Sunscreen Calculator
Fair skin that burns in 15 minutes gets a theoretical 450 minutes (7.5 hours) of protection from SPF 30, but in practice you still reapply every 2 hours. This sunscreen calculator multiplies your skin type's unprotected burn time by the SPF to show the theoretical protection window, then reminds you why the 2-hour reapplication rule wins: sweat, water, and rubbing strip sunscreen long before the math runs out.
Quick answer
SPF multiplies the time your bare skin takes to burn. Burn in 15 minutes, and SPF 30 stretches that to 450 in theory.
Theoretical protection
450 minutes theoretical (reapply every 2 hours)
Protected hours
7.5
Unprotected burn time
15 min
Reapply every
120 min
What this tells you
- •SPF multiplies the time your bare skin takes to burn. Burn in 15 minutes, and SPF 30 stretches that to 450 in theory.
- •Skin type sets the starting point, from about 10 minutes for very fair skin to 60 for deeply pigmented skin.
- •The theoretical number assumes a perfect, full-thickness layer that never wears off. Real life falls far short.
- •Dermatologists' rule stands regardless of SPF: reapply every 2 hours, and after swimming or heavy sweating.
How to Use
- 1Pick your Fitzpatrick skin type. When in doubt, choose the fairer option.
- 2Enter the SPF on your sunscreen bottle.
- 3Read the theoretical protection time, then plan around the 2-hour reapplication rule anyway.
- 4Apply enough: a full shot glass (about 35 ml) covers an adult body, a quarter teaspoon covers the face.
How It Works
Formula
theoretical protection = unprotected burn time x SPFSPF is defined in lab conditions as the ratio of protected to unprotected burn time. Type II skin burning in 15 minutes with SPF 30 gives 15 x 30 = 450 minutes on paper. Labs apply 2 mg per square centimeter, roughly double what most people use, and a half dose can cut effective SPF by far more than half, which is why the reapplication rule beats the arithmetic.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Fair skin, SPF 30
15 minutes of natural protection times 30. Still reapply at the 2-hour mark.
Very fair skin, SPF 50
Even the highest common SPF starts from just 10 unprotected minutes for Type I skin.
Olive skin, SPF 15
A darker skin type with a modest SPF matches fair skin with a high one, on paper.
Unprotected Burn Time by Skin Type
Approximate minutes to burn in strong midday sun with no sunscreen.
| Fitzpatrick type | Description | Burn time |
|---|---|---|
| I | Very fair, always burns, never tans | 10 min |
| II | Fair, burns easily, tans minimally | 15 min |
| III | Medium, sometimes burns | 20 min |
| IV | Olive, rarely burns | 30 min |
| V | Brown, very rarely burns | 40 min |
| VI | Dark brown or black | 60 min |
Common mistakes
- Trusting the theoretical time. Sunscreen rubs, sweats, and washes off, so the 2-hour reapplication rule applies whatever the math says.
- Applying too little. Most people use half the tested dose, which drops an SPF 30 to an effective 10 or less.
- Skipping reapplication after swimming. Water resistant means 40 or 80 minutes in water, not the whole afternoon.
- Assuming dark skin needs no sunscreen. Type V and VI burn less but still take UV damage and need protection.