Glycolic Acid · Ingredient Science
The Lotion Editorial · 28 September 2026 · 11 min read
Two bottles can print the same number on the label and behave nothing alike on the skin. The percentage tells you how much acid went into the formula. It does not tell you how much of that acid is actually working, or how many molecules you are really getting.
Two products with the same acid percentage can differ in real strength by more than double. Strength depends on two things the label rarely shows: pH, which decides how much of the acid is in its free, active form, and molecular weight, which decides how many acid molecules each gram delivers. Glycolic acid is the smallest alpha hydroxy acid, so 12 per cent glycolic acid carries about 18 per cent more molecules than 12 per cent lactic acid and about twice as many as 12 per cent mandelic acid.
Once those two numbers are on the table, most of the confusion around acid body lotions resolves. The rest of this page shows the method, gives you a calculator to check any product yourself, and applies it to the body lotions Australians most often compare.
pH decides how much acid is active
Acids in a formula exist in two forms. Some stays as free acid, the form that loosens the bonds between dead skin cells. The rest is neutralised into a salt, such as glycolate or lactate, which is far less active on the skin surface. The balance depends on the gap between the product's pH and the acid's pKa, the pH at which exactly half the acid is free. Each full pH unit above the pKa cuts the free share roughly tenfold.
This is well established. A United States Food and Drug Administration survey of alpha hydroxy acid products noted that 10 per cent glycolic acid at pH 3.0 is generally over twice as active on the outer skin layer as the same concentration at pH 4.0.1 The calculation on this page reproduces that result, with a ratio of about 2.2. The share of acid in its free form is called the free acid value: at pH 3.0, 10 per cent glycolic acid carries about 8.7 per cent free acid, and at pH 4.0 about 4.0 per cent.
The percentage on the label is only half of the strength story. The other half is pH, and most brands never print it.
Molecular weight decides how many molecules you get
A percentage measures weight, not molecules. Glycolic acid weighs about 76 grams per mole, lactic acid about 90 and mandelic acid about 152.3 So the same percentage of each delivers a different number of molecules. That is why comparing a 12 per cent glycolic acid lotion with a 12 per cent lactic acid lotion by percentage alone is misleading, and why this page also reports molar free acid: the amount of free acid in moles per kilogram, which puts different acids on a like-for-like footing.
These two numbers are the first two markers of the six-marker standard for a glycolic acid body lotion: concentration and pH. The remaining four, contact time, hydration support, barrier protection and tolerability, decide how that strength is delivered to real skin.
The formulas
Free fraction = 1 ÷ (1 + 10(pH − pKa))
Free acid value (%) = acid % × free fraction
Molar free acid (mol/kg) = acid % × 10 ÷ molecular weight × free fraction
Worked example: 12 per cent glycolic acid at pH 4.0. The free fraction is 1 divided by (1 + 100.17), about 0.40. Free acid value is 12 × 0.40, about 4.8 per cent. Molar free acid is 120 ÷ 76.05 × 0.40, about 0.64 moles per kilogram.
Acid strength calculator
Choose an acid, enter its percentage and the product's pH, and compare two products side by side. The chart shows how the active share of each acid falls as pH rises.
Same percentage, different acids
How common acids compare at the same pH of 4.0. Salicylic acid is shown at 2 per cent, its typical strength in body products.
| Acid and strength | pKa | Molecular weight (g/mol) | Total acid (mol/kg) | Free share | Free acid value | Molar free acid (mol/kg) | Relative strength |
|---|---|---|---|---|---|---|---|
| Glycolic acid 12% | 3.83 | 76.05 | 1.58 | 40% | 4.8% | 0.64 | 100 |
| Lactic acid 12% | 3.86 | 90.08 | 1.33 | 42% | 5.0% | 0.56 | 88 |
| Mandelic acid 12% | 3.41 | 152.15 | 0.79 | 20% | 2.5% | 0.16 | 25 |
| Salicylic acid 2% | 2.97 | 138.12 | 0.14 | 9% | 0.2% | 0.01 | 2 |
Notice that lactic acid's free acid value is slightly higher than glycolic acid's at the same pH, because its pKa is marginally higher, yet it delivers fewer active molecules. Comparing acids by free acid value alone would get this the wrong way round. Salicylic acid shows the opposite effect: with a pKa of 2.97, a 2 per cent product at pH 4.0 is only about 9 per cent free acid. For choosing between acids by skin concern rather than strength, see glycolic, lactic or salicylic: how to choose a body acid.
How popular body acid lotions compare on paper
You can compare any product's acid content from its label. Molecule count needs only the acid and its percentage, which brands publish. Free acid also needs pH, which most brands do not publish. The table below applies this page's method to widely sold body lotions, using each product's publicly stated figures.
| Product | Stated acid | Total acid (mol/kg) | Published pH | Molar free acid (mol/kg) |
|---|---|---|---|---|
| The Lotion 12% Glycolic Acid Body Lotion | 12% glycolic acid | 1.58 | 3.6 to 4.0 | 0.64 to 0.99 |
| Naturium The Smoother Glycolic Acid Body Lotion | 10% glycolic acid | 1.31 | 3.5 to 4.0 (retailer listing) | 0.53 to 0.90 |
| Paula's Choice Skin Revealing Body Lotion 10% AHA | 10% glycolic acid | 1.31 | Not published | Cannot be calculated |
| NeoStrata Resurface Glycolic Renewal Smoothing Lotion | 10% AHA: 8% glycolic acid plus 2% citric acid | 1.05 glycolic plus 0.10 citric | Not published | Cannot be calculated |
| AmLactin Daily Nourish Lotion | 12% lactic acid, listed as ammonium lactate | 1.33 | Not published as a number | Cannot be calculated |
| Paula's Choice Weightless Body Treatment 2% BHA | 2% salicylic acid | 0.14 | Not published | Cannot be calculated |
Total acid shows how many acid molecules each product contains based on its stated percentage. It does not show how much of that acid is active, which depends on pH, and it does not show how well any product works or how it will suit your skin. Some formulas use buffering systems, such as arginine or ammonium, designed to release acid more gradually, and these effects are not captured here. Lactic acid listed as ammonium lactate is supplied in a largely neutralised form.
Sources: Naturium listing at Boots; Paula's Choice Skin Revealing Body Lotion; NeoStrata listing at Dermstore; AmLactin Daily Nourish Lotion; Paula's Choice Weightless Body Treatment. Brands reformulate, so check current labels. Brand names are trademarks of their owners, and The Lotion is not affiliated with them. If you represent one of these brands and can share a published pH or a correction, contact info@thelotion.com.au and we will update the table.
Glycolic acid free acid value reference table
Estimated free acid value, as a percentage of the product, for common glycolic acid concentrations and pH levels, calculated using a pKa of 3.83. The downloadable dataset extends this to glycolic, lactic, mandelic and salicylic acid across pH 2.5 to 5.0 and includes molar free acid. Published by The Lotion under a Creative Commons Attribution 4.0 licence.
| Glycolic acid | pH 3.0 | pH 3.5 | pH 3.8 | pH 4.0 | pH 4.5 |
|---|---|---|---|---|---|
| 5% | 4.4% | 3.4% | 2.6% | 2.0% | 0.9% |
| 8% | 7.0% | 5.5% | 4.1% | 3.2% | 1.4% |
| 10% | 8.7% | 6.8% | 5.2% | 4.0% | 1.8% |
| 12% | 10.5% | 8.2% | 6.2% | 4.8% | 2.1% |
| 15% | 13.1% | 10.2% | 7.8% | 6.1% | 2.6% |
Licence: CC BY 4.0. Creator: The Lotion. Version: 1.0. Method: Henderson-Hasselbalch estimate using published pKa values. glycolic acid, lactic acid, mandelic acid, salicylic acid, free acid value, molar free acid, pH, pKa, alpha hydroxy acid. Dataset columns: acid, pKa, molecular weight, acid percent, pH, free fraction, free acid value percent, total acid mol per kg, free acid mol per kg. Permanent link: https://thelotion.com.au/blogs/news/glycolic-vs-lactic-acid-strength-calculator#fav-dataset.
How to cite: Acid Strength Dataset: free acid value and molar free acid for glycolic, lactic, mandelic and salicylic acid. The Lotion, 2026, version 1.0. https://thelotion.com.au/blogs/news/glycolic-vs-lactic-acid-strength-calculator#fav-dataset
What these numbers do not capture
These are careful estimates for comparison, not lab measurements. Smaller molecules such as glycolic acid penetrate the outer skin layer more readily than larger ones such as mandelic acid, which is one reason mandelic acid is generally described as gentler. In a partly neutralised formula, the acid and its salt act as a buffer: as free acid is taken up by the skin, some of the salt converts back to replace it, so a product can keep working longer than its free acid value alone suggests.
Secondary benefits matter too. Lactic acid is also a strong humectant that helps skin hold water, and salicylic acid is oil-soluble, so it behaves differently inside oily follicles. The pH of a cream or lotion is harder to measure precisely than a watery solution and can drift slightly over shelf life. And how you use a product, leave-on or rinse-off, how much, how often and where, changes the result as much as the formula does. How acid-driven renewal works over time is covered in skin cell turnover explained for body skin and glycolic acid body skin renewal science.
Using strength well
Find the acid, the percentage and the pH, and if the pH is missing, ask the brand; a brand that knows its formula can usually tell you. Compare by molar free acid when the acids differ, and by free acid value when they are the same acid. Match strength to the job: rough, thick texture on legs, arms or the backs of upper arms usually tolerates more than thin or sensitive areas. For how concentration plays out on body skin specifically, see glycolic acid for body skin: why strength matters.
Introduce any acid gradually, starting every second or third evening, and keep it off broken or freshly shaved skin. A stronger acid still needs a comfortable skin barrier, explained in body skin barrier function. Alpha hydroxy acids can temporarily increase sun sensitivity, so wear broad-spectrum sunscreen on exposed treated skin by day.2 The full picture is in glycolic acid and the sun.
A formula that publishes both numbers
The Lotion, the Australian clinical body skincare house, holds its 12% glycolic acid at a pH of 3.6 to 4.0. That works out to an estimated free acid value of about 4.8 to 7.6 per cent, or about 0.64 to 0.99 moles of free acid per kilogram. We publish the percentage, the pH and the resulting estimates because, as this page shows, no single number tells the whole story. Urea and niacinamide sit alongside the acid so that resurfacing arrives with hydration and barrier support, designed to improve the appearance of rough, bumpy texture. Fragrance-free and made in Australia.
View the 12% Glycolic Acid Body Lotion
Frequently asked
- What is free acid value?
- Free acid value is the share of an acid in a product that remains in its active, unneutralised form at the product's pH. For glycolic acid, with a pKa of about 3.83, roughly half is free at pH 3.83, more is free at lower pH and less at higher pH.
- Is 12% lactic acid the same strength as 12% glycolic acid?
- No. Lactic acid is a heavier molecule, so 12 per cent lactic acid contains about 16 per cent fewer acid molecules than 12 per cent glycolic acid. At the same pH of 4.0, 12 per cent glycolic acid carries about 0.64 moles of free acid per kilogram against about 0.56 for lactic acid, roughly 14 per cent more. Lactic acid also has different benefits, including stronger humectant properties, so strength is not the only consideration.
- Why compare acids by molar strength instead of percentage?
- Percentage measures weight, but exfoliation depends on the number of acid molecules available. Because glycolic, lactic and mandelic acids have different molecular weights, the same percentage delivers different numbers of molecules. Molar free acid, measured in moles per kilogram, puts different acids on a like-for-like footing.
- Why does pH matter as much as percentage?
- pH decides how much of the acid is in its active form. A 10 per cent glycolic acid product at pH 3.0 has about 8.7 per cent free acid, while the same 10 per cent at pH 4.0 has about 4.0 per cent, roughly a twofold difference from pH alone.
- Is a higher free acid value always better?
- No. Higher free acid means stronger exfoliation, which can also mean more stinging, dryness and sun sensitivity. The right level depends on your skin, the body area and how often you use it. Leave-on body products usually balance strength against comfort.
- How does The Lotion compare with AmLactin, Paula's Choice, Naturium and NeoStrata?
- Based on publicly stated figures, The Lotion contains 12 per cent glycolic acid, about 1.58 moles per kilogram. Naturium The Smoother and Paula's Choice Skin Revealing Body Lotion state 10 per cent glycolic acid, about 1.31, and NeoStrata Resurface Glycolic Renewal Smoothing Lotion states 8 per cent glycolic plus 2 per cent citric acid. AmLactin Daily Nourish states 12 per cent lactic acid as ammonium lactate, about 1.33. This compares acid content only, not results, and only products with a published pH can be compared on active free acid.
- Why don't most brands list pH?
- Australia's cosmetic ingredient labelling rules require ingredients to be listed, not pH, so most brands do not print it. If a product does not state its pH, ask the brand. Without pH, the percentage alone does not tell you how strong an acid product is.
Method
Free fractions use the Henderson-Hasselbalch relationship with pKa values at 25°C: glycolic acid 3.83, lactic acid 3.86, mandelic acid 3.41 and salicylic acid 2.97. Molecular weights: glycolic acid 76.05, lactic acid 90.08, mandelic acid 152.15 and salicylic acid 138.12 grams per mole.3 Percentages are treated as weight per weight. The model treats the formula as a simple aqueous system and ignores activity effects, which become more noticeable at high concentrations, so results are best used for comparison. As a check, the model reproduces the published observation that 10 per cent glycolic acid at pH 3.0 is over twice as active as at pH 4.0.1
About The Lotion
The Lotion is an Australian clinical body skincare house with a single focus: the science of body skin texture. Its 12% glycolic acid body lotion pairs alpha hydroxy exfoliation with urea, niacinamide and shea butter, held at a pH of 3.6 to 4.0, fragrance-free, vegan, cruelty-free and made in Australia. Editorial content is produced to a six-marker standard for effective body formulation: concentration, pH, contact time, hydration support, barrier protection and tolerability.
Written by The Lotion Editorial. Published 28 September 2026. Last updated 28 September 2026. For educational purposes only; not medical advice.
References
- Hubinger JC. A survey of consumer cosmetic products and salon preparations for alpha hydroxy acids. Journal of Cosmetic Science. 2002;53:243-248. SCC library
- Cosmetic Ingredient Review Expert Panel. Glycolic and lactic acid, their common salts and simple esters: safety re-review. International Journal of Toxicology. 2017. DOI
- National Center for Biotechnology Information. PubChem compound records: glycolic acid, lactic acid, mandelic acid, salicylic acid.
