Sunscreen Amount Calculator — How Much Sunscreen to Use
Most people use far too little sunscreen, getting a fraction of the rated SPF protection. Enter your height, weight and which parts of your body you're covering to find out exactly how many millilitres you need per application and for a full day.
cm
kg
Body coverage
Amount for full SPF protection at 2 mg/cm² (the lab-test standard)
- 1
Body surface area (DuBois)
0.007184 × 175⁰˙⁷²⁵ × 70⁰˙⁴²⁵ = 1.8481 m²DuBois & DuBois formula, the standard BSA estimate used in clinical medicine. - 2
Exposed skin area
1.8481 m² × 100% × 10,000 = 18,481 cm² - 3
Sunscreen needed per application
18,481 cm² × 2 mg/cm² ÷ 1,000 = 37 mL2 mg/cm² is the lab-test standard at which SPF ratings are measured.
How does this calculator work?
For full SPF protection you need 2 mg of sunscreen per cm² of skin. Using the DuBois BSA formula and a typical adult BSA of ~1.7 m², that's about 34 mL per full-body application. Use the teaspoon rule (1 tsp per body region) as a practical guide — and reapply every 2 hours.
Formula
How this is calculated
SPF ratings are determined in laboratory tests using an application density of 2 mg per cm² of skin. For an average adult body surface area of about 1.7 m² this works out to 34 g (about 34 mL for a typical lotion) for full-body coverage. In practice most people use only 25–50% of this amount — half a teaspoon where a full teaspoon is needed — which means they effectively get the square root of the SPF: SPF 50 applied at half the dose behaves closer to SPF 7.
Your personal BSA is calculated using the DuBois & DuBois formula: BSA (m²) = 0.007184 × H^0.725 (cm) × W^0.425 (kg), one of the most widely used BSA formulae in clinical medicine. The exposed fraction depends on your clothing: full body, swimsuit, half-body or face and arms only.
The teaspoon rule is a practical shortcut: about 1 teaspoon (5 mL) per major body region — face/neck, each arm, chest, abdomen, upper and lower back, each leg — totalling roughly 7 teaspoons (35 mL) for a full body. This calculator gives you the same answer derived from your actual BSA, so tall or heavier people get a proportionally correct figure.
Frequently asked questions
SPF is rated at 2 mg/cm², which is a surprisingly thick layer. An average adult needs about 30–35 mL for full-body coverage, roughly 2 tablespoons. Using less dramatically reduces protection — applying half the amount cuts effective SPF from 50 down to about 7.
Dermatologists recommend reapplying every 2 hours and immediately after swimming or heavy sweating. For a full beach day with 3 applications you may need 90–105 mL, so a 100 mL bottle barely covers one person for one day.
The teaspoon-per-region rule (1 tsp each for face, left arm, right arm, chest, abdomen, upper back, lower back, left leg, right leg) gives about 35–45 mL, which is close to the calculated amount for most adults and is much easier to measure at the beach than weighing lotion.
Also known as
TG we-Calculate Editorial Team. (2026). Sunscreen Amount Calculator — How Much Sunscreen to Use [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/sunscreen-amount-calculator
TG we-Calculate Editorial Team. "Sunscreen Amount Calculator — How Much Sunscreen to Use." TG we-Calculate. 2026. https://we-calculate.com/calculator/sunscreen-amount-calculator.
TG we-Calculate Editorial Team, "Sunscreen Amount Calculator — How Much Sunscreen to Use," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/sunscreen-amount-calculator
@misc{wecalculate_sunscreen_amount_calculator, title = {Sunscreen Amount Calculator — How Much Sunscreen to Use}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/sunscreen-amount-calculator}}, year = {2026}, note = {TG we-Calculate} }
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