How Skin Tone Is Measured: The Scale Beauty Uses Was Built for Sunburn

Ask a brand what skin tone a shade is for and you will often get a Fitzpatrick number. Ask what Fitzpatrick measures and the answer is not color. It is how your skin behaves in sunlight, which is a different property, measured for a different purpose, by a scale that says so in its own definition.

There is a system that does measure skin color as a number, used in dermatology research. Knowing which one you are being handed is the difference between a shade recommendation with something behind it and one with a borrowed label on it.

How skin tone is measured in the clinic, and what that scale is for

DermNet's definition, in full: The Fitzpatrick skin type (or phototype) describes a way to classify the skin by its reaction to exposure to sunlight.

Reaction to sunlight. Not shade, not depth, not undertone.

The scale does involve pigment, but through that lens: The Fitzpatrick skin type (or phototype) depends on the amount of melanin pigment in the skin. This is determined by constitutional colour (white, brown, or black skin) and the effect of exposure to ultraviolet radiation (tanning).

So constitutional color is an input, and what comes out the other end is a burn-and-tan category. Two people whose skin looks identical in a mirror can land in different phototypes if one burns and the other tans, and two people in the same phototype can need different foundation shades.

DermNet also notes that Fitzpatrick skin phototype is a constitutional characteristic present at birth, which is worth knowing because it means a summer tan does not move you up the scale, even though it certainly moves your foundation shade.

Is Fitzpatrick a color scale?

No, and the mismatch has a practical shape.

It was designed to answer a clinical question: how much ultraviolet exposure this skin tolerates, which is what dosing phototherapy and advising on sun protection requires.

It is a six-step ordinal scale, which is far too coarse for foundation, where a single brand routinely offers thirty or forty shades and the difference between two adjacent ones is visible on a jaw.

It says nothing about undertone at all. Warm, cool, neutral and olive are not phototype properties, and no Fitzpatrick number distinguishes them. The popular substitutes do not work either, for reasons covered in why undertone tests fail.

Using it as a color scale answers a question the instrument was not built to answer, which is why so much shade advice organized around it fails in the mirror.

What is the individual typology angle?

Skin color is measurable, and dermatology research does measure it, using colorimetry rather than categories.

The color space is CIE L*a*b*, which describes any color with three coordinates: L* for lightness, a* on a red to green axis, and b* on a yellow to blue axis. A colorimeter or spectrophotometer reads those values off skin directly.

From two of those coordinates, researchers compute the individual typology angle, usually written ITA. It is derived from lightness and the yellow-blue value with an arctangent, producing an angle in degrees: higher angles correspond to lighter skin, lower and negative angles to deeper skin. The commonly used bands run from very light through light, intermediate, tan and brown to dark, with the boundaries placed at specific degree values.

Marking the sourcing on this, because it matters. The definition and the category boundaries appear in the dermatology and cosmetic-science literature, and the papers I could find are on publisher sites that cannot be read without a subscription. So I am describing ITA rather than quoting it, and you would not be able to check a quotation against those pages. The Fitzpatrick material above is quoted because DermNet is readable by anyone.

What ITA gives you that a phototype does not: a continuous number rather than six buckets, derived from an instrument rather than from a questionnaire about how you burn.

How skin tone is measured without an instrument

You do not have a spectrophotometer, so the question becomes what a reasonable substitute looks like.

Depth and undertone are separate readings. Depth is how light or deep the skin is, roughly what ITA quantifies. Undertone is the hue underneath, which is closer to the a* and b* axes and is not captured by any phototype number. Treating them as one variable is the most common shade-matching error, and skin tone vs undertone is a different question again from skin type, which describes oil and sensitivity rather than color.

Daylight, no makeup, and the jaw rather than the wrist. Foundation has to match the face it sits on, and forearm skin is a different color from facial skin on most people.

Compare against something known. Three candidate shades side by side on the jawline, blended, viewed in daylight, beats any single swatch and beats any of the usual undertone tests.

Season moves depth, not undertone. A tan changes where you sit on the depth axis while leaving the hue underneath alone, which is why people who buy one shade a year end up wrong for half of it. Where the vocabulary fits worst is at the deep end, covered in foundation shade for deep skin.

Why any of this belongs on a skincare site

Because two things that follow from skin color are documented, and both change what you should do rather than what you should buy.

Pigmented marks after inflammation are not evenly distributed. DermNet notes that postinflammatory hyperpigmentation is mostly observed in darker skin types, and DermNet's phototype page makes the same point from the other side: darker skin is also more prone to develop postinflammatory pigmentation after injury (brown marks).

That is why prevention outranks correction for anyone in that group, and why the calculation on picking at a spot or choosing an aggressive treatment is genuinely different, as dark spots after acne sets out.

Baseline pore appearance differs too. DermNet: Certain ethnic groups may have larger pores, particularly those of African and Indian ancestry. A starting point rather than a problem, and a reason that comparing your skin to someone else's tells you very little.

What a shade recommendation needs to be worth anything

A short test for any advice that claims to match you:

1. Does it distinguish depth from undertone? If not, it is working with one variable where there are at least two.
2. Is it using a phototype as a color? Then it is using a sunburn scale to pick a foundation.
3. Does it account for the product changing after application? Foundation darkening is real, measured, and caused by something other than what it is named after.
4. Was the reading taken on the face, in daylight? Wrist swatches and store lighting are two of the most reliable ways to buy the wrong shade.
5. Does it acknowledge that it cannot see you? No article, quiz or chatbot can measure your skin. What writing can do is tell you which comparisons are worth making.

FAQ

How is skin tone measured? In research, with colorimetry: skin is read in the CIE L*a*b* color space and reduced to an individual typology angle, a continuous number derived from lightness and the yellow-blue axis. In consumer contexts it is usually described with the Fitzpatrick phototype, which classifies skin by its reaction to sunlight rather than by color.

Is the Fitzpatrick scale a color scale? No. DermNet defines it as a way to classify skin by its reaction to exposure to sunlight. Constitutional color is one input, but the output is a burn-and-tan category with six steps, and it says nothing about undertone.

What is the individual typology angle? A number computed from two coordinates of the CIE L*a*b* color space, lightness and the yellow-blue value, giving an angle in degrees where higher means lighter. It sorts skin into bands from very light to dark and is used in dermatology research because it is continuous and instrument-based.

What is the difference between skin tone and undertone? Depth is how light or deep the skin reads and is what ITA quantifies. Undertone is the hue underneath, warm, cool, neutral or olive, and no phototype number captures it. They move independently, which is why a tan changes your shade but not your undertone. Olive is the awkward case, because ITA is computed without the red-green axis it sits on: olive undertone works through what that leaves out.

How do I find my skin tone at home? Bare skin, daylight, on the jawline rather than the wrist, comparing at least three candidates side by side. Repeat it seasonally, because depth moves with sun exposure.

Does my skin tone change what skincare I need? Not the products so much as the priorities. Postinflammatory hyperpigmentation is mostly observed in darker skin types, so prevention carries more weight, and the risk calculation on aggressive treatments differs.

---

The useful correction here is narrow and load-bearing: how skin tone is measured depends on which question is being asked, and the number the beauty industry borrowed answers a question about sunlight. If a shade recommendation is built on a phototype, it is built on a scale that was never measuring the thing you are trying to match.