Why is glycerin not on the list?

Because the published comedogenic literature this site draws on does not give it a rating. Plenty of other sites will tell you glycerin is non-comedogenic, and they may well be right, but that is a claim this site has no published rating to support and so does not make.

Will you add these ingredients?

Only if a published rating for them turns up in the sources this site already cites. Adding an ingredient with a number nobody published would be inventing the thing the whole list exists to avoid.

When an ingredient is not on the list

Paste a label into the checker and some of what you typed will come back with nothing against it. That is worth understanding, because it is the part people most often read the wrong way round.

This site publishes 285 ingredients that carry a comedogenic rating in the dermatology literature. It is a list of things that have been rated, and most of what has been rated was rated because somebody suspected it of blocking pores. So an ingredient missing from it has no published rating here, and that is a different statement from a rating of 0.

Absent could mean it was tested and did not block pores. It could mean nobody has tested it. From this list alone the two look identical, and this site will not guess which.

The ones people ask about most

Checked against the same list the checker matches on, every time this page is built.

Vaseline also written as Petroleum jelly, Petrolatum: No rating is published for it here. An occlusive, and the single most searched of these.
Squalane: No rating is published for it here. Not the same substance as squalene, which IS on the list at a rating of 3.
Glycerin also written as Glycerol: No rating is published for it here. A humectant, and one of the most common ingredients in skincare.
Sunflower seed oil also written as Helianthus annuus seed oil: No rating is published for it here. An oil, on a list that carries 82 of them.
Meadowfoam seed oil also written as Limnanthes alba seed oil: No rating is published for it here. An oil, on a list that carries 82 of them.
Dimethicone: No rating is published for it here. A silicone, used to give a product its slip.
Rosehip oil also written as Rosa canina fruit oil: No rating is published for it here. An oil, on a list that carries 82 of them.

Three of those are oils, on a list carrying 82 entries whose composition names an oil. It is reasonable to read that as an oil nobody found a reason to rate, and it is still not a rating.

Squalane is worth a sentence of its own. The list carries Squalene, spelled with an e, at a rating of 3, and they are different substances: squalane is the hydrogenated form. Reading one as the other would give you a number for something you did not ask about, which is why the checker does not match them to each other.

What to do with an ingredient that is not here

Read the rest of the label. A product is a formula, and the ingredients that do carry a rating tell you more than the ones that do not. Where a rated ingredient sits in the list matters too, because the order is by descending concentration.

If an ingredient is missing and you want to know about it specifically, a dermatologist can answer questions this list cannot, and a patch test on your own skin answers the only question that is really about you.

Check a label against the list, or read where these ratings come from.

Common questions

Does not being on the list mean an ingredient is safe?: No. The list holds ingredients that carry a published comedogenic rating, so being absent from it means no rating has been published, and that is a different statement from a rating of 0. It might be absent because it was tested and found not to block pores, or because nobody has tested it, and this site cannot tell you which.
Why is glycerin not on the list?: Because the published comedogenic literature this site draws on does not give it a rating. Plenty of other sites will tell you glycerin is non-comedogenic, and they may well be right, but that is a claim this site has no published rating to support and so does not make.
Will you add these ingredients?: Only if a published rating for them turns up in the sources this site already cites. Adding an ingredient with a number nobody published would be inventing the thing the whole list exists to avoid.

Where the ratings come from

Kligman AM, Kwong T. “An improved rabbit ear model for assessing comedogenic substances.” British Journal of Dermatology, June 1979;100(6):699–702. (opens in a new tab) The rabbit-ear assay that produced most of the 0–5 numbers still quoted today.
Fulton JE Jr, Pay SR, Fulton JE 3rd. “Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear.” Journal of the American Academy of Dermatology, January 1984;10(1):96–105. (opens in a new tab) The published ingredient-by-ingredient ratings that most comedogenic lists, including this one, trace back to.
Draelos ZD, DiNardo JC. “A re-evaluation of the comedogenicity concept.” Journal of the American Academy of Dermatology, March 2006;54(3):507–512. doi:10.1016/j.jaad.2005.11.1058 (opens in a new tab) Found that animal-model results did not reliably predict what happened on human skin, and that a rating for a single ingredient does not tell you how a finished product will behave.
U.S. Food and Drug Administration, “Cosmetics Labeling Claims”. (opens in a new tab) States that the FDA does not keep a list of approved or accepted cosmetic claims; a label must simply be truthful and not misleading. There is no FDA definition of “non-comedogenic” for a brand to meet.