Corneometry: the science that really measures your skin's hydration

All skincare products promise hydrated skin. Very few are willing to prove it with an instrument. Corneometry is one of the methods that turns a promise into a measurement, and it deserves to be understood, because it changes the way a moisturizer is judged.

Here is how it works, what it measures exactly, how a reliable test is carried out, and why we use it to validate the effect of our formulas.

What exactly is corneometry?

Corneometry assesses the hydration level of the outermost layer of the skin, the stratum corneum. It is based on a simple physical principle: water conducts electricity. The more water the skin surface contains, the more it changes the electrical behavior of a probe placed on it.

The device, called a corneometer, applies a probe to the test area for a few seconds. It measures capacitance, an electrical value directly linked to the water content of the surface. The method is painless, fast, and non-invasive, which makes it possible to repeat readings without altering the skin's condition.

Results are expressed in arbitrary units, which can be compared from one measurement to the next as long as the conditions remain the same. What matters is not the absolute value, but the difference between before and after, or between a treated area and a control area.

What it measures, and what it does not measure

This is the most important point in this article, because it separates honest information from misleading claims.

Corneometry measures the hydration of the upper layers of the epidermis. It provides information about the skin's surface condition, its immediate suppleness, and its comfort. It does not measure hydration in deeper layers, and no reputable brand should imply otherwise.

Skin whose surface is better hydrated looks smoother, feels less tight, and reflects light better. It is a real, visible, measurable effect, even if it is not a transformation of the deeper layers.

This distinction does not diminish the value of the result. On the contrary, it gives it its proper scope. A gain in surface hydration is exactly what you expect from a cleansing product or a body lotion: immediate and lasting comfort, where that tight, uncomfortable feeling begins.

Corneometry, TEWL: do not confuse the measurements

Corneometry is not the only instrumental method used in dermocosmetics. Another measurement comes up often: transepidermal water loss, abbreviated as TEWL. The two complement each other, but they do not say the same thing.

• Corneometry measures the amount of water present on the surface at a given moment. It answers the question: is the skin hydrated right now?
• Transepidermal water loss measures the rate at which water escapes through the skin. It answers a different question: is the skin barrier properly retaining moisture?

An ideal formula improves surface hydration while respecting the barrier that retains it. Understanding this nuance protects you from oversimplified claims: a hydration number alone does not tell you anything about the health of the skin barrier.

How a reliable measurement is carried out

The rigor of a corneometry test depends as much on the protocol as on the device. Several conditions are checked before each session, otherwise the result would lose all value.

• A prior acclimatization of the volunteers in a room with controlled temperature and humidity, to neutralize the environment's effect on the skin.
• A baseline measurement on a control area, before any application, serving as a reliable point of comparison.
• Readings taken at set intervals after application, for example immediately and then a few hours later, to track how the effect changes over time rather than at a single moment.
• A sufficient number of volunteers so the result does not depend on a few individual cases.

The improvement is calculated by comparing the treated area with the control area on the same volunteer. This internal comparison limits bias linked to differences in skin from one person to another and strengthens the reliability of the result.

What our measurements reveal

Our corneometry studies, conducted by an independent laboratory, show up to +36 % hydration of the upper layers of the epidermis, on panels of 12 to 22 volunteers depending on the product, in compliance with Regulation (EC) No 1223/2009.

This result mainly concerns our cleansing and nourishing products, those whose daily use justifies checking the hydrating effect.

• The Reddition Body Cleansing Gel and the Insurrection Hand Wash Gel illustrate a point that is often overlooked: a rinse-off product can leave skin more hydrated than it was before washing.
• The Elégance Cleansing Body Gel applies the same logic to skin that calls for extra softness.
• To extend this effect, the Nourishing Body & Hand Lotion Imminence and the Face Cream Resilience help maintain surface comfort throughout the day.

Why this measurement should matter in your choice

When you compare two hydrating products, the ingredient list tells you about the formula's intent, the price tells you about its positioning, and reviews tell you about other buyers' experiences. Corneometry adds data that none of these benchmarks provide: measured proof that the effect exists, and that it has been tested under controlled conditions.

That is why we integrated it into our approach, rather than settling for appealing wording. A word like hydrating becomes much more convincing when backed by a number.