Where the number comes from
The pendulum tester was developed in the United Kingdom for road surfaces and adopted worldwide for pedestrian floors. A weighted arm with a rubber slider on its foot is released from horizontal. As it swings through the bottom of its arc the slider drags across a wetted patch of floor for a fixed length, and the friction it meets slows the arm. A pointer records how high the arm swings on the far side. A frictionless surface would let the arm swing to the same height it started from and read zero. A very grippy surface stops it short and reads high. The scale is calibrated so that the reading is called the British Pendulum Number, or BPN.
What the test does and does not measure
The BPN is a measure of the friction between a specific rubber and a specific surface, wet, under a specific load and speed. It is not a general property of the material; it is a property of the surface in its condition at that moment. That is exactly what makes it useful for installed floors. The same tile can read 50 when new and 28 after five years of polish and detergent film, and the difference is real.
Why the slider matters
Two slider rubbers are used. Slider 96 is the harder rubber representing shod feet, and P ratings are defined from Slider 96 results. Slider 55 is softer and represents bare feet, used for pool surrounds, change rooms and showers. A report should always say which slider was used, because the numbers are not interchangeable.
How readings become a result
AS 4663:2013 requires conditioning swings to bed the slider in, then five recorded swings on the same wetted spot. The five are averaged to give the mean BPN for that location. If readings spread widely, the technician looks for a reason, such as uneven wear or contamination, and may relocate slightly or record the spread.
From BPN to P class
AS 4586:2013 defines the classes. A mean BPN above 54 is P5. From 45 to 54 is P4. From 35 to 44 is P3. From 25 to 34 is P2. From 12 to 24 is P1, and below 12 is P0. Whether a class is acceptable depends on the location under HB 198:2014: P3 meets the recommendation for a covered entry that gets wet and misses it for a commercial kitchen.
What makes the number fall
Polishing by foot traffic, which rounds off the micro-texture that grips a wet shoe. Sealers and coatings applied for appearance that fill the texture in. Detergent film that dries on the surface and turns slick when re-wetted. Fine dust or flour that lubricates the surface. And on external surfaces, algae and organic growth.
What makes it rise
Removing the film with the right cleaning chemistry. Etching treatments that recreate micro-texture in tile and stone. Anti-slip coatings that add texture with aggregate. Honing polished stone back to a matt finish. All of these show up directly in a retest, which is how the improvement is proven.