How vacuum suction actually works

Why a vacuum with huge suction numbers can still clean poorly

A spec sheet with a big pascal figure looks like it should settle the question of which vacuum cleans better, but pascals describe a test condition, not a cleaning condition. Sealed suction is measured with the inlet completely blocked, so no air is moving at all. The number tells you how hard the motor can pull against a wall, which is a useful engineering figure but not a description of what happens when the nozzle is sitting on a rug with air flowing freely through it.

Once the inlet opens back up and air starts moving, the sealed pressure figure drops fast and a different set of factors takes over. On carpet, the dominant factor is agitation from a driven brush roll flicking dirt up out of the pile so airflow can carry it away. On a hard floor, agitation barely matters and the result comes down to airflow volume and how well the nozzle seals against the floor. A machine can have an enormous sealed suction rating and a worn or hair-bound brush roll, and it will still leave carpet looking dull, because the brush roll, not the pressure rating, was doing the work of loosening embedded dirt.

What each unit on a spec sheet is actually measuring

Three units show up on vacuum listings and they are not interchangeable. Pascals (Pa), and the older water lift measurement it replaced, describe sealed pressure: how much vacuum the motor can pull with airflow stopped. One inch of water lift is defined as approximately 249 Pa, so a spec sheet quoting either unit is describing the same physical quantity, just in a different scale. CFM (cubic feet per minute) describes airflow alone, with no reference to pressure at all: a fan can move a lot of air at low pressure, or a little air at high pressure, and CFM only tells you about the volume moving.

Air watts (AW) is the figure meant to combine the two into something closer to real-world cleaning performance. It is calculated from airflow and pressure together and is meant to represent the useful work being delivered at the cleaning head, not just at the motor. This is why air watts is the more common headline figure on cordless sticks and uprights, while sealed suction in pascals shows up more on robot vacuums, where the nozzle geometry and airflow path are fixed by the robot chassis rather than swappable attachments.

• Pascals (Pa) or water lift: sealed pressure, inlet blocked, no airflow
• CFM: airflow volume alone, no reference to pressure
• Air watts (AW): combined airflow and pressure at the cleaning head
• 1 inch of water lift is approximately 249 Pa

There is no conversion between pascals and air watts

Because pascals measure pressure with zero airflow and air watts is calculated from pressure and airflow together, the two figures describe physically different conditions and cannot be converted into each other with a formula. A chart that claims "X pascals equals Y air watts" is applying a made-up ratio, usually backed into from one specific machine, and that ratio does not hold for a different motor, fan design or nozzle. If a listing gives you sealed suction in Pa for one model and air watts for another, you cannot line them up on a single scale. Treat any site offering that conversion as guessing.

The practical takeaway is to compare like against like. Two robot vacuums both quoted in Pa can be reasonably compared to each other. Two cordless sticks both quoted in AW can be reasonably compared to each other. A robot quoted in Pa against a stick quoted in AW cannot be compared at all from the spec sheet, because they are not measuring the same thing.

Why carpet and hard floor reward different things

Carpet pile hides dirt down at the base of the fibres, below where airflow alone can reach it. A driven brush roll (also called a beater bar) spins fast enough to physically flick the pile open and knock embedded dirt loose so the airstream can then lift it into the machine. This is why a vacuum with a passive, non-driven brush roll, or a driven roll that has gone bald or has a broken belt, will consistently underperform on carpet even with strong sealed suction, and why cordless sticks intended mainly for hard floors often carry a visibly softer roller than upright models built for carpet.

Hard floor removes agitation from the equation almost entirely, since there is no pile to open up. What matters instead is airflow volume, reaching corners and edges, and how well the nozzle head seals against the floor surface. A worn or warped nozzle that no longer sits flush lets air leak in around the sides, which drops the effective airflow at the surface even if the motor itself is unchanged. This is also why the same machine often needs a different floor-head attachment for hard floor versus carpet: the seal and bristle stiffness the two surfaces reward are different.

Reading a spec sheet without over-trusting the headline number

Headline suction and air watt figures are almost always taken in the machine's highest power setting and with a specific attachment fitted, conditions that do not match how most owners actually run the vacuum day to day. There is no independent standard body requiring manufacturers to test a fixed way, so figures from different brands, and even different product lines from the same brand, are not guaranteed to be measured identically. Treat the number as a rough indicator of the motor's capability class rather than a precise prediction of cleaning outcome on any specific floor.

A more reliable read on whether a machine is actually cleaning well day to day comes from checking the parts that do the mechanical work: is the brush roll still driven and free of wrapped hair, is the belt (where fitted) still gripping rather than slipping, and is the filter clear enough that the motor is pulling full airflow rather than fighting a clog. Those checks tell you more about real performance than any single number on the box.