The Bottom Line

SPF measures protection against UVB rays (sunburn). SPF 30 blocks 97% of UVB, SPF 50 blocks 98%. The difference between high SPF numbers is tiny — what matters far more is applying enough and reapplying every 2 hours. Most people apply only 25-50% of the recommended amount, effectively cutting their protection in half. "Broad spectrum" on the label means it also protects against UVA (aging) rays.

SPF Numbers Demystified

SPF stands for Sun Protection Factor. It's a measure of how much UVB radiation it takes to produce sunburn on protected skin compared to unprotected skin. Practical translation:

  • SPF 15: Blocks 93% of UVB
  • SPF 30: Blocks 97% of UVB (recommended minimum)
  • SPF 50: Blocks 98% of UVB
  • SPF 100: Blocks 99% of UVB

The jump from SPF 15 to 30 is meaningful (93% → 97%). The jump from 30 to 100 is just 2%. This is why dermatologists say SPF 30 is the practical sweet spot — higher numbers offer diminishing returns.

What SPF Doesn't Measure

  • UVA protection: SPF only measures UVB. UVA causes aging and contributes to cancer. Look for "broad spectrum" to ensure UVA coverage.
  • Duration: SPF 50 does NOT last longer than SPF 30. ALL sunscreens need reapplication every 2 hours.
  • Real-world performance: Lab testing uses 2 mg/cm² thickness. Most people apply 0.5-1.0 mg/cm². Your SPF 30 may function as SPF 10-15 in practice.

How to Apply Correctly

  • Amount for face + neck: 1/4 teaspoon (nickel-sized dollop)
  • Amount for full body: 1 ounce (shot glass full)
  • Timing: Apply 15 minutes before going outside (for chemical sunscreens). Mineral sunscreens work immediately.
  • Reapply: Every 2 hours outdoors, or immediately after swimming, sweating, or toweling off
  • Don't forget: Ears, back of neck, tops of feet, and scalp (if thin/bald)

Frequently Asked Questions

Is SPF in my makeup enough?

Only if you apply enough. Most people apply far less foundation than the 2 mg/cm² used in SPF testing. A light application of SPF 30 foundation might deliver only SPF 5-10. Use a dedicated sunscreen underneath; consider SPF makeup as a bonus layer, not your primary protection.

Do I need sunscreen indoors?

If near windows: yes, for UVA protection (UVA penetrates glass). If in a windowless room: not for UV, though those with melasma may benefit from blocking visible light from indoor lighting. Most people don't need sunscreen in windowless indoor spaces.

Can I use last year's sunscreen?

Check the expiration date. Sunscreens maintain their SPF for 3 years from manufacture if stored properly. Heat, direct sunlight, and leaving sunscreen in a hot car can degrade the UV filters before the expiration date. When in doubt, replace it.

  1. Gabros S, et al. "Sunscreens and photoprotection." StatPearls. 2023.
  2. Lim HW, et al. "Current challenges in photoprotection." JAAD. 2017;76(3S1):S91-S99.
  3. Ou-Yang H, et al. "High-SPF sunscreens (SPF ≥70) may provide ultraviolet protection above minimal recommended levels." JAAD. 2012;67(6):1220-1227.