Sunbathing Calculator

Estimate how long you can sunbathe safely before burning, based on your Fitzpatrick skin type, today's UV Index, and your sunscreen's SPF.

Quick Facts

Formula
Unprotected burn time = skin-type base time ÷ UV Index
With sunscreen, that time is multiplied by the SPF value.
Reapplication
Every 2 hours, sooner after swimming or heavy sweating
Recommended regardless of SPF, since sunscreen wears off physically.

Your Results

Calculated
Unprotected burn time
-
Bare skin at today's UV Index
Protected burn time
-
With sunscreen applied at rated SPF
Reapply sunscreen by
-
Whichever comes first
Remaining safe time today
-
After time already spent in sun

Ready

Set your skin type, UV Index, and sunscreen SPF, then press Calculate.

About the Sunbathing Calculator

This tool estimates how long you can stay in the sun before your skin is likely to burn, using the standard skin-type / UV Index burn-time model that underlies most sunburn-time guidance. It combines your Fitzpatrick skin type, today's UV Index, and your sunscreen's SPF rating to produce both an unprotected and a protected safe-exposure estimate, plus a reapplication reminder.

The formula

Each Fitzpatrick skin type has a widely used approximate baseline: the time it takes bare skin to show minimal reddening (erythema) at UV Index 1. Because burn risk scales roughly linearly with UV Index, the unprotected time at any UV Index is:

  • Unprotected burn time = base time at UV Index 1 ÷ current UV Index.

The baseline times used here are approximately 67 minutes for Type I, 100 for Type II, 200 for Type III, 300 for Type IV, 400 for Type V, and 500 for Type VI. Sunscreen's Sun Protection Factor (SPF) is defined as the multiple of unprotected time that properly applied sunscreen allows before skin reddens, so:

  • Protected burn time = unprotected burn time × SPF.

The calculator then subtracts any minutes you say you have already spent in the sun today from whichever time applies (protected if you entered an SPF above 0, unprotected otherwise) to show your remaining safe window, and caps its reapplication recommendation at 2 hours — the standard dermatological guidance — regardless of how high the SPF math runs.

Why "properly applied" matters

The SPF multiplier assumes sunscreen is applied at about 2 milligrams per square centimeter of skin — roughly a full shot glass (30 mL, about 1 ounce) for an average adult body per application. Most people apply far less in practice, which can cut real-world protection well below the number on the bottle. The calculator's protected-time estimate assumes correct, even application.

Practical context

This is a planning estimate, not a medical guarantee. Cloud cover, time of day, altitude, reflective surfaces (sand, water, snow), medications that increase photosensitivity, and individual variation all affect real burn risk. Use the result as a starting point, reapply sunscreen on schedule, seek shade during peak UV hours, and stop before you expect to reach your estimated limit.

Frequently Asked Questions

What formula does this sunbathing calculator use?
It uses the standard skin-type / UV Index burn-time model: an unprotected minimal-erythema (sunburn) time is looked up for your Fitzpatrick skin type at UV Index 1, then divided by today's UV Index. If you apply sunscreen, that unprotected time is multiplied by the SPF value, since SPF is defined as how many times longer properly applied sunscreen lets skin tolerate UV exposure before reddening.
What are the Fitzpatrick skin types?
The Fitzpatrick scale classifies skin by how it reacts to sun exposure, from Type I (very fair, always burns, never tans) to Type VI (deeply pigmented, never burns). Lower types have shorter baseline unprotected burn times and higher sun sensitivity; higher types tolerate longer unprotected exposure before reddening.
Why does the calculator still tell me to reapply sunscreen even at high SPF?
Dermatology guidance recommends reapplying broad-spectrum sunscreen at least every two hours, and immediately after swimming, heavy sweating, or towel-drying, regardless of the SPF number. Sunscreen wears off from friction and water long before the SPF math alone would predict, so the calculator caps its reapplication guidance at two hours.
How accurate is the safe sunbathing time estimate?
It is a planning estimate, not a medical guarantee. Real burn risk also depends on sunscreen application thickness and evenness, cloud cover, time of day, reflective surfaces like sand water or snow, altitude, medications, and individual skin sensitivity. Use the result as a starting point and reapply sunscreen and seek shade well before the estimated time runs out.

Related calculators