STANDARDS

DFT measurement: SSPC-PA 2 vs ISO 19840

By a certified coating inspector · Published 23 September 2026

Dry film thickness is the most measured property in coating inspection, and the two standards used to judge it work differently. SSPC-PA 2, used on US and AMPP-based specifications, averages readings into spots and areas. ISO 19840, used on ISO 12944-based work, judges individual readings against a nominal thickness. Mix up their rules and a coat that passes under one fails under the other.

The short answer: use the standard your specification names. Under SSPC-PA 2, three or more gauge readings in a 4 cm circle make a spot, and five spots per roughly 10 m² (100 ft²) make an area. With the default restriction level (Level 3), each spot must be at least 80 % of the specified minimum and no more than 120 % of the maximum, and each area average must fall within the specified range. Under ISO 19840, you take individual readings (with a profile correction deducted) to a sampling plan based on area. The mean must reach the nominal DFT, every reading must be at least 80 % of nominal, and fewer than 20 % of readings may fall between 80 % and 100 %.

Side by side

	SSPC-PA 2	ISO 19840
Thickness target	A specified minimum and maximum per coat. If only a single value is given, the default range is ±20 % of it.	A nominal DFT (as defined in ISO 12944-5), plus a maximum if one is specified. Applies to nominal DFTs of 40 µm and above.
Gauge set-up	Calibration, accuracy verification and adjustment per ASTM D7091. Roughness is dealt with by adjusting the gauge with shims on the prepared surface, or by subtracting a base metal reading (average of at least 10 readings on the bare steel).	Verify at zero and with foils on smooth steel, then subtract a correction value for the profile: 10 µm for a fine, 25 µm for a medium and 40 µm for a coarse ISO 8503-1 profile, and 25 µm when the profile is unknown and no uncoated sample is available. Annex A (adjusting on a rough surface) applies only if agreed.
What is judged	Spots (average of ≥3 readings within a 4 cm circle) and areas (average of 5 spots).	Individual readings.
How many	Up to about 28 m² (300 ft²): every 10 m² area. 28–93 m² (300–1,000 ft²): three random areas. Above that: three areas in the first 93 m², plus one for each further 93 m² or part of it.	By inspection area: 5 readings up to 1 m², 10 up to 3 m², 15 up to 10 m², 20 up to 30 m², 30 up to 100 m², then 10 more per extra 100 m². Split areas above 1,000 m². Difficult areas get extra readings.
Pass criteria	Five restriction levels. Level 3 (default): spots ≥80 % of min and ≤120 % of max, area averages within min–max.	Mean ≥ nominal; every reading ≥80 % of nominal; fewer than 20 % of readings between 80 % and 100 % of nominal; every reading ≤ the specified maximum (if none is specified, ISO 12944-5 sets it). These criteria already allow for measurement uncertainty.
Outliers	An unusually high or low reading that can't be repeated can be discarded within a spot.	Only a limited number of measurements may be repeated, set by the sampling table.

A worked example under each

ISO 19840. An inspection area of 25 m² has a nominal DFT of 250 µm. The sampling plan calls for 20 readings, corrected for profile. To pass, the mean must be at least 250 µm, no reading may be below 200 µm, and no more than 3 of the 20 readings (fewer than 20 %) may fall between 200 and 249 µm.

SSPC-PA 2. A coat is specified at 200–300 µm, Level 3, over 450 m² of steel (about 4,800 ft²). That means three areas in the first 1,000 ft² plus four more (one for each further 1,000 ft² or part of it): seven areas, 35 spots, at least 105 gauge readings. Each spot must lie between 160 µm (80 % of 200) and 360 µm (120 % of 300), and each area average between 200 and 300 µm.

Traps inspectors and candidates fall into

Adding back the profile twice. If you adjusted the gauge on the blasted surface, or already subtracted a base metal reading, don't deduct a correction value as well.
Treating 120 % as a licence. Under PA 2 the 120 % tolerance applies to individual spots. Area averages must still sit within the specified range, and the manufacturer's maximum still applies.
Misreading the 20 % rule. The rule is about how many readings fall between 80 % and 100 % of nominal. A reading below 80 % fails the area outright, however few there are.
Zeroing on rough steel. "Zero" an electronic gauge only on a smooth surface. On a blasted surface, use shims or a base metal reading.
Mixing standards. An ISO 12944 specification with PA 2 spot rules, or the other way round, is a question to raise before measuring, not after.

DFT rules are among the most-tested topics in both CIP levels and in FROSIO, and our questions on them explain which rule applies and why. Try the free sampler to see the format.

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Sources

KTA-Tator — Measuring coating thickness according to SSPC-PA 2 — gauge types, BMR and shim adjustment, spot and area rules, area counts, restriction levels
ISO 19840:2012 — thickness of dry films on rough surfaces — scope, correction values, sampling plan, acceptance criteria
DeFelsko — ISO 19840 acceptance criteria — the mean, 80 % and 20 % rules

Checked on 23 September 2026.

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