ISO 4406 vs ISO 4407: The Code vs the Counting Method

What is ISO 4406?

ISO 4406 is the international cleanliness coding standard. It takes the number of solid particles in a fluid and turns it into a compact, comparable three-number code such as 18/16/13, based on particle counts per millilitre at ≥4 µm, ≥6 µm and ≥14 µm. Lower numbers mean cleaner fuel, and because the scale is logarithmic, each step up roughly doubles the particle count in that band. The current edition is ISO 4406:2021. It says nothing about how you count the particles — only how to report the result. See what ISO 4406 is and the full ISO 4406 code chart.

What is ISO 4407?

ISO 4407 is a measurement method: "Hydraulic fluid power — Fluid contamination — Determination of particulate contamination by the counting method using an optical microscope." A fluid sample is drawn through a membrane filter that captures its particles, and those particles are then sized and counted on the membrane under an optical microscope — either manually or with image-analysis software. Particles of roughly ≥2 µm can be counted this way, though resolution and accuracy depend on the optics and the operator. The current edition is ISO 4407:2025. It produces particle counts; those counts are typically then expressed using the ISO 4406 code.

The key difference: a code vs a way to count

The confusion comes from the similar numbers, but the two standards sit at different steps of the same workflow. ISO 4407 (or another counting technique) measures the particles; ISO 4406 classifies the result into a code. They are not competing alternatives — a lab following ISO 4407 still reports its answer as an ISO 4406 code. The important practical point is that ISO 4407 microscope counting is a manual, periodic lab procedure, so the result describes the fluid as it was when the sample was taken, not as it is now.

Editions cited: ISO 4406:2021 (coding) and ISO 4407:2025 (optical-microscope counting method). Automatic particle counters are calibrated to ISO 11171, traceable to NIST reference material. Always work to the standard edition and method your quality process specifies.

How the counts are actually obtained today

ISO 4407 microscope counting is a recognised reference method, but most cleanliness numbers — in the lab and especially in the field — come from automatic optical or laser particle counters. Fluid flows through a sensing cell where a light source and detector size and count each particle, and the instrument converts the counts into the ISO 4406 code. Because these counters are calibrated to the ISO 11171 reference, a "4 µm(c)" particle means the same thing on any calibrated instrument, so results are comparable whether they came from a microscope or an automatic counter. For a fuller comparison of continuous versus periodic measurement, see IoT particle counting vs lab sampling.

Which applies to live diesel monitoring?

For stored diesel, the number that matters is the ISO 4406 code — and it matters continuously, because water, dust and ageing push the code up between samples. Trendfuel measures that code live with automatic laser particle counters across a 4–70 µm range, streaming to a secure cloud dashboard with sub-three-second latency and email and SMS alarms, while its strategic partner DieselPure cleans the fuel with single-pass SAE J1488_201010 filtration. ISO 4407 microscope counting still has a place for periodic, independent lab validation — but it can't watch fuel in real time. See the ISO 4406 monitoring solution or the live data.