SunGuard

SPF Protection Calculator

SPF × baseline burn minutes = estimated protection window.

SPF rating

Unprotected burn time (min)

Protected time (minutes)

450

Reapply every 2 hours regardless — sweat and water rapidly reduce protection.

Protected time (hours): 7.5

What you entered

SPF rating: 30Unprotected burn time (min): 15

1
SPF multiplier applied to unprotected burn time
30 × 15 min= 450 min

Result

Protected time (minutes): 450

SPF 30 on skin that burns in 15 minutes theoretically protects for about 450 minutes — but reapply every 2 hours regardless.

How long sunscreen protects you by SPF

SPF multiplies the time your skin takes to burn unprotected. If you would burn in 10 minutes, SPF 30 theoretically extends that to 300 minutes.

SPF	Time to burn unprotected	Theoretical protection	Hours
15	10 min	150 min	2.5 h
30	10 min	300 min	5.0 h
50	10 min	500 min	8.3 h
100	10 min	1,000 min	16.7 h
30	15 min	450 min	7.5 h
50	20 min	1,000 min	16.7 h

These are laboratory multiples and real protection is far shorter. Dermatologists advise reapplying every two hours regardless of SPF, and immediately after swimming or sweating. SPF 15 blocks about 93% of UVB, SPF 30 about 97%, and SPF 50 about 98%, so the numbers rise much faster than the actual protection. General wellness information only, not medical advice. Discuss any health decision with a qualified clinician.

What the SPF number actually multiplies

SPF is a multiplier on how long it takes your unprotected skin to burn — if you'd normally burn in 10 minutes, SPF 30 theoretically extends that to roughly 300 minutes, though real-world factors shrink this considerably.

Why reapplication still matters

Sweat, water, and simple time degrade sunscreen's effectiveness well before the theoretical window runs out — dermatologists generally recommend reapplying every 2 hours regardless of what the SPF math suggests.

Frequently asked questions

Is SPF 50 twice as good as SPF 25?

Not in practice. SPF 15 blocks 93% of UVB rays, SPF 30 blocks 97%, SPF 50 blocks 98%, and SPF 100 blocks 99%. The jump from 30 to 50 is only 1% more protection. SPF 30 is sufficient for most people; the bigger factor is applying enough (most people use only 25–50% of the recommended amount) and reapplying every 2 hours.

How often should I reapply sunscreen?

Every 2 hours when outdoors, regardless of SPF. Reapply immediately after swimming, sweating heavily, or toweling off. Even 'water-resistant' sunscreen lasts only 40–80 minutes in water. For a full day outdoors, expect to use about 1 oz (a shot glass full) per application for full body coverage.

Does sunscreen prevent tanning?

It reduces tanning but doesn't prevent it entirely. SPF 30 still lets through 3% of UVB rays, and most sunscreens offer incomplete UVA protection (UVA causes tanning and aging). No sunscreen blocks 100% of UV rays. If you're tanning through sunscreen, you're still getting UV damage — a tan itself is a sign of DNA damage.