The Lotion Standard · Marker Two in depth

The half-told number: why a glycolic percentage means nothing without pH

A figure on a jar tells you how much acid is in the bottle. It does not tell you how much of that acid is awake. Those are two very different numbers, and only one of them does anything to your skin.

Walk the body care aisle and the percentages shout at you. Ten per cent. Twelve. Fifteen. They are printed large because they are easy to compare and easy to believe: more must mean stronger, stronger must mean better. It is a tidy story. It is also, on its own, close to meaningless, and the reason is a piece of chemistry that almost no label mentions.

An alpha hydroxy acid such as glycolic does its work only when it is in its free acid form, the form that is chemically reactive at the skin's surface. But in a finished lotion, not all of the acid stays free. Depending on how the formula is balanced, much of it can be converted into a neutral, inactive salt. What decides the split is pH.

What pH is actually doing

Every acid has a tipping point, a pH at which exactly half of it sits in its active free form and half in its neutral inactive form. For glycolic acid that point is around pH 3.8.¹ Raise the formula's pH above that and the balance tips toward inactive: the acid is still in the bottle, still on the ingredient list, but increasingly asleep. Lower it well below that and almost all of the acid is awake at once, which is potent but can be needlessly harsh on large areas of body skin.

The practical consequence is a working window. For glycolic acid on the body, that window sits in the region of pH 3.5 to 4.0: low enough that a meaningful fraction of the acid is active and doing its job, controlled enough that it remains tolerable across the arms, thighs and torso rather than just a small patch of face.²

The free acid value: the number that should be on the jar

Put concentration and pH together and you arrive at the figure that genuinely matters: the free acid value. It describes not how much acid is present, but how much of it is actually available to act. It is why a lower percentage at the right pH can comfortably outperform a higher percentage formulated badly, and why two lotions can carry the same proud number on the front and behave like entirely different products.

Consider two lotions, each labelled the same:

Same number on the front. Different chemistry behind it. Only one is doing the work the label implies.

Why lower is not automatically gentler, and higher is not automatically better

This is where the percentage-only mindset misleads in both directions. A high number at a careless pH can be either harsh or hollow. A modest number at a considered pH can be quietly effective. The honest way to read an acid is never the percentage alone; it is the percentage understood through its pH. A formula that discloses one without the other is asking to be judged on half the evidence.

A percentage is a promise. A pH is whether the promise was kept.

This is the second of the six markers in the six-marker clinical body lotion standard: a working pH that keeps the acid active. The Lotion's 12% glycolic acid body lotion is formulated within that window and pairs the acid with urea, niacinamide and shea so that the active fraction resurfaces while the barrier is supported, rather than relying on a large number alone.

If the percentage on the label has ever led you astray, it is worth understanding why acid strength is so often misread on body skin, where surface area and barrier condition change what a given number actually does.

Concentration is the other half of this story. If pH decides how much acid is awake, the percentage decides how much is there to begin with, and the two are read together rather than in isolation. It is worth understanding why a higher strength is not automatically a better lotion, and why the terrain it lands on, explored in how body skin differs from the face, changes what any number actually does.

Updated May 2026