A cap feels like the hat problem is handled. Scalp covered. Peak over the eyes. Then the burn line shows up where the bill stops: rims of the ears, the back of the neck, the strip of cheek the visor never reaches.

That is the geometry, not bad luck. A baseball cap is a small shade for the forehead. It is not a sun hat.

What the numbers say

ARPANSA publishes typical protection factors from the shade a hat casts—not the UPF of the cloth on the crown. Higher is more cover. About 1 means that site is getting roughly the same UV as with no hat.

On a baseball cap:

• Forehead — about 9
• Nose — about 4
• Cheeks, ears, chin, and neck — about 1 (essentially no extra shade)

On a broad-brim (adult brim about 7.5 cm / 3 in):

• Forehead — 15+
• Nose — about 7
• Ears — about 6
• Cheeks and neck — about 2
• Chin — still about 1

A bucket hat tracks the brimmed numbers. A legionnaire with a neck flap is the one that actually lifts the neck—ARPANSA puts that site above 4, with the nose around 10.

The Skin Cancer Foundation’s rule is the same geometry in inches: a brim at least 3 in all the way around, or a 3 in bill with a permanently attached drape over the ears and neck. CDC’s travel advice is the same: brim ≥3 in / 75 mm circumferential. Do not rely on a standard baseball cap. Fabric on the crown should be dense; the Foundation’s seal asks for UPF 50.

Cancer Council NSW puts a hat’s cut of UV to the eyes at about 50%. Add close-fitting sunglasses and Cancer Council Australia puts the combined cut at up to about 98%. The cap still does not cover the ears.

Two ratings sit on one hat. UPF is how much UV goes through the cloth on the scalp. SunSmart’s fabric example: a UPF 30 cloth lets about 1/30 of the UV through—roughly 3%—so the crown can look well rated. The ARPANSA factors above are how much shade reaches skin the cloth never touches. A baseball cap’s ears still sit at about 1. A UPF 50 crown does not close that gap. SunSmart notes that caps and visors cannot claim a UPF rating, because the design does not cover face, ears, and neck.

ARPANSA’s other blunt line: hats rarely give enough cover on their own when the UV index is 3 or higher. The brim is a helper. It is not the whole kit.

Why the cap fools you

Three traps:

1. “I’m wearing a hat.” Coverage of the crown is not coverage of the face. The visor is a small awning. Ears, temples, and the nape sit outside it—the same sites Cancer Council flags as common for skin cancers.
2. Comfort as the cue. Caps stay on in wind and don’t flop during a run. Brims get ditched. If a cap is the only hat that stays on for the sport, keep it—and treat ears and neck as uncovered skin.
3. Bounce from below. Every style in the ARPANSA set leaves the chin near 1. Pale ground, sand, and water send UV up under the brim. That path is the same one in shade does not cancel UV. The hat does not cancel it.

The cloth on the crown can still fail on its own terms—thin, wet, or loose-knit fabric lets UV through the top of the head. That is the cotton T-shirt problem applied to a hat. This piece is the missing brim, not the weave.

Before you put it on

Use the live UV index first, then judge the brim:

1. Check today’s peak. If it is High (6–7) or above, do not treat a cap as enough for a long midday block.
2. Measure the brim, not the logo. Adult wide brims around 7.5 cm / 3 in all the way around. Bucket brims at least about 6 cm. A visor-only cap fails the ears and neck by design. Close weave; a dark under-brim cuts bounce onto the face.
3. Cover the nape. A legionnaire flap, a sewn-on drape, or a collared shirt plus sunscreen on the back of the neck. The brim alone only about doubles cover there.
4. Sunscreen what the hat misses — ears, neck, lips, the part in your hair, and the chin. Even a brimmed hat only about doubles cover on the cheeks and neck. That is not a sunscreen replacement. If the cap is all that stays on, those sites are bare.
5. Add sunglasses. A brim cuts glare. UV-blocking lenses close the rest of the eye dose.

Sources

Hat shade at the face, including baseball-cap ears and neck near 1, and hats rarely enough alone at UV 3+: ARPANSA, sun protection using hats. 3 in brim all around, or 3 in bill plus a permanently attached drape; fabric UPF 50: Skin Cancer Foundation Seal. Same ≥3 in / 75 mm circumferential brim, and “do not rely on standard baseball caps”: CDC Yellow Book, Sun Exposure in Travelers. UPF 30 cloth passing about 1/30 of UV; caps cannot claim a UPF rating: SunSmart, sun-protective hats. Eye figures: Cancer Council sunglasses. A crown UPF label does not cover the ears.