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Glycolic, lactic and mandelic acid are all alpha hydroxy acids (AHAs), and all three loosen the bonds between dead surface cells so skin looks smoother and brighter. They are not interchangeable. Each one has a different molecular size, a different strength as an acid, and a different feel on the skin. As a cosmetic formulator, I choose between them based on three numbers: molecular weight, pKa and the final pH of the product. This page compares the three side by side so you can decide which AHA belongs in your moisturizer.
If you already know you want glycolic acid and need help with a 10% glycolic moisturizer routine, start with our 10% glycolic moisturizer guide, which covers how to use it, how often, and what to pair it with. This page answers a different question: which acid to choose in the first place.
The Short Answer
- Glycolic acid is the smallest and fastest. Choose it for rough texture and dullness on skin that already tolerates acids.
- Lactic acid is slightly larger and is often described as gentler. Choose it for dry or easily flushed skin, or as a first AHA.
- Mandelic acid is about twice the size of glycolic acid. Choose it when you want the slowest, mildest exfoliation and are willing to accept slower visible change.
Side-by-Side Comparison: Glycolic vs Lactic vs Mandelic
| Property | Glycolic acid | Lactic acid | Mandelic acid |
|---|---|---|---|
| Molecular formula | C2H4O3 | C3H6O3 | C8H8O3 |
| Molecular weight (g/mol) | 76.05 | 90.08 | 152.15 |
| pKa | 3.83 | 3.86 | 3.41 |
| Typical origin | Sugarcane or synthetic | Fermentation (milk, sugars) | First isolated from bitter almond extract |
| Relative speed | Fastest | Moderate | Slowest |
| Feel on skin | Most tingle | Less tingle, more humectant feel | Least tingle in most users |
| Best fit | Experienced acid users, texture | Dry skin, first-time AHA users | Reactive skin, slow and steady routines |
Molecular weight and pKa values come from standard chemical references for each compound. "Relative speed" and "feel" describe the general pattern formulators work with, and individual skin varies.
Why Molecular Size Matters
Smaller molecules move into the outer layer of skin faster. At 76 g/mol, glycolic acid is the smallest of the common AHAs, which is why it produces the quickest visible smoothing and also the most tingle. Lactic acid adds one carbon and a methyl group, bringing it to 90 g/mol. Mandelic acid carries a benzene ring, which doubles its weight to 152 g/mol and slows how quickly it moves into the skin.
Think of it as dosing speed. The same 10% concentration of each acid does not deliver the same amount of acid to the skin in the same amount of time. Glycolic acid reaches the surface layers quickly, lactic acid a little more slowly, and mandelic acid gradually. Slower delivery usually means a softer feel and slower visible change.
Why pH and pKa Matter More Than the Percentage
The percentage on the label is only half the story. What does the exfoliating work is the "free acid," the portion of the acid that has not been neutralized. How much free acid exists depends on the product's pH compared with the acid's pKa.
- When product pH equals the acid's pKa, about half of the acid is in its free form.
- Each full pH unit above the pKa cuts the free acid roughly tenfold.
- Each full pH unit below the pKa raises the free acid, along with the tingle.
A study published in Dermatologic Surgery in 1998 tested this directly with lactic acid. In 26 women, 10% lactic acid was tested at pH 2.0, 3.0 and 4.0, and lactic acid at 5%, 10% and 15% was tested at pH 3.0. Skin renewal was measured with the dansyl chloride staining method. At a fixed concentration, the effect was highly pH dependent. At a fixed pH, it was concentration dependent. The authors concluded that the free acid concentration is the active portion.
Practical takeaway: a 10% glycolic product at pH 4.5 and a 10% glycolic product at pH 3.5 are very different products, even though the label percentage is identical.
The Safety Ceiling for Leave-On AHA Products
The Cosmetic Ingredient Review (CIR) Expert Panel, as summarized by the FDA, concluded that glycolic and lactic acid are safe in consumer cosmetics when three conditions are met:
- The AHA concentration is 10% or less.
- The final product pH is 3.5 or higher.
- The product either protects skin from increased sun sensitivity or its directions tell users to use daily sun protection.
This is why so many leave-on AHA moisturizers sit at exactly 10% and around pH 3.5 to 4.0. When I formulate a leave-on AHA moisturizer, I set 10% at pH 3.5 as the outer edge, not the target. For lactic and mandelic acid, I hold the same 10% ceiling as a house rule.
Sun Sensitivity: The Same Rule for All Three
An FDA-sponsored clinical study by Kaidbey and colleagues, published in 2003, tested 10% glycolic acid at pH 3.5 against a placebo on the backs of 29 adults. The cream was applied once daily, 6 days per week, for 4 weeks. Glycolic acid increased sunburn cell formation and lowered the amount of UV needed to redden the skin. The FDA reports that sensitivity to UV reddening rose about 18%. One week after stopping, the differences were no longer seen.
That study tested glycolic acid, not lactic or mandelic. Because all three work the same way, by thinning the outermost dead cell layer, I apply the same rule to every AHA: broad-spectrum sunscreen every morning while using it and for one week after stopping. The FDA's recommended "Sunburn Alert" label language says the same.
Glycolic Acid: Fastest Visible Smoothing
What it does well
Glycolic acid gives the most noticeable change in the look of rough texture and dullness in the shortest time. It is the most studied AHA, and it is the acid in most of the research on AHA moisturizers.
Where it falls short
Its small size is also its downside. Glycolic acid produces the most stinging, especially on freshly shaved or waxed skin, around the nose and mouth, and on skin that is already dry.
Who should choose glycolic
- Skin that already tolerates acids without lasting redness
- Rough, dull or uneven-looking texture as the main concern
- Users who want visible change within the first month
Lactic Acid: The Moisture-Friendly AHA
What it does well
Lactic acid sits between the two others in size. It also carries humectant behavior, meaning it holds water, so lactic acid products often feel more moisturizing. A review on lactic acid peeling published in 2024 in Clinical, Cosmetic and Investigational Dermatology describes a study that compared glycolic, L-lactic, D-lactic, citric, hydroxybutyric and malic acids at equal molar concentration and pH. L-lactic and glycolic acid showed the best results, and L-lactic acid caused significantly less irritation than the others.
That "L" matters. Lactic acid exists in two mirror-image forms, L and D. Products labeled "chirally correct" use the L form, which is the form the review found best tolerated. Our Alpha Hydroxy Probiotic Cleanser Chirally Correct uses lactic acid with salicylic acid in a rinse-off gel, which is a low-commitment way to see how your skin responds to lactic acid before moving to a leave-on product.
Where it falls short
Visible change is usually slower than glycolic acid at the same percentage and pH.
Who should choose lactic
- Dry or dehydrated-looking skin
- First-time AHA users
- Skin that flushes easily with glycolic acid
Mandelic Acid: The Slow, Gentle Option
What it does well
Mandelic acid has a molecular weight of 152 g/mol, about twice that of glycolic acid. It moves into skin more slowly, which is why many people find it the most comfortable of the three. It also has a lower pKa (3.41), which makes it a slightly stronger acid. At the same product pH, a slightly smaller share of mandelic acid stays in the free acid form than glycolic or lactic acid, so pH has to be set with mandelic acid's own pKa in mind.
Where it falls short
The evidence base is thinner. Most of the published mandelic acid research I could find tests it as a professional peel, not as a leave-on moisturizer, so I do not quote specific results for mandelic acid moisturizers here. Expect slower, more gradual visible change than with glycolic acid.
Who should choose mandelic
- Skin that reacts to glycolic and lactic acid
- Users who prefer slow, steady change over fast change
- Anyone who wants an AHA they can use more often with less tingle
Choosing by Skin Type
| Skin description | First choice | Starting frequency |
|---|---|---|
| Normal, acid-experienced | Glycolic, 10% | 2 to 3 evenings per week |
| Dry or dehydrated-looking | Lactic, 10% | 2 evenings per week |
| Easily flushed or reactive | Mandelic, 10% | 2 evenings per week |
| Oily, uneven texture | Glycolic 10%, or a lactic and salicylic rinse-off cleanser | 2 to 3 evenings per week |
| First AHA ever | Lactic 10%, or a rinse-off lactic cleanser first | 2 evenings per week |
Every option in this table is set at 10%, the CIR ceiling for leave-on consumer AHA products. I keep the percentage at that ceiling and adjust comfort with frequency instead of diluting the acid. The frequencies are starting schedules from formulating practice, not study results. Increase frequency only after two weeks without lasting redness, peeling or stinging that continues after application.
Can You Combine Them?
Yes, and many professional formulas do. A blend lets a formulator use a small amount of glycolic acid for speed with lactic or mandelic acid for comfort. The rule is that the total AHA content still counts toward the 10% ceiling. A blend of 5% glycolic and 5% lactic is a 10% AHA product, not a 5% one.
What I avoid is layering two separate AHA products, for example a glycolic toner followed by a lactic serum. That stacks the free acid without anyone controlling the final pH or the total percentage.
For how acids fit alongside scrubs and other methods, see our guide to exfoliating your face properly, and if dullness is your main concern, our guide to exfoliating dull skin.
Formulator Notes for DIY Makers
If you make your own AHA moisturizer, these are the checkpoints I use:
- Measure pH with a calibrated meter, not strips. Strips are not accurate enough in the 3.5 to 4.0 range.
- Add the acid to the cooled emulsion, below 40°C (104°F), and adjust pH upward with a dilute sodium hydroxide or arginine solution until you reach 3.5 to 4.0.
- Check your preservative and thickener. Many carbomer gels and some emulsifiers thin out at low pH. Choose thickeners and emulsifiers rated for use at pH 3.5 to 4.0.
- Recheck pH after 24 hours and after 1 week. Emulsions can drift as they settle.
- Add hydration support. Glycerin and sodium hyaluronate help offset the dry feel acids can leave. Use sodium hyaluronate at 2%, the highest rate in the 0.1% to 2% range on its product page. At 2%, high molecular weight sodium hyaluronate forms a thick gel, so build the formula as a gel-cream. Our Sodium Hyaluronate Powder Pure Hyaluronic Acid is a cosmetic ingredient for this purpose. It hydrates; it does not exfoliate.
Frequently Asked Questions
Is lactic acid weaker than glycolic acid?
At the same percentage and pH, lactic acid usually acts more slowly and feels milder because its molecule is larger (90 vs 76 g/mol). It is not weaker as an acid; their pKa values are almost identical (3.86 vs 3.83). The difference is speed, not acid strength.
Is mandelic acid good for sensitive skin?
Mandelic acid is the slowest of the three because it is about twice the size of glycolic acid, and many people with reactive skin find it the most comfortable. Patch test on the inner forearm or behind the ear for 3 nights before using it on the face.
Which AHA gives the fastest results?
Glycolic acid, because it is the smallest. It also causes the most stinging, so speed comes with a trade-off.
Do I need sunscreen with lactic or mandelic acid?
Yes. The FDA-sponsored sun sensitivity study tested glycolic acid, but all AHAs thin the outer dead cell layer the same way. Use broad-spectrum sunscreen daily while using any AHA and for one week after stopping.
What percentage of AHA should a leave-on moisturizer have?
For consumer leave-on products, the CIR safety conclusion is 10% or less at a final pH of 3.5 or higher, with sun protection directions. For a first AHA, choose the gentler acid, lactic or mandelic, at 10%, and control comfort with frequency: start at 2 evenings per week.
These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease.
About the Author
Written by Kari Thomas, cosmetic formulator and founder of Skin Perfection.
Sources
- U.S. Food and Drug Administration. Alpha Hydroxy Acids. Summary of the CIR Expert Panel safety conclusion and the FDA-sponsored sun sensitivity research.
- Kaidbey K, et al. Topical glycolic acid enhances photodamage by ultraviolet light. Photodermatology, Photoimmunology & Photomedicine. 2003;19(1):21-27. Randomized, double-blind study in 29 adults, 10% glycolic acid at pH 3.5, 4 weeks.
- The roles of pH and concentration in lactic acid-induced stimulation of epidermal turnover. Dermatologic Surgery. 1998;24(6):641-645. Double-blind, placebo-controlled study in 26 women.
- Lactic Acid Chemical Peeling in Skin Disorders. Clinical, Cosmetic and Investigational Dermatology. 2024. doi:10.2147/CCID.S455700. Review including lactic acid molecular data and the comparison of AHAs at equal molar concentration.
- Kornhauser A, Coelho SG, Hearing VJ. Applications of hydroxy acids: classification, mechanisms, and photoactivity. Clinical, Cosmetic and Investigational Dermatology. 2010;3:135-142. doi:10.2147/CCID.S9042.