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Understanding Micron Ratings

De-mystifying the measurement scale that determines exactly what your filter is capable of blocking.

What Exactly is a Micron?

A micron (short for micrometer) is a unit of measurement equal to one-millionth of a meter. To put that into perspective:

  • A single grain of beach sand is about 100 to 1,000 microns.
  • A human hair is roughly 70 microns thick.
  • A red blood cell is about 8 microns across.
  • Most bacteria are between 0.5 and 2 microns in diameter.

When a water filter is advertised as a "1 Micron Filter," it means the microscopic pores acting as a net are 1 micrometer wide. Anything larger than 1 micron will be trapped, while anything smaller will pass through.

Nominal vs Absolute Ratings

This is the biggest trick in water filter marketing. You must know the difference before buying:

Nominal Rating

A "Nominal 1 Micron" rating means the filter will catch roughly 85% of particles that are 1 micron or larger. Because the pores are irregularly shaped, a fair amount of 1-micron particles will still slip through.

Absolute Rating

An "Absolute 1 Micron" rating is a strict scientific standard. It means the filter has been laboratory certified to trap 99.9% of all particles 1 micron or larger. If you are dealing with dangerous pathogens, you must use an Absolute rated filter.

Standard Ratings by Technology

Standard Sediment (PP)
5 to 10 Microns. Catches visible rust and mud.
Carbon Block (CTO)
0.5 to 1 Micron. Catches chlorine and microplastics.
Ultrafiltration (UF)
0.01 Micron. Catches heavy bacteria and colloidal clay.
Reverse Osmosis (RO)
0.0001 Micron. Catches everything, including dissolved heavy metals and microscopic viruses.

Read Next:

RO Membranes Deep Dive → Sediment Filters →