How much sunscreen is actually enough?
The SPF number on a bottle is measured in a lab at a fixed dose. Almost nobody applies that much, so almost nobody gets the number they paid for. This works out what your face needs, and shows you the gap.
When to put more on
The standard advice. Reapply at least every two hours, and after swimming, towelling or heavy sweating.
What the modelling adds. If you are going to be out for hours, the model says reapply 15 to 30 minutes after you go out, not only at the two-hour mark.
Where each number comes from
- Measured2 mg/cm2 What the SPF on the bottle was tested at. ISO 24444:2019, the in vivo SPF method. Cited in our own knowledge base on the Zinc Oxide and Ethylhexyl Triazone rows.
- Measured1.04 mg/cm2 What people actually apply, without instructions. Le Digabel J, Questel E, Lauze C, Carballido F, Josse G. In vivo evaluation of sunscreen application by multispectral imaging. Skin Res Technol 2023;29(8):e13320. PMID 37632173. 26 healthy volunteers, single unguided application to the face.
- Measured1.45 mg/cm2 What people apply when given an application guide. Le Digabel et al. 2023, PMID 37632173, same 26 volunteers, application according to an instruction guide. Reported as around 40 percent higher than the unguided single application.
- Measured1.51 mg/cm2 Median dose on the face six months after being taught. Azurdia RM, Pagliaro JA, Rhodes LE. Sunscreen application technique in photosensitive patients. Photodermatol Photoimmunol Photomed 2000;16(2):53-6. PMID 10823312. Median face thickness rose from 0.33 mg/cm2 at baseline to 1.51 at six months, in 6 photosensitive patients.
- MeasuredSPF 30 blocks approximately 97% of UVB rays. Above that, the difference is marginal; SPF 50 blocks about 98%. Our own guide, How to build a simple skincare routine, citing the American Academy of Dermatology.
- MeasuredIf you are going to be out for hours, the model says reapply 15 to 30 minutes after you go out, not only at the two-hour mark. Diffey BL. When should sunscreen be reapplied? J Am Acad Dermatol 2001;45(6):882-5. PMID 11712033. A mathematical model of application amount and substantivity; reapplication at 20 minutes gave 60 to 85 percent of the UV exposure of reapplication at 2 hours.
- AssumedGrams and teaspoons need a surface area, and there is no clean published figure for an adult face as a thing you put cream on. The areas below are our round assumptions, not measurements, and they are the numbers to argue with if the result looks wrong. Faces differ by more than the precision anyone wants here.
- AssumedTeaspoons assume sunscreen weighs about the same as water. A real emulsion sits close enough that the difference is smaller than the accuracy of a kitchen spoon. Density taken as 1 g/mL.
- AssumedWe could not find a study behind the two-finger rule. It is a widely repeated dermatology teaching aid, not a measured equivalence, and how much it actually delivers depends on the width of the nozzle and the size of your hands. Use it as a floor to beat, not as a measurement.
Questions people ask
How much sunscreen should I apply to my face?
At the dose the SPF on the bottle was tested at, a face of about 600 square centimetres needs around 1.2 grams, which is about a quarter teaspoon. The area is our stated assumption; the 2 mg per square centimetre test dose is the measured anchor.
How much sunscreen for face and neck?
About 1.8 grams, which is about between a quarter and half a teaspoon, using our round assumption of 900 square centimetres for a face plus the front and sides of the neck.
Is this SPF calculator free?
Yes. No sign-up and no email; it runs entirely in your browser, and the working behind every number is printed on this page.
Can I apply less if I use a higher SPF?
The number on the bottle was measured at the same tested dose whatever the SPF, so applying less undercuts a high SPF exactly as it undercuts a low one. For the numbers themselves: SPF 30 blocks approximately 97% of UVB rays. Above that, the difference is marginal; SPF 50 blocks about 98%.
What is the two finger rule for sunscreen?
Two finger lengths of sunscreen, squeezed along your index and middle fingers, is the usual rule of thumb for a face and neck. We could not find a study behind the two-finger rule. It is a widely repeated dermatology teaching aid, not a measured equivalence, and how much it actually delivers depends on the width of the nozzle and the size of your hands. Use it as a floor to beat, not as a measurement.
What this does not do: it does not know your face, your sunscreen or your day, and it will not tell you what SPF you are actually getting. It tells you how much product a tested dose works out to, and how far the usual amount sits from it.
Wondering what the SPF number itself means? That is its own page, and the 30-versus-50 question has an honest answer. Want to know what is in the sunscreen itself? Paste its ingredient list into the free label decoder. Wondering whether an ingredient makes skin more sun-sensitive? That is a column in the ingredient glossary, and it says "unknown" more often than you would think.