ISO 4406 vs NAS 1638: The Modern Code vs the Legacy Standard

Q: What is the difference between ISO 4406 and NAS 1638?

ISO 4406 is the modern global cleanliness code: it uses cumulative particle counts at 4, 6 and 14 µm and reports a three-number code such as 18/16/13. NAS 1638 is an older US aerospace standard that uses differential counts across five size bands (5–15, 15–25, 25–50, 50–100 and over 100 µm) and reports a single class from 00 (cleanest) to 12 (dirtiest).

Q: Can you convert NAS 1638 to ISO 4406?

Not exactly. Because the two systems count particles at different sizes and in different ways (cumulative vs differential, per millilitre vs per 100 millilitres), there is no precise mathematical conversion. Approximation charts exist to cross-reference the two, but they are estimates, not exact equivalents.

Q: Which standard should I use for diesel fuel?

ISO 4406 is the modern standard used for diesel cleanliness. The Worldwide Fuel Charter and engine makers such as Caterpillar express diesel targets in ISO 4406 (for example 18/16/13). NAS 1638 still appears in older hydraulic and aerospace manuals, so you may need to reference it, but new diesel targets are set in ISO 4406.

Q: Is NAS 1638 still used?

NAS 1638 is largely legacy — it has been superseded in aerospace by SAE AS4059 and in most industries by ISO 4406 — but it is still referenced in older equipment manuals and specifications, so it has not disappeared entirely.

Both grade how dirty a fluid is by counting particles — but they count differently, report differently, and don't convert cleanly. Here's how they compare and which one matters for diesel.

ISO 4406 is the modern global cleanliness code; NAS 1638 is an older US aerospace standard. ISO 4406 reports cumulative particle counts at ≥4, ≥6 and ≥14 µm as a three-number code (e.g. 18/16/13). NAS 1638 reports differential counts across five size bands as a single class from 00 (cleanest) to 12 (dirtiest). Because they count particles differently, there is no exact conversion between them.

What is ISO 4406?

ISO 4406 is the international cleanliness coding standard. It counts the particles per millilitre at or above three sizes — ≥4 µm, ≥6 µm and ≥14 µm(c) — and reports them as a three-number code such as 18/16/13, where lower is cleaner and each step up the scale roughly doubles the count. It is the standard used across fuel, lubrication and hydraulics today. See what ISO 4406 is and the full ISO 4406 code chart.

What is NAS 1638?

NAS 1638 is the National Aerospace Standard 1638, a US cleanliness classification that predates ISO 4406. Instead of cumulative counts at fixed sizes, it counts particles in five differential size bands — 5–15 µm, 15–25 µm, 25–50 µm, 50–100 µm and over 100 µm — and assigns a single class number from 00 (cleanest) to 12 (dirtiest) based on the worst-case band. Counts are typically expressed per 100 millilitres. NAS 1638 was widely used in aerospace and hydraulics for decades and still appears in older equipment manuals.

The key differences

	ISO 4406	NAS 1638
Origin	International standard (current)	US aerospace standard (legacy)
Counting method	Cumulative (particles at or above a size)	Differential (particles within a size band)
Sizes used	≥4, ≥6, ≥14 µm(c)	5–15, 15–25, 25–50, 50–100, >100 µm
Result format	Three-number code (e.g. 18/16/13)	Single class (00 cleanest → 12 dirtiest)
Basis	Per millilitre	Typically per 100 millilitres
Status	Preferred standard today	Legacy; largely superseded

The core difference: ISO 4406 gives three numbers (fine, medium and coarse particles), while NAS 1638 collapses everything into one class based on the worst size band — so ISO 4406 carries more information about what is actually in the fluid.

Can you convert between them?

Not exactly. Because the two systems measure at different particle sizes and count in different ways — cumulative versus differential, per millilitre versus per 100 millilitres — there is no precise mathematical conversion from a NAS 1638 class to an ISO 4406 code. Approximation charts exist to cross-reference the two (for example, they often place a NAS class near an ISO 4406 code of a similar cleanliness level), but they are estimates. If a specification is written in one system, measure and report in that system rather than converting.

Which should you use for diesel?

For stored diesel, ISO 4406 is the standard that matters. Diesel cleanliness targets are written in ISO 4406 — the Worldwide Fuel Charter and engine makers such as Caterpillar specify targets like 18/16/13 at the dispensing point, and modern common-rail injection commonly targets around 12/9/6. You may still encounter NAS 1638 in older hydraulic or aerospace manuals, but new diesel and injection-system targets are set in ISO 4406. For the cited detail, see the ISO 4406 & modern engine requirements guide and acceptable ISO 4406 levels for diesel. You may also want the related ISO 4406 vs ISO 4407 comparison.

Measuring it live

Whichever code a standard uses, the value only matters if it reflects the fuel now. 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 the ISO 4406 monitoring solution or the live data.

Common questions

ISO 4406 vs NAS 1638, answered.

What is the difference between ISO 4406 and NAS 1638?

ISO 4406 is the modern global code: cumulative particle counts at ≥4, ≥6 and ≥14 µm reported as a three-number code such as 18/16/13. NAS 1638 is an older US aerospace standard: differential counts across five size bands (5–15, 15–25, 25–50, 50–100 and over 100 µm) reported as a single class from 00 (cleanest) to 12 (dirtiest).

Can you convert NAS 1638 to ISO 4406?

Not exactly. The two systems count particles at different sizes and in different ways, so there is no precise conversion. Approximation charts exist for cross-reference, but they are estimates — report in whichever system your specification uses.

Which standard should I use for diesel fuel?

ISO 4406. Diesel targets from the Worldwide Fuel Charter and makers such as Caterpillar are written in ISO 4406 (e.g. 18/16/13 at the dispensing point, ~12/9/6 for common-rail). NAS 1638 mainly survives in older hydraulic and aerospace manuals.

Is NAS 1638 still used?

It is largely legacy — superseded in aerospace by SAE AS4059 and in most industries by ISO 4406 — but it still appears in older equipment manuals and specifications, so you may still need to reference it.

See your diesel's ISO 4406 code — live.

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