Sunscreen is one of the few beauty products whose label carries a measurable promise, and one of the few that most people read incorrectly. The number on the front is not a general “strength” score, and the stars on the back are not decoration. They measure two different kinds of ultraviolet light, and a bottle that does well on one can do poorly on the other. Knowing which is which makes shopping simpler, and makes it much easier to ignore the marketing.
SPF: the number on the front
SPF stands for sun protection factor, and it measures protection against UVB, the type of ultraviolet radiation that is the main cause of sunburn. NHS guidance describes the scale as running from 2 up to 50+, with the higher numbers offering stronger UVB protection, and recommends choosing at least factor 30.
It helps to understand what the number does not say. It is not a measure of how long you can stay in the sun, and it is not a promise that doubling the factor doubles the protection. The gains at the top of the scale are smaller than the numbers suggest, which is why a well-applied factor 30 is a sensible baseline rather than a compromise. What matters far more than the difference between 30 and 50 is whether enough product is actually on the skin.
The stars: the part people skip
The second rating is the UVA star rating, which appears as up to five stars on the back or side of the pack. UVA is the longer-wavelength radiation that penetrates more deeply and is associated with skin ageing, among other effects. A high SPF on its own does not tell you about it, which is why NHS advice is to look for at least four-star UVA protection, or the UVA symbol in a circle, which indicates that the product meets the EU standard for UVA protection.
So the shorthand is simple: SPF 30 or higher for UVB, four or five stars for UVA. A label that shows only the first and is silent on the second is telling you half the story.
Amount matters more than anything else
The most common mistake is not the choice of product; it is the quantity. Ratings are tested at a generous layer of sunscreen, and people usually apply much less. NHS advice for adults covering the whole body is around six to eight teaspoons, which is considerably more than most people use.
That figure is for the full body, as on a beach holiday. For everyday use on the face and neck, the principle is the same: apply generously and evenly, and do not rub in a thin smear and consider the job done.
When to reapply
Sunscreen does not stay put. NHS guidance is to reapply every two hours, and also after swimming, towel-drying or sweating, even when the product is labelled water-resistant. “Water-resistant” describes how well a product holds up after a certain amount of water exposure, not how long it lasts for, and it does not mean you can skip the next application.
If you wear make-up, reapplying is awkward, and that is a real obstacle for many people. The practical answer is to build your protection from more than one layer: a daily sunscreen under make-up for ordinary days, and a hat, shade and clothing for longer days outside.
Sunscreen is one tool, not the whole plan
The most important line in the NHS guidance is the least glamorous one: do not rely on sunscreen alone. Suitable clothing, time in the shade when the sun is at its hottest, and a hat do work that a product cannot. This matters in the UK, where it is easy to assume that cloud means safety. Ultraviolet light is not the same thing as heat, and the strength of the sun, not the temperature, is what matters.
Reading the small print
A few other things on a label are worth noticing.
- The open-jar symbol. This shows how many months a product stays usable once opened. An old half-bottle from a previous summer may be past its best.
- Formulations. Mineral and chemical filters are both used in sunscreens that meet the same standards; the better choice is the one you will actually wear, and apply properly, every day. Feel, scent and finish matter because they decide whether a product is used.
- Marketing claims. Terms such as “all-day”, “invisible” or “natural” are not substitutes for the two ratings above.
Who should be more careful
Some people need a higher level of caution than the general advice, including those with very fair skin, a history of sunburn, many moles or a family history of skin cancer, and anyone taking medicines or using skincare that makes the skin more sensitive to light. If any of that applies to you, or you notice a mole that changes in size, shape or colour, speak to a pharmacist or your GP.
The short version
Choose factor 30 or higher, check for four or five stars, apply more than feels necessary, reapply on schedule, and treat shade and clothing as part of the plan. It is not an elaborate routine, but it is a rare example of beauty advice where the label is a reliable guide, once you know what the label is saying.
This article is general information, not medical advice. If you are concerned about your skin or a mole, speak to a pharmacist or your GP.
Sources: NHS (Sunscreen and sun safety)
