# L-04 — The number nobody publishes

Every sunscreen on earth is tested at 2 mg/cm². Almost nobody applies 2 mg/cm². The gap is the whole product.

- **Canonical:** https://www.30plusskin.care/ledger/the-number-nobody-publishes
- **Site:** 30+/SKIN — Skincare, published as a spec sheet.
- **Issue:** ISSUE NO. 12 / REV. 2026.08
- **Standing terms:** INDEPENDENT · AFFILIATE-FUNDED · NOT MEDICAL ADVICE

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- **Published:** 2026-07-02
- **Reading time:** 6 min
- **Tags:** SPF, DOSE, METHOD

SPF is not a property of a formula. It is a property of a formula at a specified thickness. The thickness is 2 milligrams per square centimetre, it is written into the standard, and it is the reason a bottle can honestly say 50.

Observed application in the field lands between 0.5 and 1.0 mg/cm². Protection does not scale linearly with dose — it collapses faster than that. Half the dose is not half the SPF.

| Field | Value |
| --- | --- |
| LABEL DOSE | 2.0 MG/CM² |
| OBSERVED MEDIAN | 0.75 MG/CM² |
| LABEL SPF | 50 |
| EFFECTIVE SPF, ILLUSTRATIVE | ~8 |
| FACE + NECK AT LABEL DOSE | ≈ 1/3 TSP |

We print the dose on the pump collar rather than in the small type on the back. It is the only specification on the product that the user has to execute themselves, and it is the one that decides whether anything else in the routine is worth buying.

## What we changed

- Pump calibrated so that four full depressions equal the label dose for a face and neck.
- Tint instead of nano-milling, so under-application is visible as patchiness.
- Reapplication interval printed as a clock face, not a sentence.

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## Elsewhere in the register

- [Review register](/reviews)
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- [Evidence register](/evidence)
- [The ledger](/ledger)
- [How we test](/how-we-test)
- [Agent guide](/llms.txt)
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