An ISO 4406 code tells you how dirty the oil in a hydraulic system really is. Read the code, compare it against the target for the most sensitive component in the circuit, and size the filter from that target. Guessing at a micron number without a target code is how circuits end up with the wrong element.
Most filter mistakes start here. A micron number gets ordered because it was the number on the old element, and nobody checks whether the oil in the circuit was ever at the cleanliness the pump and valves needed. This guide explains how the code is built, what targets look like, and how to work backwards from a target code to a filter.
What the three numbers count
ISO 4406:1999 expresses cleanliness as three numbers separated by slashes, for example 19/17/14. Each number is a code for the particle count per millilitre at one size threshold:
| Position | Size threshold | What it picks up |
|---|---|---|
| First | Over 4 µm | Silt-sized particles, the fraction that settles in clearances and drives silting wear |
| Second | Over 6 µm | The mid range, where much of the damage-causing population sits |
| Third | Over 14 µm | Coarse particles, usually ingested dirt or wear debris |
The numbers are codes on a doubling scale. Each step up roughly doubles the particle count at that size:
| Code | Particles per millilitre |
|---|---|
| 14 | 80 to 160 |
| 16 | 320 to 640 |
| 18 | 1,300 to 2,500 |
| 20 | 5,000 to 10,000 |
| 21 | 10,000 to 20,000 |
| 22 | 20,000 to 40,000 |
| 23 | 40,000 to 80,000 |
| 24 | 80,000 to 160,000 |
So 19/17/14 means somewhere between 2,500 and 5,000 particles over 4 µm per millilitre, 640 to 1,300 over 6 µm, and 80 to 160 over 14 µm. The code is a range, not a count, which is why two laboratories reporting 19/17/14 can have measured different actual numbers. Ranges above are per ISO 4406:1999.
Two-number codes are a different standard
If a machine file or an old oil report shows a two-number code such as 20/17, that is the superseded ISO 4406:1987 format, which reported only two size ranges. It cannot be converted by adding a third number. When you find one, treat it as a historical reading and take a fresh sample rather than assuming the oil is still at that level. Anything quoted today should be the three-number code from ISO 4406:1999.
What target the circuit should hold
The target comes from the component that is most sensitive to particles, not from the pump size or the tank volume. Published targets vary between sources, so treat this table as a starting point and confirm against the manufacturer’s own recommendation for the exact component:
| Circuit | Common target | Notes |
|---|---|---|
| Gear pump, industrial | 19/17/14 | Tolerates more contamination than pressure-compensated designs |
| Vane pump | 18/16/13 | One step tighter than gear |
| Piston pump, swashplate | 17/15/12 | Clearances and speed make it less forgiving |
| Servo or proportional valve | 16/14/11 | The valve sets the target, not the pump |
| Mobile equipment, dirty duty | Per OEM | Often a compromise between cleanliness and cost of ownership |
From a target code to a filter
Once the target code is fixed, the filter becomes a means to an end. Three decisions follow:
- Where to filter. Protect the sensitive component at its inlet: a pressure-line filter on the pump outlet for servo and proportional circuits, a return-line filter for general industrial duty, and only a coarse strainer on the suction side.
- What rating to ask for. Convert the target into a micron rating and an efficiency. The efficiency part is the beta ratio: a glass-fibre element rated beta 200 at its micron point removes about 99.5 percent of particles at or above that size in one pass, and beta 1000 about 99.9 percent. Servo and proportional circuits justify the higher figure; standard return duty works at beta 200. The method is set out in our guide to micron ratings, beta ratios and bypass valves.
- Which media. With MP Filtri coding, A is glass fibre with an absolute rating (A03, A06, A10), P is impregnated cellulose with a nominal rating (P10, P25), and M is wire mesh for coarse duty (M60, M90). Match the media letter to the circuit: A for servo and proportional valves, P for standard return lines, M for suction strainers only.
One rule carries most of the weight: the tighter the target code, the lower the micron number and the more likely you want glass fibre (A) rather than cellulose (P). A P10 element in a circuit specified for 16/14/11 is unlikely to hold the target, however well the element itself is made.
How to find out where the oil stands now
A target code is only useful against a measurement. Two routes cover most machines:
- Send a sample for particle counting. Ask the laboratory to report to ISO 4406:1999 in three-number format. Sample from a running system at a representative point; a sample drawn from the tank bottom reads the sludge, not the oil the pump sees.
- Fit or read a clogging indicator. MP Filtri housings take VST visual and VSP electric indicators that signal when the element reaches its change point. An indicator tells you the filter is loaded; it does not tell you the cleanliness of the oil in the tank. On a servo circuit, wire the electric version so the machine alarms or halts rather than running on bypassed oil.
The two readings together, the oil’s code and the filter’s state, are what a contamination-control routine actually runs on.
Where the code gets misread
Four patterns account for most of the confusion:
- Comparing codes of different vintages. A two-number 1987-format reading sitting next to a three-number 1999 reading looks like contamination doubled. It did not; the formats differ.
- Treating the first number as the whole story. The 14 µm count can look acceptable while the 4 µm silt fraction is what wears the valve.
- Chasing the code with a finer element alone. Pressure drop, cold-start bypass and suction starvation all get worse. Fix the target first, then size the element.
- Assuming a new fill is clean. Oil in storage picks up particles, so new oil is not delivered at target cleanliness. Filter it on the way in, or the first hours of running undo the change.
Sourcing filters against a target code
We supply MP Filtri housings, elements and clogging indicators, and cross-reference equivalent elements from Hydac, Parker, Donaldson, Pall and Mahle. Send the target code, the component list or the existing element code, and we will confirm media, micron rating and beta ratio against the current official datasheet before anything is ordered. Every element is original and new, bought through the manufacturer or established channels, with procurement documentation for verification on arrival.
- Browse the MP Filtri range we carry.
- Compare the wider hydraulic filter range across all brands.
- Read more selection guides, including the element cross-reference guide.
- Or start from the complete brand directory.
- Send the target code straight to the inquiry form.
Cleanliness codes quoted are per ISO 4406:1999. Target values are typical published figures; confirm ratings, beta values and micron ratings against the current official component documentation before ordering.