Why does each ISO 4406 code number double the particle count?

ISO 4406 uses a logarithmic scale, so each range number represents roughly double the particle concentration of the number below it. That is why a one-number reduction removes about half the particles in that size band and is a large, real improvement in cleanliness.

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ISO 4406 Code Chart: Particle Counts Per Range Number

Q: What ISO 4406 code is clean enough for diesel?

The Worldwide Fuel Charter and manufacturers such as Caterpillar recommend 18/16/13 at the point diesel is dispensed, while modern high-pressure common-rail injection commonly targets around 12/9/6 at the injector. Always confirm the requirement in your engine or injection-system manufacturer's manual.

Every ISO 4406 range number and the band of particles per millilitre it stands for — plus how to read a three-number code and a worked example. Bookmark it as a quick reference.

In ISO 4406, each range number stands for a band of particles per millilitre, and each step up the scale roughly doubles the count. For example, code 18 means 1,300–2,500 particles/ml, code 16 means 320–640, and code 13 means 40–80. A three-number code such as 18/16/13 gives the range codes for particles ≥4 µm, ≥6 µm and ≥14 µm, in that order — lower is cleaner.

The full ISO 4406 range-number chart

ISO 4406 assigns a range number (also called a range code) to a fluid based on how many particles per millilitre fall at or above each measured size. The table below lists each range number from 6 to 24 with the particle-count band it represents. The count is stated as "more than" the lower bound "up to and including" the upper bound, per millilitre of fluid.

ISO 4406 code (range number)	More than (particles/ml)	Up to and including (particles/ml)
24	80,000	160,000
23	40,000	80,000
22	20,000	40,000
21	10,000	20,000
20	5,000	10,000
19	2,500	5,000
18	1,300	2,500
17	640	1,300
16	320	640
15	160	320
14	80	160
13	40	80
12	20	40
11	10	20
10	5	10
9	2.5	5
8	1.3	2.5
7	0.64	1.3
6	0.32	0.64

Source: the range-number allocation defined in ISO 4406:2021. The full standard extends below code 6 and above code 24; the range above covers the codes seen in real diesel and hydraulic cleanliness reporting. Values are per millilitre.

How to read a three-number ISO 4406 code

An ISO 4406 result is written as three range numbers separated by slashes — for example 18/16/13. Each position corresponds to a particle size, always reported smallest first:

First number — particles ≥4 µm(c): the smallest, most numerous particles per millilitre.
Second number — particles ≥6 µm(c): a subset of the above, at 6 µm and larger.
Third number — particles ≥14 µm(c): the larger, more damaging particles at 14 µm and larger.

The "(c)" means the particle counter was calibrated to the ISO 11171 reference, so a "4 µm(c)" particle means the same thing on any calibrated instrument. Because each code counts particles at or above that size, the first number is always the highest and the third the lowest.

Worked example. A diesel sample contains 4,500 particles ≥4 µm, 1,100 particles ≥6 µm and 110 particles ≥14 µm, all per millilitre. From the chart: 4,500 falls in the 2,500–5,000 band → code 19; 1,100 falls in 640–1,300 → code 17; 110 falls in 80–160 → code 14. The ISO 4406 code is 19/17/14.

Why the scale is logarithmic

Each range number represents roughly double the particle concentration of the number below it. That is why the chart's bands double at every step (…20–40, 40–80, 80–160…). The practical consequence: dropping any number by one removes about half the particles in that size band, so even a single-digit improvement is a large, real gain in cleanliness — and a code that looks "only a few numbers higher" can hold many times more contamination.

What code is clean enough for diesel?

Cleanliness targets depend on the injection system. The Worldwide Fuel Charter and manufacturers such as Caterpillar recommend 18/16/13 at the point fuel is dispensed into the machine, while modern high-pressure common-rail injection commonly targets around 12/9/6 at the injector — roughly 64 times cleaner at 4 µm. Always confirm the specific requirement in your engine or injection-system manufacturer's manual. For the fully-cited detail, see the ISO 4406 & modern engine requirements guide and acceptable ISO 4406 levels for diesel.

From a code on a chart to a live reading

A chart tells you what a code means; it does not tell you your fuel's code right now. Stored diesel drifts as water, dust and ageing push the count up, so a number from last month's lab report may already be wrong. Trendfuel measures the ISO 4406 code continuously — laser particle counters across a 4–70 µm range streaming to a secure cloud dashboard with sub-three-second latency, with email and SMS alarms — while its strategic partner DieselPure cleans the fuel with single-pass SAE J1488_201010 filtration, routinely taking stored mining diesel from 22/20/14 to 9/6/0. See it on the ISO 4406 monitoring page or watch the live data.

Common questions

The ISO 4406 chart, answered.

What is the ISO 4406 code chart?

The ISO 4406 code chart maps each range number to a band of particles per millilitre. For example, code 18 means 1,300–2,500 particles/ml of that size and code 16 means 320–640. Each step up the scale roughly doubles the count because the scale is logarithmic.

How do you read an ISO 4406 code like 18/16/13?

The three numbers are the range codes for particles per millilitre at ≥4 µm, ≥6 µm and ≥14 µm, in that order. For 18/16/13 that is up to 2,500 particles at ≥4 µm (code 18), up to 640 at ≥6 µm (code 16) and up to 80 at ≥14 µm (code 13). Lower numbers are cleaner.

Why does each code number double the particle count?

ISO 4406 uses a logarithmic scale, so each range number represents roughly double the particle concentration of the number below it. A one-number reduction removes about half the particles in that size band, so it is a large, real improvement in cleanliness.

What ISO 4406 code is clean enough for diesel?

The Worldwide Fuel Charter and makers such as Caterpillar recommend 18/16/13 at the dispensing point, while modern common-rail injection commonly targets around 12/9/6 at the injector. Always confirm the requirement in your engine or injection-system manufacturer's manual.

Does ISO 4406 use a 1 ml or 100 ml basis?

The range-number chart is defined per millilitre. Some particle counters report the underlying counts per 100 ml before converting to the ISO 4406 range code, but the standardised code itself is based on particles per millilitre.

See your real ISO 4406 code — live.

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