How Much Sunscreen to Use on Face? A Dermatologist on 10 Myths

I'm Dr. Hongseok Kim, a board-certified Korean dermatologist with 21 years of practice and the medical director of VOS Dermatology Clinic in Gangnam, Seoul, in front of Dosan Park. At VOS I see every patient myself, from the consultation to the treatment.

How much sunscreen to use on face is the question behind most of the sunscreen mistakes I see, because the number on the bottle only holds if you apply the amount it was tested with. In this video I went outside on a bright spring day and worked through ten common beliefs about sunscreen, from "30 minutes before" to vitamin D. Spring matters here: UV rises sharply from April into May, so waiting until summer to start is already a little late. This page is based on my YouTube video from April 2024, rewritten for readers abroad, and the video above can be watched with translated captions.

How much sunscreen to use on face: the tested amount

Short answer: SPF is measured at 2 mg per square centimetre, which for the face is about a kidney-bean-sized amount, close to filling one finger joint (in the video at 09:47).

Most people know this figure, and almost nobody actually applies it. That matters because every SPF value on a label assumes that amount. It is also why make-up cannot replace sunscreen. A BB cream or foundation marked SPF 30 only gives SPF 30 if you put on just as much of it, and a face covered in that much foundation would look completely different, far beyond a white cast (in the video at 10:15). So I treat sunscreen as the base layer and make-up with SPF as a top-up: it can boost protection for a while during the day, but it should not be thought of as your sunscreen (in the video at 10:39).

Too little sunscreen means protection runs out early

Short answer: apply less than the tested amount and the protection time shrinks, so a morning layer can be gone before lunch (in the video at 11:20).

The way it is usually explained, SPF 30 multiplied by about 15 minutes gives around 450 minutes, roughly seven to eight hours: leave for work at 8 a.m. and you are covered until mid-afternoon (in the video at 11:03). That only holds at the recommended amount. With a typical thin layer, if you go out at 9 a.m. the protection may already be over by 11, and then you walk out for lunch at 1 or 2 feeling safe and stay out longer, with no real protection at all (in the video at 11:35).

Ideally, sunscreen is reapplied about every two hours when you are out (in the video at 11:48). If that is too much, women who wear make-up can touch up with a product containing sunscreen, and men can apply another layer before going out. I also suggest two layers in the morning: a second layer adds protection time and gets you much closer to lasting through the working day (in the video at 12:12). For how I handle top-ups in daily life, including over make-up, see my note on how often to reapply sunscreen.

The label myths: 30 minutes before, and SPF 100 vs SPF 50

Short answer: only chemical sunscreen needs the 30-minute wait, and SPF 100 is not twice as strong as SPF 50 (in the video at 00:24).

There are two types of sunscreen. Chemical (organic) sunscreen is absorbed and acts inside the skin to absorb UV, so it needs time: apply it at least 30 minutes before you go out. Mineral (physical) sunscreen reflects and scatters UV on the surface and works as soon as it is on (in the video at 01:05). Check which type you have.

SPF refers to UVB, the shorter, higher-energy wavelength behind sunburn and redness; UVA, the longer wavelength, is known for driving ageing with long exposure (in the video at 01:33). Applied at the recommended amount, SPF 50 blocks about 98% of UVB and SPF 100 about 99%. The difference is mainly in how long protection lasts, not in how much is blocked, which brings us back to using the full amount (in the video at 02:00). I compare filter types and textures in more depth in my guide to choosing a Korean sunscreen for face.

Indoors and under lights: when you can skip it

Short answer: indoors it depends on large windows; ceiling lights are not a reason to wear sunscreen (in the video at 02:30).

Glass blocks most UVB, but UVA passes through windows into the room (in the video at 02:44). Many offices now use UV-blocking film, and blinds or curtains also cut UVA well. If you sit next to a big window and are unsure, wear sunscreen, and since UVA is the concern, look at the PA rating rather than SPF: PA+++ or PA++++, or "broad spectrum" on products from abroad (in the video at 03:42). Without large windows, you do not need it indoors.

Fluorescent tubes are coated with titanium dioxide on the inside, which works much like sunscreen built into the bulb (in the video at 04:08). There are reports of skin problems at about 10 cm for around eight hours, but ordinary ceiling lighting is not a problem, and LED lighting does not emit UV. Very bright light held very close can still affect skin through visible and blue light; if that is your situation, ingredients such as iron oxide help block some visible light (in the video at 05:42). For most people it is not needed.

Clothing, shade and clouds give less protection than you think

Short answer: a thin cotton T-shirt lets about 20% of UV through, shade blocks little, and broken cloud can raise UV (in the video at 06:38).

Clothing is rated by UPF, measured the same way as SPF. A rash guard or parasol marked UPF 50 blocks about 98%, but an ordinary cotton T-shirt is around UPF 5, letting about a fifth of UV through (in the video at 06:53). Dark clothing blocks more UV than white, though it also absorbs heat. With a thin tee, apply sunscreen to exposed areas and anywhere you are concerned about.

In shade, scattered and reflected UV from the ground and surroundings still reaches you; the protection value of shade is only about 5 to 15 (in the video at 07:57). On cloudy days, a 2012 study from Granada reported that UV rose when clouds covered about three quarters of the sky and the sun broke through between them (in the video at 08:38). Full cloud cover cuts UVB by about half and rain over 5 mm by about 75%, but UVA reaches the ground at a similar level (in the video at 09:03). Long-term UV exposure is behind many of the spots I treat, which my guide to Korean dark spot treatment covers.

Does sunscreen age skin or cause bone loss?

Short answer: sunscreen does not age skin when enough is used, and it is very unlikely to cause osteoporosis through low vitamin D (in the video at 12:38).

Vitamin D matters for bones and metabolism and is made in the skin with UV. In the tests I have run, close to 80% of people were below the normal range anyway, so I recommend taking vitamin D as a supplement rather than exposing skin to UV and accepting ageing, oxidative damage and skin cancer risk (in the video at 13:16). Getting enough from food would mean eating something like 30 eggs a day (in the video at 13:43). Most people also leave the neck, ears and backs of the hands uncovered, and vitamin D is made there too; unless you coated your whole body at the full amount, sunscreen-driven bone loss is not something to worry about (in the video at 14:20).

If you want to know how your own skin reacts to sun and products, skin type analysis with VOS Skin MBTI is how we assess it at the clinic. My overview of which skin care steps need a dermatologist explains where daily habits like sunscreen end and clinic care begins, and our creator experiences at VOS show how visitors combined the two.

Results vary from person to person, and the right plan for your skin is decided at an in-person consultation.

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Results, swelling and recovery vary from person to person. This page is general information from VOS Dermatology Clinic based on Dr. Kim's video and does not replace an in-person consultation.