Sunscreen Calculator

Enter Calculation

Body Surface Area (cm²)

Hours Outside

SPF Rating

Baseline Burn Time (min)

Results

Sunscreen Per Application

32

grams (at 2 mg/cm²)

Reapplications needed	2 times
Total sunscreen for the day	64 g
Effective protection time	300 min

What Is the Sunscreen Calculator?

This tool estimates three things that matter for proper sun protection: how much sunscreen you should apply, how many times you need to reapply during your time outdoors, and roughly how long your chosen SPF can theoretically protect you. It is based on the dermatology standard application rate of 2 mg of sunscreen per square centimeter of skin — the same density used when SPF is measured in the lab.

Sunscreen is applied across the body's exposed zones at about 2 mg per square centimeter.

How to Use It

Enter the total skin surface area you plan to cover in square centimeters. As a reference, an average adult's whole body is roughly 16,000–18,000 cm², while the face, neck and ears together are about 600 cm². Then enter how many hours you'll be outside, your sunscreen's SPF rating, and your baseline burn time — the number of minutes your bare skin takes to redden in the sun (often 10 minutes for fair skin).

The Formula Explained

The amount per application is \(\text{Area} \times 2\ \text{mg/cm}^2\), converted to grams by dividing by 1000. Because sunscreen degrades and rubs off, you should reapply every two hours, so the number of applications is \(\left\lceil \text{hours} \div 2 \right\rceil\). Total product needed is the per-application amount multiplied by that count. Finally, theoretical protection time is \(\text{SPF} \times \text{baseline burn time}\).

$$\text{Total} = \frac{2 \cdot \text{Area (cm}^2\text{)}}{1000} \times \left\lceil \frac{\text{Hours}}{2} \right\rceil$$

$$\text{Protection} = \text{SPF} \times \text{Burn Time (min)}$$

The calculator covers three things: how much to apply, when to reapply, and how long SPF lasts.

Worked Example

For a full body (16,000 cm²), 4 hours outside, SPF 30 and a 10-minute baseline: amount = \(16{,}000 \times 2 = 32{,}000\ \text{mg} = \) 32 g per application. Reapplications = \(\left\lceil 4 \div 2 \right\rceil = \) 2. Total = \(32 \times 2 = \) 64 g. Protection time = \(30 \times 10 = \) 300 minutes in ideal conditions.

FAQ

Why 2 mg/cm²? That's the lab density used to certify SPF labels. Most people apply only a quarter to half of that, getting far less protection than the label implies.

Does SPF × burn time mean I'm safe that long? No — it's a theoretical maximum. Sweat, water, and rubbing reduce it, so always reapply every two hours.

How much is a shot-glass of sunscreen? About 30 ml, which is the commonly cited full-body adult dose and matches the ~32 g this calculator estimates.