Sunscreen Calculator
Enter your skin type, the UV index, your SPF level, and which body areas you are covering. The calculator tells you how long your sunscreen protects you, exactly how many milliliters to apply per session, how many times to reapply during the day, and how many bottles to pack for a trip. All math is based on the 2 mg/cm2 dermatological testing standard and the WHO UV index model.
Why the 2 mg/cm2 standard matters
When sunscreen manufacturers certify an SPF rating, they apply exactly 2 milligrams of product per square centimeter of skin during laboratory testing. Most people apply far less, often 0.5-1 mg/cm2, which means their real-world protection is much weaker than the label says. At 0.5 mg/cm2, an SPF 30 product delivers roughly SPF 5-7. This calculator uses the full 2 mg/cm2 standard so you know the correct amount to apply to actually get the protection you are paying for.
How SPF protection time is calculated
SPF stands for Sun Protection Factor. Your unprotected skin has a minimum erythemal dose (MED), the amount of UV radiation that causes the first visible redness. At UV index 1, a very fair skin type (Fitzpatrick I) reaches MED in about 67 minutes; a deep skin type (Fitzpatrick VI) can take over 600 minutes. As the UV index rises, these times shorten proportionally. Applying SPF 30 multiplies the time by 30, theoretically extending protection 30-fold. However, no sunscreen is 100 percent absorbed, and sweating, rubbing, and swimming all reduce effectiveness, which is why reapplication every 2 hours is a standard recommendation regardless of how high the theoretical protection time calculates out.
Reapplication: the rule most people miss
Even if your SPF mathematically protects for 4 or 6 hours at your UV index, dermatologists universally recommend reapplying every 2 hours when outdoors, and immediately after swimming, sweating heavily, or towelling off. Water-resistant sunscreens are tested at 40 or 80 minutes of water exposure before retesting, so "water-resistant" does not mean waterproof. The 2-hour reapplication rule is a safety buffer that accounts for real-world wear-off that laboratory conditions cannot fully simulate.
How much sunscreen to pack for a trip
Multiply your milliliters per application by the number of applications per day, then by the number of days. An average full-body application for an adult (face, arms, and legs, about 7,350 cm2 total) is roughly 14.7 mL. With 3 applications per day over 7 days, that is about 309 mL, or roughly 2 standard 150 mL bottles. Most people drastically underpack because they underestimate both how much to apply and how often. Use this calculator to generate a realistic shopping list before you leave.
Fitzpatrick skin type and sun sensitivity
| Type | Description | Reaction to sun | Typical ancestry |
|---|---|---|---|
| I | Very fair, often freckled | Always burns, never tans | Northern European |
| II | Fair | Usually burns, tans minimally | European |
| III | Medium, light brown | Sometimes burns, gradually tans | Southern European, some Asian |
| IV | Olive, moderate brown | Rarely burns, tans easily | Mediterranean, Hispanic, some Asian |
| V | Brown | Very rarely burns, tans darkly | Middle Eastern, South Asian, some African |
| VI | Deep brown to black | Almost never burns | Sub-Saharan African |
The Fitzpatrick scale (1975) classifies skin by its reaction to UV radiation. Higher types have more melanin and a longer unprotected burn time.
Frequently asked questions
How much sunscreen should I apply to my face?
The standard is 2 mg per square centimeter of skin. For an average adult face, head and neck (about 450 cm2), that works out to around 0.9 mL, roughly two pumps from a typical 0.5 mL dispenser. A common mnemonic is a quarter-teaspoon (about 1.25 mL) for the face alone, which is a reasonable practical target.
How often should I reapply sunscreen?
Every 2 hours when you are outdoors, and immediately after swimming, sweating heavily, or towelling off. No sunscreen lasts indefinitely in real-world conditions: sweat, rubbing, and UV degradation all reduce effectiveness. Even if your SPF theoretically protects for longer based on your skin type and UV index, the 2-hour rule is a safety buffer that all major dermatology bodies recommend.
What SPF should I use?
The American Academy of Dermatology recommends at least SPF 30, which blocks about 97% of UVB. SPF 50 blocks about 98%, and SPF 100 blocks about 99%. The difference between SPF 50 and SPF 100 is small in percentage terms but can matter for people with very fair skin or conditions that increase UV sensitivity. Choose SPF 30 as a minimum, SPF 50 if you are fair-skinned or will be at the beach or high altitude.
Does UV index affect how long my sunscreen lasts?
Yes. The higher the UV index, the faster your unprotected skin would burn, and the shorter the window of meaningful protection even with sunscreen. At UV index 10, an SPF 30 lotion that might protect a type II skin for over an hour at UV index 3 gives a much narrower safety window. This calculator adjusts protection time for UV index, but the 2-hour reapplication ceiling still applies.
Is water-resistant sunscreen good enough for swimming?
Water-resistant means the product has been tested to maintain its SPF rating after 40 or 80 minutes of water immersion (the label specifies which). After that window, you must reapply. It is not waterproof. Reapply as soon as you get out of the water and towel off.
Do darker skin tones still need sunscreen?
Yes, though they have a higher natural protection (Fitzpatrick types V-VI have a longer unprotected burn time). All skin tones are still susceptible to UV-induced DNA damage, skin cancer, and photoaging even without visible burning. The Skin Cancer Foundation recommends SPF 30 or higher for all skin tones.
When should I apply sunscreen before going outside?
Apply sunscreen 15 to 30 minutes before sun exposure. Chemical sunscreen filters need time to be absorbed into the skin and begin absorbing UV radiation. Mineral sunscreens (zinc oxide, titanium dioxide) work immediately upon application, but 15 minutes before going out is still a good habit to ensure even coverage.