Also, only knowing one of these values does not give you a full picture of the vac strength.

A strong vacuum has both a high CFM and water lift rating. These two values should be balanced because if one of the values is low you will definitely notice it in the performance.

A simpler way to think about CFM and water lift is as air flow and suction. Without both elements the vac strength lowers.

The actual suction (water lift) of a vacuum should remain unchanged because it is not affected by the airflow but the airflow does change. The airflow, or CFM, of a vacuum is affected by the filter.

The cleaner the filter the better the performance so naturally a filter that has not been changed or cleaned for a long while lowers the CFM. That is why a good water lift is needed; it compensates for an older or unclean filter.

At the same time, a vacuum with a strong airflow but poor suction won’t be useful when cleaning up a lot of debris. The vacuum will be able to move the debris around but little will actually end up inside the collection bag.

Bonus Tip

BOOSTING CFM

There is a simple hack that can get you greater CFM from the same vacuum but it will depend on the model. More advanced and professional vacs might have the ability to join together two vacuum units which should increase the CFM.

To summarize, the most important thing to remember when shopping for a vacuum, specifically a shop vac, is that the CFM and water lift should be balanced. This ensures the best performance.