Reference · Sunscreen
SPF protection chart
Exactly what percentage of UVB each SPF blocks — and why the jump from 30 to 50 is smaller than the marketing makes it sound.
SPF to percent-of-UVB-blocked, exactly
These figures are not estimates or brand claims. They come straight from the SPF definition, where SPF is the fraction of UVB that reaches skin: an SPF of N lets through 1/N of UVB and blocks the rest.
| SPF rating | % UVB blocked | % UVB that passes | What it means |
|---|---|---|---|
| SPF 2 | 50% | 1/2 (50%) | Effectively no meaningful protection |
| SPF 8 | 87.5% | 1/8 (12.5%) | Below the dermatology daily minimum |
| SPF 15 | 93.3% | 1/15 (≈6.7%) | Old minimum; fine indoors, low for outdoors |
| SPF 30 | 96.7% | 1/30 (≈3.3%) | The recommended daily minimum for most people |
| SPF 50 | 98% | 1/50 (2%) | Best for extended sun, fair skin, post-procedure |
| SPF 100 | 99% | 1/100 (1%) | Marginal gain; useful buffer for under-application |
✦The number on the bottle is the ceiling, not the floor. Apply too thin and SPF 50 can perform like SPF 15.
The one formula behind every SPF number
- SPF 30: (1 − 1/30) × 100 = 96.7%
- SPF 50: (1 − 1/50) × 100 = 98.0%
- SPF 100: (1 − 1/100) × 100 = 99.0%
No sunscreen reaches 100%, and no realistic SPF gets you meaningfully past 99%. That is why dermatologists emphasize broad-spectrum coverage, generous application, and reapplication over chasing ever-higher numbers.
Why your real SPF is lower than the label
- Under-application. SPF is tested at 2 mg/cm². Most people apply 25–50% of that, which roughly halves the SPF they actually get.
- Missed reapplication. Filters degrade and rub off; reapply every 2 hours outdoors, and after swimming or sweating.
- SPF ≠ UVA. SPF only rates UVB (burning). For aging-and-cancer UVA protection, the label must say broad spectrum.
This page presents the established SPF-to-UVB math for education. It is not medical advice. If you have a history of skin cancer, photosensitivity, or a specific condition, talk to a board-certified dermatologist about the right sun-protection plan for you.
Frequently asked
What percent of UV does SPF 30 block? +
SPF 30 blocks about 96.7% of UVB rays, meaning roughly 1/30 (about 3.3%) gets through. The math is fixed: percent blocked equals (1 minus 1 divided by the SPF number) times 100.
Is SPF 50 really better than SPF 30? +
Slightly. SPF 30 blocks about 96.7% of UVB and SPF 50 blocks about 98%, a difference of roughly 1.3 percentage points. The bigger real-world reason to choose SPF 50 is that almost everyone applies too little sunscreen, which sharply lowers the SPF you actually get, so a higher number gives you a buffer.
Does SPF measure UVA protection? +
No. SPF only measures UVB protection (the rays that cause sunburn). UVA rays, which drive aging and also contribute to skin cancer, are covered by the words "broad spectrum" on the label, not by the SPF number. Always choose a broad-spectrum product.
Why does doubling SPF not double protection? +
Because SPF is not a linear scale. Going from SPF 30 to SPF 60 does not double protection; it moves you from blocking about 96.7% to about 98.3% of UVB. Each increase past 30 adds smaller and smaller gains as you approach 100%.
How much sunscreen do I actually need? +
Most people apply only 25–50% of the tested amount, which roughly halves their real SPF. The standard guideline is about 1/4 teaspoon for the face and neck and about 1 ounce (a shot glass) for the whole body, reapplied every 2 hours outdoors.
Is a higher SPF safe to rely on all day? +
No SPF lasts all day. Even SPF 100 needs reapplication every 2 hours of sun exposure (or after swimming or sweating). Higher SPF buys margin for under-application, not freedom from reapplying. When in doubt, ask a dermatologist about the right routine for your skin and history.
Related: The sunscreen guide · How much sunscreen you need · Mineral vs chemical sunscreen · Sunscreen cost calculator · All reference charts