If you change one thing after reading this site, make it this. Sunscreen is the highest-return product in any routine and the one most consistently used wrong — not by choosing badly, but by applying too little.
The number that matters
SPF is measured in a lab at 2 mg per square centimetre of skin. For a face and neck that is roughly:
- Two finger-lengths — product squeezed along the full length of your index and middle fingers, or
- A generous half-teaspoon
Most people apply about a quarter of that.
Why a quarter does not give you a quarter of the protection
Protection does not scale linearly with thickness. Research consistently finds that applying roughly half the tested amount delivers substantially less than half the labelled SPF — an SPF 50 used at typical real-world thickness performs somewhere closer to an SPF 15.
The practical consequence: the amount you apply matters more than the number on the bottle. A generously applied SPF 30 beats a thin SPF 50 every time.
Measure two finger-lengths once, against your own fingers. It will look like far too much. It is not.
Reapplication
Filters degrade in UV light and rub off on hands, phones and collars. A morning application is not protection at 4pm.
- Outdoors or beside a window: every two to three hours.
- Indoors, away from windows: one morning application is genuinely fine. The panic about reapplying at a desk is overdone.
- After swimming or sweating: immediately, whatever the bottle claims about water resistance.
Over makeup, a powder or stick SPF is the practical compromise. Not as good as reapplying properly, far better than nothing.
What PA++++ means, and why it matters more than SPF
SPF measures UVB — the wavelength that burns. It says almost nothing about UVA, which penetrates deeper, drives pigmentation and photo-ageing, and passes through window glass.
The PA rating measures UVA protection, from PA+ to PA++++. Look for a PA rating or the words "broad spectrum".
This is also the answer to "why isn't my pigmentation treatment working". UVA through a window, every day, refills pigment as fast as your serum fades it.
The places everyone misses
In rough order of how often they show damage: ears, neck and the back of the neck, chest, backs of hands, eyelids, and the hairline or parting.
Chemical vs mineral
Neither is safer or more effective. Modern broad-spectrum chemical filters are well studied. Choose on how it feels, because how it feels determines whether you use enough of it.
The rule underneath all of this
The best sunscreen is the one you will apply generously, every morning. Elegance is not vanity here — a formula you dislike is one you will under-apply and skip on rushed days, and that costs more protection than any difference between two well-made products.
Frequently asked
Two finger-lengths, or a generous half-teaspoon, for face and neck. Most people apply roughly a quarter of that, which reduces the real-world protection of an SPF 50 to somewhere nearer SPF 15.
Beside a window, yes — glass blocks most UVB but far less UVA, which is what drives pigmentation and ageing. Away from windows, a single morning application is fine.
No. The jump from SPF 30 to SPF 50 is small compared with the difference between applying enough and applying too little. A generously applied SPF 30 outperforms a thin SPF 50.
