Does SPF 30 block 30 times more UV than bare skin?

No. SPF is a ratio, and the fraction blocked is 1 - 1/SPF. For SPF 30 that is about 96.7%; for SPF 50, about 98%. The two labels differ by roughly 1.3 percentage points of UVB.

Why does my sunscreen seem to underperform?

Most often because less than the tested dose was applied. SPF values are determined at 2 mg/cm², which is roughly 1.2-2 g for a face. Applied thinner, the effective protection drops well below the label.

Does SPF tell me anything about UVA?

No. SPF describes UVB, which covers about 280-320 nm. UVA covers about 320-400 nm and is addressed by a broad spectrum claim, not by the SPF number.

Is a soap bar bad for the skin barrier?

A soap bar sits around pH 9-10, while skin surface is about pH 4.5-5.5. It shifts the surface pH during washing; mild syndet cleansers near pH 5.5 sit closer to the skin's own baseline.

SPF 30 blocks about 96.7% of UVB

Skinpu — Skin Barrier and SPF Basics | skinpu.com

An SPF number is a fraction, not a feeling: SPF 30 blocks roughly 96.7% of UVB and SPF 50 roughly 98%. The same test that produces those percentages assumes 2 mg of sunscreen per square centimetre of skin — about 1.2 to 2 g for a face.

The surface

Skin surface sits at about pH 4.5-5.5 — the acid mantle. Cleansers near pH 5.5 sit close to that baseline; soap bars around pH 9-10 push it upward. The stratum corneum holds about 20-30% water by weight, and normal transepidermal water loss stays under about 10 g/m²/h.

The spectrum

UVB covers 280-320 nm and UVA covers 320-400 nm. An SPF value describes UVB blocking only, computed as 1 - 1/SPF: about 96.7% for SPF 30 and about 98% for SPF 50. UVA is a separate question, addressed by a broad spectrum claim rather than by the SPF number.

The dose

SPF is determined at 2 mg/cm². Scaled to a face, that is roughly 1.2-2 g of product. Applied below that dose, the effective protection falls well under the label figure — which is why the grams matter as much as the number printed on the bottle.

SPF 30 blocks about 96.7% of UVB; SPF 50 about 98%. The gap between the labels is smaller than the numbers suggest.

The test assumes 2 mg/cm². For a face, that is roughly 1.2-2 g — measured, not guessed.

What does SPF 30 actually block?

SPF 30 blocks about 96.7% of UVB under the test conditions — the number is a fraction, not a shield rating.

The arithmetic is 1 - 1/SPF. For SPF 30 that is 1 - 1/30, or roughly 0.967 — about 96.7% of UVB blocked. For SPF 50 it is 1 - 1/50, about 98%. The difference between the two labels is only around 1.3 percentage points of UVB, even though the numbers look far apart.

SPF is defined against UVB, which covers roughly 280-320 nm of the spectrum. UVA covers about 320-400 nm and is not described by the SPF number at all. A sunscreen that only carries an SPF value tells you nothing about UVA from the label alone — that is what a broad spectrum claim addresses.

All of this holds only at the tested dose: 2 mg of product per square centimetre of skin. Applied thinner, the effective SPF falls well below the printed number. The label is a laboratory figure, not a promise about how a person actually applies the bottle.

SPF 30 → 1 - 1/30 → about 96.7% UVB blocked
SPF 50 → 1 - 1/50 → about 98% UVB blocked
UVB range: 280-320 nm; UVA range: 320-400 nm
Test dose: 2 mg/cm² across the tested area

Figures from the notes

Diagrams and tables used across the reference: the pH scale, the UV bands, the test dose conversion, and the water-loss cross-section.

The key terms of this guide, drawn to one scale

Reference values

Key barrier and SPF values used throughout this reference
Quantity	Typical value	Unit	What it describes
Skin surface pH	4.5-5.5	pH	The acid mantle on intact skin
Mild syndet cleanser pH	about 5.5	pH	Close to the skin's own surface
Traditional soap bar pH	9-10	pH	Alkaline; shifts surface pH during washing
Stratum corneum water content	20-30	% by weight	Water held in the outer layer
Transepidermal water loss	under about 10	g/m²/h	Normal rate; rises when the barrier is disrupted
Sunscreen test dose	2	mg/cm²	Amount assumed when SPF is determined
Sunscreen for a face	1.2-2	g	Conversion of the test dose to a typical face
UVB blocked by SPF 30	about 96.7	%	From 1 - 1/SPF at the test dose

Key barrier and SPF values used throughout this reference

Questions that come up first

Does SPF 30 block 30 times more UV than bare skin?: No. SPF is a ratio, and the fraction blocked is 1 - 1/SPF. For SPF 30 that is about 96.7%; for SPF 50, about 98%. The two labels differ by roughly 1.3 percentage points of UVB.
Why does my sunscreen seem to underperform?: Most often because less than the tested dose was applied. SPF values are determined at 2 mg/cm², which is roughly 1.2-2 g for a face. Applied thinner, the effective protection drops well below the label.
Does SPF tell me anything about UVA?: No. SPF describes UVB, which covers about 280-320 nm. UVA covers about 320-400 nm and is addressed by a broad spectrum claim, not by the SPF number.
Is a soap bar bad for the skin barrier?: A soap bar sits around pH 9-10, while skin surface is about pH 4.5-5.5. It shifts the surface pH during washing; mild syndet cleansers near pH 5.5 sit closer to the skin's own baseline.