How Much Sunscreen Do You Need, and When Should You Reapply?
How much to use on your face, when to reapply, and whether you need it on cloudy days or indoors.
Written and reviewed by Dr. ScenePublished Updated
What SPF and PA protect against
Sunlight contains two main types of ultraviolet light.
- UVB burns the skin and turns it red. Protection against it is shown as an SPF number.
- UVA reaches deeper into the skin and is linked to pigmentation and skin aging. Korean products label it PA+ to PA++++ — the more plus signs, the stronger the protection.
The basic rule is to choose a product that covers both. For everyday use, SPF 30 or higher with PA+++ or higher is usually enough.
How much you need for the labeled protection
SPF is measured with 2 mg of sunscreen per square centimeter of skin. Most people apply less than half of that, so the protection they get is far lower than the number on the label.
- Whole face: two finger-lengths squeezed along your index and middle fingers (about 1/4 teaspoon)
- Don't forget your neck and ears — they are the most commonly missed areas.
- If that much feels heavy, apply it in two thin layers.
When to reapply
| Situation | When to reapply |
|---|---|
| Outdoors | Every 2 hours |
| Heavy sweating, after swimming, after towel-drying | Right away |
| Indoors, near a window for a long time | Once in the afternoon |
| Indoors, away from windows | Once in the morning is usually enough |
A "water-resistant" label means the protection holds for 40 or 80 minutes in water — not for the whole day.
Do you need it on cloudy days and in winter?
Yes. Clouds let a large share of UV light through, and UVA changes less with the seasons than UVB does. Snow, water and sand reflect UV and increase your exposure. Window glass blocks most UVB but lets UVA through, so if you sit by a window or drive for long periods, it is worth wearing sunscreen indoors too.
If you have a skin condition or have just had a treatment, your sun care may need to be different. Follow the advice you were given at your appointment first.
Frequently asked questions
How do I reapply over makeup?
A sunscreen stick, cushion or spray is the easiest way. A powder with SPF alone rarely gives you enough product, so use it to top up rather than to replace the sunscreen you apply in the morning.
Is a higher SPF always better?
SPF 30 blocks about 97% of UVB and SPF 50 about 98%. A higher number does not make it last longer, so applying enough and reapplying on time matters more than choosing the highest number.
Do I need sunscreen if I stay indoors?
If you sit near a window or drive for a long time, yes — window glass lets UVA through. If you are mostly indoors away from windows, applying once in the morning is usually enough.
This article was written and reviewed by Dr. Scene. Treatment results and recovery time vary from person to person, and side effects such as swelling, bruising or inflammation may occur. Please consult our medical team thoroughly before any treatment.
Temporary image · to be replaced with actual photosAbout the author
Dr. Scene
Medical director, Dermia Clinic
Dr. Scene, the medical director of Dermia Clinic with over 10 years of clinical practice, is a highly experienced physician.