Hydraulics & Fluid Power Reference Library · Fluids, Filtration & Contamination
ISO 4406 Cleanliness Codes Explained
How to read a three-number ISO 4406 fluid cleanliness code such as 18/16/13, what each number measures, and how codes are used to set and check cleanliness targets.
Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.
What it is
ISO 4406 is a shorthand for the amount of solid particles in a fluid sample. The particles are counted, and each count is converted to a range (scale) number, giving a code like 18/16/13. The three numbers stand for particles at or above three sizes per millilitre of fluid. The scale is logarithmic: each step up roughly doubles the allowed particle count. So 19/17/14 is about twice as dirty as 18/16/13 in each size band, not slightly dirtier. Particle sizes are written in µm(c), which means the counter was calibrated per ISO 11171.
How to identify
- Lab report or portable counter: the code is usually given with the raw particle counts per mL for each size.
- Three numbers come from automatic particle counters. Two numbers with a dash in front (e.g. –/16/13) usually mean a microscope count, which cannot see the smallest size.
- Check the sample bottle label and sampling point. A code means little if the sample was taken badly (see "How to Take an Oil Sample Correctly").
What the three numbers mean
| Position | Particle size counted | What it tells you |
|---|---|---|
| First number | ≥ 4 µm(c) | Fine silt: drives wear, varnish and sticking of tight-clearance valves |
| Second number | ≥ 6 µm(c) | Small particles that cause wear in pumps and valves |
| Third number | ≥ 14 µm(c) | Larger particles: sudden failures, scoring, jammed spools |
Key points
- Lower is cleaner. One step lower roughly halves the particles in that band.
- Targets come from the component makers. Servo and proportional valves and high-pressure piston pumps need cleaner fluid than gear pumps and cylinders. The system's target is set by its most sensitive component, then adjusted for pressure and duty. Use the component manufacturers' published targets.
- New oil is not clean oil. New fluid from a drum or tote is often dirtier than the target for a modern system. Filter it before or during filling.
- Trend the results. A code that rises sample after sample shows that filters, breathers or seals are failing before damage is visible.
- Improving cleanliness usually means better filtration (beta ratio per ISO 16889), sealed reservoirs with desiccant breathers, and clean fill and transfer practices.
Common mistakes
- Reading 18/16/13 as "eighteen particles".
- Comparing a microscope count (two numbers) directly with an automatic count (three numbers).
- Taking samples from a reservoir drain or right after filling, which misrepresents what is in the system.
- Setting one cleanliness target for every machine in a plant.
Governing standards: ISO 4406 (coding the level of contamination by solid particles); ISO 11171 (calibration of automatic particle counters); ISO 4021 (sampling from operating systems); ISO 16889 (filter performance).
Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.