Glossary
PSI (Pounds Per Square Inch)
Definition
PSI, pounds per square inch, measures the pressure a washer produces - the force of the water - and is only half of what determines real cleaning power.
High psi cuts and strips, which is useful on hard flatwork and dangerous on siding, shingles, and wood. Many beginners chase a big psi number and then damage surfaces that should have been soft washed instead.
Pressure works with flow. A machine with high psi but low flow rinses slowly, while balanced psi and gpm clean faster and gentler. For most exterior work, the right answer is the lowest pressure that gets the job done, with chemistry doing the heavy lifting.