# Glycolic vs Lactic Acid: Same Family, Different Depth, and the pH That Decides

Source: https://beautyanswerbook.com/2026-11-10-glycolic-vs-lactic-acid-ph-molecular-size.html

November 10, 2026

*Ask which acid is stronger and everyone says glycolic. Ask which is gentler and everyone says lactic. Both answers are true and both miss the point, because the difference between these two acids is not strength, it is depth, and the number that decides how either one behaves is the pH on the bottle, not the percentage.*

## Two molecules, one mechanism, different depth

Glycolic and lactic acid are both alpha hydroxy acids. They loosen the bonds between dead corneocytes at the surface so the stratum corneum sheds faster, and they do it the same way. What differs is size. Glycolic acid weighs 76.05 grams per mole, the smallest alpha hydroxy acid used in skincare, and it slips between corneocytes faster. Lactic acid weighs 90.08 grams per mole, a bigger molecule that stays mostly in the stratum corneum at consumer concentrations. Smaller and faster is not stronger, it is deeper, and deeper is why glycolic shows up in clinic peels at 30 percent and above while lactic rarely does.

The head to head trial says the two land closer than the reputation suggests. In 1996, Stiller and colleagues ran 8 percent L-lactic cream against 8 percent glycolic cream against vehicle for 22 weeks on the face and forearms of 74 women aged 40 to 70. On the face, 76 percent of glycolic users and 71 percent of lactic users improved at least one grade on a nine point scale, and lactic actually beat vehicle on more endpoints on the forearm. At the same concentration and the same pH, the two acids are close on photodamage. The reputation gap is mostly a depth gap, and depth matters more for texture and peels than for daily brightening.

## The pH decides whether the percentage means anything

Neither acid penetrates meaningfully in its salt form. The free acid fraction, determined by pH and the acid's dissociation constant, is the active portion. Glycolic acid has a pKa of 3.83 and lactic acid 3.86, which means both need formulation below pH 4 to deliver real exfoliation. A product labeled 10 percent glycolic acid at pH 7 contains virtually no free acid and performs no differently than a non-acid moisturizer. A 5 percent glycolic toner at pH 3.2 can sting more than a 10 percent lactic serum buffered to pH 4.0. The percentage on the front of the bottle is a marketing number. The pH is the chemistry.

Lactic acid has one property glycolic does not share. In 1996 Rawlings and colleagues showed that L-lactic acid, but not its mirror image D-lactic acid, is incorporated as a substrate into keratinocyte ceramide synthesis, raising ceramide levels in the stratum corneum and improving barrier function. In vitro work puts the effect at roughly a 300 percent increase in ceramide production at concentrations as low as 4 percent. Glycolic is the exfoliator. Lactic is the exfoliator that also feeds the barrier, which is why it is the better default for dry or reactive skin even though the two acids look equivalent on a label. For the retinoid ladder that pairs with either acid, see [Retinal vs Retinol vs Tretinoin: The Ladder, Explained](2026-11-08-retinal-vs-retinol-vs-tretinoin-ladder.md), and for the concentration question in general, [Niacinamide: What the Evidence Actually Supports at 5 Percent](2026-11-08-niacinamide-evidence-5-percent.md).

## How to choose, without a spreadsheet

Pick glycolic if the goal is texture, stubborn pigmentation, or sun damage, and the skin is resilient and already tolerating acids. Pick lactic if the skin is dry, sensitive, or new to acids, or if the barrier needs support alongside the exfoliation. Either way, start at 5 to 8 percent, two or three nights a week, at a pH below 4, and add frequency before adding strength. And use sunscreen, because an acid that speeds shedding also leaves newer skin more exposed to UV, and the pigmentation you are trying to fade will come back faster without it.

## Frequently asked questions

### Is glycolic acid stronger than lactic acid?

It penetrates deeper because it is smaller, 76 grams per mole against 90, but at the same concentration and pH the two land close. A 1996 trial at 8 percent for 22 weeks found 76 percent of glycolic users and 71 percent of lactic users improved on photodamage.

### Does the percentage on the bottle tell me how strong it is?

Not on its own. The free acid fraction depends on pH, and both acids need a pH below 4 to exfoliate. A 10 percent glycolic product at pH 7 has almost no free acid, while a 5 percent product at pH 3.2 can sting.

### What does lactic acid do that glycolic does not?

L-lactic acid is incorporated into keratinocyte ceramide synthesis, raising ceramide levels in the stratum corneum and supporting the barrier. Glycolic acid has not been shown to do that.

### Which acid should a beginner start with?

Lactic, at 5 to 8 percent, two or three nights a week. It is more forgiving at the same concentration, it moisturizes while it exfoliates, and it supports the barrier instead of only stripping it.