Sunscreen, decoded: SPF, broad spectrum and how much to use

How to choose and use sunscreen: SPF 30+, broad spectrum, the shot-glass rule, reapplying, tinted options for every skin tone, and babies under 6 months.

Written by Dr. Neera Nathan, MD, MSHS, board-certified dermatologist Last updated Sources cited Editorial policy

Sun protection 7 min readPublished Reviewed

Key takeaways

  • Choose broad-spectrum, water-resistant sunscreen with SPF 30 or higher.
  • Most adults need about 1 ounce, roughly a shot glass, for the body, and about a teaspoon for the face.
  • Reapply every two hours and right after swimming or sweating, whatever the SPF.
  • Tinted sunscreens with iron oxide also help against visible light, which can worsen dark spots.

Why it matters, for every skin tone

Ultraviolet (UV) light causes sunburn, skin aging and most skin cancers. At least one in five Americans will develop skin cancer by age 70.1 Sun protection also helps with uneven pigment and dark spots, which matter to many people with brown and Black skin, who can still get skin cancer.2

Sunscreen is one layer of protection. The American Cancer Society also recommends seeking shade, covering up with clothing and a hat, wearing sunglasses and avoiding tanning beds.3

Reading the label

  • Broad spectrum means the sunscreen protects against both UVA and UVB rays.4
  • SPF mainly measures protection against UVB, the rays that cause sunburn. The FDA advises choosing a broad-spectrum sunscreen with an SPF of at least 15.5 The American Academy of Dermatology recommends SPF 30 or higher.4
  • Water resistant labels must say whether protection lasts 40 or 80 minutes in water or while sweating. No sunscreen is waterproof.5

How much, and how often

Most people use far less sunscreen than the SPF test assumes. The AAD's guidance:6

  • About 1 ounce, enough to fill a shot glass, to cover an adult's exposed skin. Larger bodies need more.
  • About a teaspoon for the face. That's roughly two finger-lengths.
  • Apply it before you go out. It takes about 15 minutes to start working.
  • Reapply every two hours, and straight after swimming or sweating.
  • Don't forget ears, neck, the tops of the feet, the scalp where hair is thin, and your lips (lip balm with SPF 30 or higher).
  • Use it on cloudy days and in winter too.

Mineral, chemical or tinted?

The best sunscreen is the one you'll use every day. Mineral sunscreens (zinc oxide, titanium dioxide) and chemical filters both work when they're broad spectrum and applied generously.

Some mineral sunscreens leave a white cast on deeper skin tones, which is one reason people stop using them. Tinted sunscreens help with that. Tinted formulas that contain iron oxide also block some visible light, which can trigger or worsen dark spots and melasma. The AAD suggests a tint that blends with your skin tone, and a non-comedogenic formula if you're acne-prone.26 Dr. Nathan has written about tinted sunscreens for Harvard Health Publishing.

Shade, clothing and timing

UV is strongest in the middle of the day. The FDA advises limiting time in the sun between 10 a.m. and 2 p.m., and the American Cancer Society suggests extra care from 10 a.m. to 4 p.m.53 Check the UV Index in your weather app. The higher the number, the faster skin can burn.3

Clothing is the simplest protection. Dark, tightly woven fabrics block more UV, and some clothing carries a UPF (ultraviolet protection factor) rating. Add a broad-brimmed hat and UV-blocking sunglasses.3

Children and babies

The FDA advises against sunscreen for infants younger than 6 months. Keep them in the shade and in protective clothing, and ask your pediatrician before using sunscreen on a young baby.5

The USPSTF recommends that clinicians counsel children, teens, young adults up to age 24 and parents of young children with fair skin about limiting UV exposure. Habits formed early last.7

When to see a doctor

See a doctor for a blistering sunburn that covers a large area, especially with fever, chills or confusion, or for a rash that appears after sun exposure. If a spot or mole changes, read about skin checks and book a visit.

Sources

  1. The Skin Cancer Foundation. Skin Cancer Facts & Statistics. Updated March 2026. View source
  2. American Academy of Dermatology. Finding skin cancer in darker skin tones. View source
  3. American Cancer Society. How to Protect Your Skin from UV Rays. View source
  4. American Academy of Dermatology. How to select a sunscreen. View source
  5. U.S. Food and Drug Administration. Sunscreen: How to Help Protect Your Skin from the Sun. Updated September 10, 2026. View source
  6. American Academy of Dermatology. How to apply sunscreen. View source
  7. US Preventive Services Task Force. Behavioral counseling to prevent skin cancer: US Preventive Services Task Force recommendation statement. JAMA. 2018;319(11):1134-1142. doi:10.1001/jama.2018.1623 View source

About the author

Dr. Neera Nathan, MD, MSHS

Harvard-trained, board-certified dermatologist and fellowship-trained Mohs surgeon with Dermatology & Skin Health.

Full profile Editorial policy

This article is general education for adults and isn't a substitute for personal medical advice. In an emergency, call 911.

Related articles