Glycolic Acid Strength Calculator
The percentage on a glycolic acid label is only half the story. Two products with the same listed concentration can have wildly different effects on your skin depending on their pH. A 10% glycolic acid product at pH 3.0 is roughly four times as potent as the same 10% at pH 4.0.
This tool does two things: it calculates the free acid value of any glycolic acid product using the Henderson-Hasselbalch equation, and it provides a personalized concentration recommendation based on your skin type, experience level, and goals. All calculations happen in your browser - we do not collect or store your inputs.
Answer four quick questions about your skin and we will suggest the right glycolic acid concentration, pH range, product type, and build-up schedule for you.
Enter any glycolic acid product's concentration and pH to find out how much active acid is actually working on your skin. The labeled percentage is only half the story - pH determines the rest.
The percentage listed on the product label
Typical range: 2.0 - 5.0
How Free Acid Value Works
Glycolic acid (C2H4O3) is a weak acid with a pKa of 3.83. In any solution, it exists in two forms: the un-ionized form (HA), which can penetrate the stratum corneum and actively exfoliate, and the ionized form (A-), which carries a charge and cannot penetrate as effectively.
The ratio between these two forms is governed by the Henderson-Hasselbalch equation:
Rearranging to solve for the fraction of free (un-ionized) acid:
This means that at pH = pKa (3.83), exactly 50% of the glycolic acid is in its free form. For every full pH unit below the pKa, the free acid fraction increases roughly tenfold. Conversely, each pH unit above the pKa reduces the free acid by about 90%.
The free acid value is simply the labeled concentration multiplied by the free acid fraction. A 10% glycolic acid product at pH 3.5 has a free acid fraction of about 0.68, so its free acid value is approximately 6.8% - meaning only 6.8% of the product is actively working on your skin.
Common Concentration Ranges
Typical glycolic acid concentrations and pH values across different product types. The free acid column shows the effective strength after accounting for pH.
| Product Type | Typical % | Typical pH | Free Acid |
|---|---|---|---|
| Cleanser | 5-10% | 3.5-4.0 | 2-6% |
| Toner | 5-8% | 3.5-3.8 | 3-5% |
| Serum | 10-15% | 3.0-3.5 | 6-12% |
| Professional peel | 30-70% | 1.5-3.0 | 25-65% |
Frequently Asked Questions
Medical Disclaimer
This calculator and the recommendations it provides are for educational and informational purposes only. They are not a substitute for professional medical advice, diagnosis, or treatment. The calculations are based on published chemical formulas and general dermatological guidelines, but they cannot account for individual skin conditions, medications, allergies, or other personal health factors. Always consult a board-certified dermatologist before starting a new active ingredient, especially if you have sensitive skin, rosacea, eczema, or any pre-existing skin condition. If you experience adverse reactions, discontinue use immediately and seek medical attention.
