Beta Ratio Calculator
A beta ratio compares how many particles of one specific size go into a filter with how many come out. It is the only honest way to state filter efficiency — and it is meaningless without the particle size it refers to.
For every 1,000 particles of 0.3 µm entering the filter, about 10 get through.
βx = particles of size x upstream ÷ particles of size x downstream. Efficiency η = (1 − 1/βx) × 100.
The particle size is part of the rating, not a detail. β75 at 5 µm and β75 at 0.3 µm describe two completely different filters.
Beta ratio to efficiency
| Beta ratio | Efficiency | Particles through per 1,000 |
|---|---|---|
| β2 | 50.0% | 500 |
| β10 | 90.0% | 100 |
| β20 | 95.0% | 50 |
| β75 | 98.7% | 13 |
| β200 | 99.5% | 5.0 |
| β1000 | 99.90% | 1.0 |
| β5000 | 99.98% | fewer than 1 |
Why beta ratio and not just a percentage
Efficiency is size-dependent. The same element can be 99.9% at 5 µm and 50% at 0.3 µm — both figures are true, and only one of them sounds good in a brochure. A beta ratio forces the size to be stated alongside the number. A rating without a size is marketing, not a specification.
Absolute vs nominal
- Absolute — efficiency above 98.7%, which is β ≈ 75 or higher.
- Nominal — efficiency between 50% and 90%, which is β between 2 and 10.
Nominal ratings are far less consistent between manufacturers: two elements both described as "10 micron nominal" can behave very differently on the same machine.
Typical micron ranges in a compressor
| Air intake filter | 1–25 µm |
| Oil filter | 1–25 µm |
| Air/oil separator | 0.3–3 µm (high-efficiency designs down to 0.01 µm) |
A smaller micron rating filters better but blocks faster — every rating is a trade with pressure drop.