SPF 30 vs SPF 50 – what does it actually mean?

In the previous posts I discussed the CHOICE sunscreen testing and how differences in testing and manufacturing may have contributed to the results.

The next practical question is what those numbers actually mean for everyday use.

Sunscreens that performed best in the testing

Sunscreen	Claimed SPF	Tested SPF	Type
La Roche-Posay Anthelios Wet Skin	50+	~72	Chemical
Neutrogena Ultra Sheer Body Lotion	50	~56	Chemical
Cancer Council Kids Sunscreen	50+	~52	Chemical
Mecca Cosmetica To Save Body	50+	~51	Chemical
*Invisible Zinc Face + Body Mineral	50	~38	Mineral

*best performing Zinc sunscreen

Most other products still provided useful protection but often tested closer to the SPF 20–40 range.

What SPF actually measures

SPF measures protection against UVB radiation, the part of sunlight that causes sunburn.

It compares how much UV exposure causes redness on protected skin compared with unprotected skin.

Another way to understand SPF is the percentage of UVB radiation blocked.

SPF	Approx UVB blocked
SPF 15	93%
SPF 30	97%
SPF 50	98%

Does SPF 50 mean you can stay in the sun longer?

In laboratory testing SPF is based on time to sunburn.

For example, if someone normally burns after 5 minutes without sunscreen:

SPF	Theoretical burn time
SPF 30	~150 minutes
SPF 50	~250 minutes

However, this assumes sunscreen is applied very thickly (2 mg/cm²).

In everyday use people usually apply less than half this amount, and sweating, swimming and missed areas reduce protection.

For this reason, sunscreen should still be reapplied every two hours regardless of SPF rating.

The practical difference between SPF 30 and SPF 50 is therefore modest but helpful, providing a slightly larger margin if sunscreen is applied thinly or unevenly.

What if a sunscreen performs below SPF 30?

The sunscreen will still give protection however, the margin for error becomes smaller, especially in a high-UV environment like Australia.

For this reason, most guidelines recommend SPF 30+ or higher.

 Simple takeaway

• SPF 30+ would be the minimum I would use

• SPF 50 gives an additional margin of safety

• application thickness matters

  • one teaspoon per limb, front and back of the body

  • half a teaspoon for the face, neck and back of ears

• reapplication matters

  • reapply every 2 hours unless stated otherwise

• chemical sunscreens performed the best in the choice study

In the next post, I’ll look at the differences between mineral and chemical sunscreens, and also discuss how sunscreen ingredients are assessed for safety.