ASTM D3273: The Mold Rating Where 10 Is the Best Score

A D3273 rating and an ASTM G21 rating both show up on datasheets as a single small number. They run in opposite directions. Read one as the other and the best result a product can get turns into the worst one it can get.

Last updated: August 26, 2026

The short version

ASTM D3273-21 is a test method for interior coatings. Coated panels hang for four weeks in a warm, humid chamber above a bed of soil that has been inoculated with mold.
Results are reported on the 0 to 10 pictorial scale from ASTM D3274. A 10 means no visible defacement. On this scale, 10 is the good number.
ASTM G21, the fungal resistance practice used for polymers and sealants, runs 0 to 4 with 0 meaning no growth. The two scales are inverted relative to each other.
Neither standard sets a pass or a fail. ASTM says a D3273 rating “does not imply any specific period of time for a fungal free coating.”
D3273 is written for interior coatings, and its own Note 1 says exterior use has not been validated.

What ASTM D3273 is and how the chamber test works

ASTM D3273-21 is titled Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber. It sits with ASTM Committee D01 on paint and related coatings, under Subcommittee D01.28 on biodeterioration. The word worth noticing in that title is interior. The first clause of the scope says the method evaluates the relative resistance of interior coatings to surface fungal growth in a severe interior environment during a four-week period.

The chamber setup is the part most write-ups get backwards. The coated panels are not inoculated. A bed of soil in the bottom of the chamber is inoculated with three fungal species and left to sporulate before the panels go in. The panels then hang above that soil without touching it, and spores reach them through the chamber air. The chamber runs at 32.5 ± 1 °C (90 ± 2 °F) at high humidity for four weeks, and testing laboratories typically rate the panels once a week across the run.

That indirect exposure is the design intent. Nothing is inoculated onto the coating and no nutrient medium is applied to it. The nutrient source sits in the soil bed below. The panel has to be colonized the way a wall in a hot, damp, spore-loaded room gets colonized.

Check the humidity figure before you quote it. It changed between editions. The current D3273-21 states a minimum, a relative humidity of greater than 93 percent. The 2016 edition stated 95 ± 3 percent, and the 2000 edition stated 95 to 98 percent. Plenty of laboratory pages and product sheets still print the older numbers as though they were current.

The 0 to 10 scale comes from ASTM D3274

D3273 does not define its own rating numbers. Results are reported on the scale from ASTM D3274, Standard Test Method for Evaluating Degree of Surface Disfigurement of Paint Films by Fungal or Algal Growth, or Soil and Dirt Accumulation, which belongs to the same subcommittee. D3274 works from pictorial reference standards: the rater compares the panel against a set of reference illustrations and assigns a number.

The scale runs 0 to 10, and 10 is the top. A 10 means no visible defacement. Numbers drop as growth spreads across the surface, so a coating reported at 10 after four weeks showed nothing, and a coating reported at 3 was substantially defaced.

Two things follow. A high number on a D3273 report is the good outcome. And 10 is the ceiling of the scale, not a bar anyone cleared. ASTM defines no rating that qualifies a coating as mold resistant.

Where the scale came from: D3274 is written around exterior exposed paint films, and it rates disfigurement from soil and dirt accumulation as well as from fungal and algal growth. The interior chamber test borrows a rating scale that was built for judging weathered exterior surfaces in the field. That is ordinary ASTM practice, and it is also why the number is a visual judgment rather than a measurement.

ASTM D3273 vs ASTM G21: the scales run opposite ways

Polymeric material datasheets, including many sealants, usually carry ASTM G21 rather than D3273. G21 is Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi, and it comes from a different committee, Subcommittee G03.04. It reports on a 0 to 4 scale where 0 is the best result: 0 is no growth, 1 is traces under 10 percent coverage, 2 is light growth from 10 to 30 percent, 3 is medium growth from 30 to 60 percent, and 4 is heavy growth from 60 percent to complete coverage.

Set the two documents side by side and the trap is easy to see.

	ASTM D3273	ASTM G21
What ASTM calls it	Standard Test Method	Standard Practice
Owning subcommittee	D01.28, Biodeterioration	G03.04
Written for	Interior coatings	Synthetic polymeric materials
Rating scale	0 to 10, taken from ASTM D3274	0 to 4, defined in G21 itself
Best possible result	10, no visible defacement	0, no growth
Worst possible result	0	4, heavy growth
How the mold reaches the specimen	Inoculated soil bed below the hanging panels	Specimen inoculated directly on carbon-free agar
Pass/fail criterion	None	None

A rating of 0 is the best result a product can earn on G21 and the worst it can earn on D3273. So a bare number with no standard beside it is unreadable. Copy a G21 zero into a D3273 column and a product that grew nothing at all now reads as completely defaced.

The reverse mistake is harder to catch, because the result still looks plausible. A 4 on G21 is heavy growth, the worst outcome that practice defines. Dropped into a discussion about D3273 it reads as a low but survivable score. It was total failure.

The two are not measuring the same thing either. G21 puts the specimen on a nutrient-salts agar that contains no carbon source, so any fungus that grows has to be feeding on the specimen itself. The current revision is direct about what it is usually feeding on: not the synthetic polymer, but the plasticizers, cellulosics, lubricants, stabilizers, and colorants around it. A G21 result describes a formulation, not a base resin. D3273 asks a different question, which is whether mold will colonize a coated surface in a hot, wet, spore-loaded room.

Neither number stands alone. G21’s reporting clause requires the organisms used, the incubation time, the visual rating, and the maximum observed change. A rating quoted without its duration is an incomplete report by the standard’s own terms, and the same caution applies to a D3273 number.

What a D3273 rating does not tell you

There is no pass and no fail

D3273 sets no minimum acceptable rating, and neither does G21. Both are comparative methods for ranking one product against another. When a submittal says a product passes ASTM D3273, somebody other than ASTM set that bar, and the useful question is who set it and at what number.

A rating is not a service life

ASTM says so in one sentence in the scope: “Performance at a certain rating does not imply any specific period of time for a fungal free coating.” The same clause adds that “a better rated coating nearly always performs better in actual end use,” which is the standard telling you the number is a ranking tool and not a warranty.

Exterior use is not validated

Note 1 in the standard says use of the method for evaluating exterior coating performance “has not been validated, nor have the limitations for such use been determined.” A laboratory that runs an exterior system through the chamber anyway has to say in the report that the result falls outside the method’s scope.

ASTM certifies nothing to D3273

There is no listing program and no certificate. A specimen was tested by a laboratory and received a rating on a date. That is the whole of it, and it is why “ASTM D3273 certified” is not a claim the standard supports.

A test result is not a mold claim. Wording such as mold proof, prevents mold, or antimicrobial describes pesticidal efficacy and is regulated under EPA pesticide rules whatever any ASTM rating says. A 10 on D3273, or a 0 on G21, shows that the surface did not support visible growth under the test conditions. It does not make the product a mold treatment.

Reading a mold number on a datasheet

Four questions turn a mold number into something you can act on.

Which standard? D3273 and G21 are the two you will see, and the direction of the scale depends entirely on which one it is.
What rating, and after how long? A rating with no duration beside it is an incomplete report.
Who ran it? An independent laboratory with a report date is a different thing from a manufacturer’s own bench, and both are better than an unattributed number.
What was actually tested? A rating attaches to the specimen that was submitted, in the color and formulation submitted, not to a whole product line.

One scope note comes up constantly with sealants. D3273 is written for interior coatings, and a cured polymeric sealant is not a coating, so the purpose-built route for a sealant is G21. Sealant manufacturers do publish D3273 numbers anyway, and those numbers are not meaningless, but the wording matters. Describe such a figure as tested per the D3273 chamber protocol. Avoid compliance wording, because a test method is not something a product can conform to.

A third designation turns up on coating datasheets as well. ASTM D5590 is the other four-week fungal method from the same subcommittee, and it uses an agar plate assay instead of a chamber, with coated specimens placed on inoculated nutrient agar. If a datasheet cites D5590, it is a different test with a different setup, and its numbers are not D3273 numbers.

Frequently Asked Questions

What is ASTM D3273?

ASTM D3273-21 is a standard test method for measuring how well an interior coating resists mold growth on its surface. Coated panels hang in an environmental chamber above a bed of soil that has been inoculated with fungi, held at 32.5 ± 1 °C (90 ± 2 °F) at high humidity for four weeks. The panels are then rated for visible growth using the pictorial rating scale from ASTM D3274. It is owned by ASTM Subcommittee D01.28 on biodeterioration.

Is a high or a low number better on ASTM D3273?

High. A D3273 result is reported on the 0 to 10 scale from ASTM D3274, where 10 means no visible defacement and 0 means the surface is completely defaced. This catches people out because ASTM G21, the fungal resistance practice used for plastics and sealants, runs the other way: its scale is 0 to 4 and 0 is the best result. A bare number is unreadable until you know which standard produced it.

What is a good ASTM D3273 rating?

There is no passing D3273 rating, because the standard does not define one. It is a comparative method. A 10 is the top of the scale, so a coating reported at 10 after four weeks showed no visible growth, but ASTM sets no minimum. Any threshold you see, such as a specification calling for a rating of 8 or better, was set by the buyer or by the manufacturer, and it should be attributed to whoever set it.

What is the difference between ASTM D3273 and ASTM G21?

They are different documents from different committees, and their scales run in opposite directions. D3273 is a test method from Subcommittee D01.28 covering interior coatings, reported 0 to 10 with 10 as the best result. G21 is a practice from Subcommittee G03.04 covering synthetic polymeric materials, reported 0 to 4 with 0 as the best result. The setups differ as well: D3273 inoculates a soil bed under the hanging panels and lets spores travel through the chamber air, while G21 inoculates the specimen directly on a nutrient-salts agar that contains no carbon source, so any growth has to feed on the specimen. Neither standard defines a pass or a fail.

Can a product be certified to ASTM D3273?

No. ASTM publishes the method and does not certify or list products to it, and there is no D3273 certification program. The defensible statement is that a specific specimen was tested to ASTM D3273 by a named laboratory and received a stated rating after four weeks. "ASTM D3273 certified" is not a claim the standard supports.

Does ASTM D3273 apply to sealants?

Not as written. The scope of D3273 covers interior coatings, and a cured polymeric sealant is not a coating. The purpose-built route for a sealant is ASTM G21, which covers synthetic polymeric materials. Sealant manufacturers do publish D3273 numbers, and those numbers are not meaningless, but the accurate description is that the product was tested per the D3273 chamber protocol. Avoid compliance wording here altogether, because a test method is not something a product can conform to.

Can ASTM D3273 be used for exterior coatings?

The standard tells you not to assume so. Note 1 states that use of the method for evaluating exterior coating performance has not been validated, and that the limitations for such use have not been determined. If a laboratory does run an exterior system through the chamber, the standard requires the test report to carry a precautionary statement saying the result falls outside the scope of the method.