How do I check whether a skincare product is right for my skin?

Why does the front of the bottle not answer this?

Because the front is an ad. Glow, clean, barrier, radiance: those words are written by the same people who set the price, and most of them mean whatever the brand needs them to mean this season.

The back is a different document. In Europe, Regulation 1223/2009 Article 19 requires a full list of ingredients on the packaging, in a fixed order and under standard INCI names. In the United States, 21 CFR 701.3 requires the same declaration in descending order of predominance. The list is the only part of the package that has to be true in a way you can check.

If a claim on the front has nothing standing behind it on the back, it is a story about the product, not the product.

Which ingredients on the list actually matter?

The ones at the top. Both rules order the list by weight, heaviest first, and both stop enforcing that order once an ingredient drops to 1 percent or less. So the first five or six lines are most of what you are buying, and everything past the 1 percent line is printed in whatever order suited the label designer.

That gives you one move worth making every time. Find the ingredient the front is selling, then find it on the back. Position 2 is a formula. Position 19 is a cameo. How the descending-order rule works, and how to find where it stops, is its own page.

How do I match a list to my own skin?

Three passes down the same list, plus one look at the rest of your shelf.

• What your skin is short of. Dehydrated skin wants humectants such as glycerin, hyaluronic acid, panthenol and urea sitting high on the list, not in the last three lines. Skin that stays greasy and still feels tight is usually short of water, not of oil.
• What has reacted on you before. If something has left your face stinging, look for the same names again. Fragrance is the single most common cause of contact reactions, which makes it a real flag on reactive skin and a non-event on skin it has never bothered. Whether fragrance is bad for skin depends entirely on whose face it is going on.
• What the claim is doing. Non-comedogenic has no legal definition in the EU or the US and no test a brand has to pass. What the claim actually means is worth knowing before you let it decide for you.
• What is already in your routine. A product can be well made and still be wrong on top of what you used last night. Two exfoliating acids, or a retinoid stacked on one, is a routine problem rather than an ingredient problem, and no single label can see it.

This is the step generic advice cannot do for you. The same bottle walks into two bathrooms and is a good idea in one of them.

What can a label never tell you?

How much of anything is in there. Article 19 sets out everything that has to be printed on a cosmetic package, and percentages are not on that list. Brands can publish a number when they want credit for it, and plenty do, but nothing obliges them to and nothing stops the rest from staying quiet.

So a percentage nobody published cannot be read off a bottle. Not by you, not by a reviewer, not by an app. Anything handing you a confident dose for a formula the brand never disclosed is guessing at you with a straight face.

The label also says nothing about how the formula is built: pH, delivery, what actually carries an active past the surface. A well built 2 percent beats a badly built 10 percent, and neither number is on the box.

What do I do with a list that looks right?

Try it small before you give it your whole face. Put a little on one discreet patch of skin, use it there for a few days, and add one new product at a time so that when something does react you know which thing did it.

A list that reads well is a good reason to try something. It is not a promise, and nothing on the back of a bottle is. Redness that keeps coming back, anything that spreads, anything painful: that belongs with a dermatologist, not with a label reader and not with an app.

Sus grades the list against your skin, not against skin in general

You set a Skin ID once: skin type, goals, what has reacted before, what you are already using. Every scan is graded against that. The same bottle can come back a B+ for one face and a D for another, and that is not a flaw in the grading. That is the entire point of it.

Point the camera at the back of the bottle, scan the barcode, or type the name. Sus reads the list, finds the ingredient the front was selling, and tells you where it actually landed. Here is what happens between the scan and the grade.

Download Sus on the App Store

Free for iPhone. Your first 3 scans are on us, no sign-up. Sus is a cosmetics tool for informational purposes. It does not diagnose or treat anything.