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Sunscreen amount calculator (grams per application)

Calculate how many grams of sunscreen you actually need per application based on body areas, body size, and exposure time.

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Why this calculator exists

Sunscreen labels are tested at a specific dose: 2 milligrams per square centimetre of skin (ISO 24444). Real-world studies measure typical application at 0.4–1.0 mg/cm² — a quarter to a half of the standard dose (Petersen & Wulf 2014; Schalka & Reis 2011). Because UV penetration through a sunscreen film is logarithmic, halving the dose can cut effective SPF by two-thirds — meaning the SPF 30 you bought routinely delivers SPF 6–15 in practice.

Most under-application is unintentional. The “right” amount feels excessive until you weigh it on a scale. This calculator gives you a concrete grams-per-application number and equivalents (teaspoons, shot glasses) so you can calibrate without measuring every time.

How the calculation works

  1. Pick your body size — small adult, average adult, large adult, child, or small child. We use approximations of body surface area (BSA): 1.5, 1.7, 2.0, 1.0, and 0.7 m² respectively.
  2. Tick the body areas you want to cover. Each region has a fixed area in cm² (e.g. face ≈ 510 cm², both arms ≈ 3,060 cm²).
  3. Add planned hours of exposure for re-application calculations (every 2 hours).
  4. Tick swim/sweat if you are in water or sweating heavily — this adds an extra application.

We multiply skin area × 2 mg/cm² × size multiplier to get grams per application, then multiply by the number of applications needed.

Quick reference (average adult)

CoveragePer applicationDaily (4-hour exposure)
Face only~1.0 g (¼ tsp)~3 g (¾ tsp)
Face + neck + ears + arms~9 g (~2 tsp)~27 g (5–6 tsp)
Full body (no scalp)~30 g (~6 tsp / 1 shot glass)~90 g (~18 tsp)

The “shot glass” rule (~30 g) corresponds to full body + face for an average adult.

Application technique that matters

  • Apply 15–30 minutes before sun exposure on dry skin. Chemical filters need lead time to bind to the stratum corneum; mineral filters work immediately but spread better with rest.
  • Use the dot-and-spread method: place 4–8 large dots over a region, then spread evenly. Spreading from a single blob almost always misses the edges.
  • Mind the missed spots: ears, behind the neck, the hairline, the tops of the feet, the back of the hands, the lips (use SPF lip balm).
  • Reapply at the 2-hour mark even if nothing visible has changed. SPF protection degrades on skin from light, sweat, abrasion, and friction.
  • Sunscreen is the last line of defence. Shade, clothing (UPF if available), wide-brimmed hats, and sunglasses come first. Sunscreen is for the skin you cannot cover.

SPF level vs. dose: what actually matters

The biggest determinant of real-world UV protection is dose, not SPF number. An SPF 30 sunscreen applied at 2 mg/cm² protects more skin than an SPF 50 applied at 1 mg/cm². SPF 50 is meaningful for very fair skin, high-UV environments (high altitude, tropical sun), photosensitivity conditions, or extended outdoor sport — but only if the dose is correct.

UVA protection is separate from SPF (which measures UVB). Look for “broad spectrum”, PA+++ / PA++++, or the EU UVA-in-circle logo. UVA causes most photoageing and contributes to skin cancer risk.

When sunscreen is not enough

  • High-UV settings (UV index 8+, snow/sand/water reflection, high altitude): combine with shade, clothing, and avoid peak hours (10 AM – 4 PM).
  • Children under 6 months: do not use sunscreen. Use shade, hats, and protective clothing. After 6 months, mineral filters (zinc oxide, titanium dioxide) are usually preferred.
  • Photosensitivity conditions (lupus, polymorphic light eruption, drug-induced photosensitivity): clothing-first protection plus SPF 50+, and avoid peak UV.
  • Wound or compromised skin: avoid chemical filters on broken skin; use mineral filters or physical barriers.

Pair with our other tools

Privacy

All calculations run in your browser. We never see, log, or store your selections. Only an anonymous event (locale, body areas count) is sent to a privacy-respecting analytics service.

Licence

This calculator implements the public ISO 24444 standard dose (2 mg/cm²) with body-region areas drawn from standard BSA reference data. The clinical guidance follows AAD and AESAN sun protection recommendations.

Frequently asked questions

How much sunscreen should I actually apply?
The international SPF testing standard (ISO 24444) is 2 mg per cm² of skin. For an average adult covering the full body and face, that works out to about 30 grams per application — roughly a shot glass, or about 6 teaspoons. For face and neck only, ~1.25 grams (a quarter teaspoon). Real-world studies show most people apply a quarter to a half of the labelled dose, which means an SPF 30 sunscreen often delivers SPF 6–15 in practice (Schalka 2011; Petersen 2014).
Why do I need so much?
SPF is measured at exactly 2 mg/cm². If you apply less, the protection drops disproportionately — not linearly — because UV penetration through a thinner film is logarithmic. Halving the dose can cut effective SPF by two-thirds, not half. Most under-application is unintentional: the standard amounts feel like a lot until you measure them.
What about reapplication?
Reapplication every 2 hours is part of the labelled SPF claim. SPF protection degrades on skin from UV, sweat, friction (clothing rubbing, towel drying), and water. After swimming, towelling off, or heavy sweating, reapply immediately regardless of the 2-hour mark. "Water-resistant" sunscreen is tested for 40 or 80 minutes of immersion — not all-day protection.
How does body size change the dose?
Body surface area (BSA) scales with height and weight. We use these approximations: small adult ~1.5 m², average adult ~1.7 m², large adult ~2.0 m², child ~1.0 m², small child ~0.7 m². The calculator multiplies the regional skin areas by the size factor for an accurate dose. Underestimating size means systematically under-applying — a common cause of sunburn in larger adults using "adult" doses sized for an average.
Is SPF 50 worth it over SPF 30?
Mathematically the difference is small: SPF 30 absorbs ~96.7% of UVB, SPF 50 absorbs ~98%. The marginal protection (~1.3 percentage points) becomes meaningful for very fair skin, in high-UV environments (high altitude, equatorial sun), or for people with photosensitivity conditions. For routine daily use in moderate climates, SPF 30 applied at the correct dose protects more skin than SPF 50 applied at half-dose.
Can sunscreen cause vitamin D deficiency?
In practice, no. Studies show typical sunscreen use does not produce vitamin D deficiency in general populations. Brief incidental exposure on hands, forearms, or face outside peak UV usually contributes enough cutaneous synthesis. People at risk of deficiency (older adults, darker skin tones, those who avoid sun for medical reasons) should discuss supplementation with a clinician — sunscreen reduction is not the answer for them.
Does my data leave the device?
No. All calculations run entirely in your browser. Only an anonymous event (locale, body areas count) is sent to a privacy-respecting analytics service. Your individual choices never leave the device.

Sources

  1. Sun protection factor: meaning and controversies — An Bras Dermatol (Schalka & Reis, 2011) (peer reviewed, retrieved 2026-04-30)
  2. Application of sunscreen — theory and reality — Photodermatol Photoimmunol Photomed (Petersen & Wulf, 2014) (peer reviewed, retrieved 2026-04-30)
  3. ISO 24444:2019 Cosmetics — Sun protection test methods — In vivo determination of the sun protection factor (SPF) — International Organization for Standardization (guideline, retrieved 2026-04-30)
  4. Sunscreen FAQs — American Academy of Dermatology (AAD) (medical society, retrieved 2026-04-30)