SPF explained
SPF 100 is not three times SPF 30. The scale is not linear, it only measures one kind of light, and it is tested at a dose almost nobody uses.
By Sawyer V. · Published
We have not tested any of this. Everything below is compiled from published manufacturer specifications, and the arithmetic is printed so you can check it. This page carries affiliate links: if you buy through one we earn a commission at no extra cost to you, and it does not change the order of the picks. Full disclosure.
The short version
SPF 30 to 50, broad spectrum, applied properly, is the whole answer. The scale is not linear - SPF 30 filters about 97 per cent of UVB and SPF 50 about 98, so the extra number buys about one percentage point. SPF measures UVB only; “broad spectrum” is the label that means UVA is covered too, and UVA is the aging one. The variable that matters far more than the number is how much you put on.
| # | Pick | Lasts | Score | Price |
|---|---|---|---|---|
| 1 | EltaMD UV Clear SPF 46 Shaved skin that breaks out | ~40 uses | 9.1/10 | $45.00 - view on Amazon |
| 2 | La Roche-Posay Anthelios Melt-In Milk SPF 60 Using enough of it | ~120 uses | 9.3/10 | $29.24 - view on Amazon |
Prices last verified September 24, 2026. They move constantly; the figure on Amazon when you click is the real one.
What the number actually is
SPF is a ratio: how much longer skin protected by the product takes to redden compared with unprotected skin, under laboratory conditions. SPF 30 means thirty times longer to burn, not thirty per cent protection.
Converting that ratio into filtration gives the number people actually want, and it is where the marketing stops being useful.
| SPF | UVB filtered | Gained over the row above |
|---|---|---|
| 15 | about 93% | - |
| 30 | about 97% | about 4 points |
| 50 | about 98% | about 1 point |
| 100 | about 99% | about 1 point |
This is why 30 to 50 is the sensible range, and why an SPF 100 product is mostly selling a number. It is also why the difference between applying SPF 50 properly and applying SPF 100 thinly runs the wrong way.
The half the number does not cover
SPF is a UVB measurement. UVB is the shorter wavelength that causes sunburn - which makes it the obvious thing to test, because burning is visible and measurable.
UVA is longer, penetrates deeper into the dermis, does not cause a visible burn, and is the wavelength associated with photoageing and cumulative damage. It also passes through window glass, which UVB largely does not.
“Broad spectrum” is the label term that means UVA has been tested for too. An SPF number without it is telling you about one of two problems. This is the single most useful thing to check on a bottle and it takes two seconds.
The dose, which is where it all falls apart
SPF is tested at 2 mg per square centimeter of skin. Almost nobody applies that much. Studies of real-world application repeatedly find people using between a quarter and half of it.
Protection does not scale linearly with the shortfall - applying half the tested dose gives considerably less than half the labeled SPF. So an SPF 50 applied at half dose is not SPF 25; it is meaningfully worse than that.
- Face and neck: about 1.25 ml. A third of a teaspoon. The two-finger method - a strip along the length of your index and middle fingers - is the usual practical measure and it is more than most people expect.
- Reapply every two hours outdoors, and after swimming or heavy sweating regardless of what the label says about water resistance.
- Do not skip the areas that get missed: ears, the back of the neck, the hairline, and the tops of the ears specifically.
The three places men under-apply
Worth naming because they are consistent and they are all skin that this site's reader specifically exposes.
- A shaved scalp. The highest point of the body, at the angle that receives the most direct sun, on skin that has spent your entire life covered. The head shaving guide treats this as the least optional step in the whole routine.
- The ears. Short hair exposes them and nobody thinks about them.
- Skin under a beard - or rather, the skin at its edges. A dense beard does block some UV, but the cheek line, the neckline and any patchy area do not. That page covers the application problem, which is real: getting a lotion to skin through hair is genuinely awkward.
What to actually buy
Broad spectrum, SPF 30 to 50, in a texture you will apply a full dose of every morning. That last clause carries more weight than any of the others - the best-formulated sunscreen in the world does nothing in the tube, and the practical difference between two products at this SPF range is almost entirely whether you keep using them.
The roundup ranks on published SPF, published filter concentrations and whether the maker publishes anything at all - which, in a category regulated as an over-the-counter drug, a surprising number still manage to avoid doing clearly.
Every pick, with the working
Each entry shows the published specifications, the axis scores they produce and what you give up. The order is the computed order, not a commercial one, and the arithmetic is open underneath every score.
Our pickShaved skin that breaks out
EltaMD UV Clear SPF 46
50 ml of SPF 46 built on 9% zinc oxide and 7.5% octinoxate, with 5% niacinamide, oil-free, fragrance-free and non-comedogenic. Freshly shaved skin is skin with its top layer taken off, and this is the sunscreen that goes on it without causing the breakouts that stop people wearing one.
- Volume
- 50 ml
- SPF
- 46
- Fragrance
- Fragrance-free
- Refill
- Nothing to refill
- Warranty
- Not applicable
- How long it lasts
- 40 uses
Published key ingredients: Zinc oxide 9%, Octinoxate 7.5%, Niacinamide 5%.
The arithmetic: 50 ml / 1.25 ml per face-and-neck dose (assumed) = about 40 applications.
What it gets right
- Published actives: zinc oxide 9%, octinoxate 7.5% — an actual disclosed concentration
- 5% niacinamide, published as a percentage rather than a claim
- Oil-free, fragrance-free, paraben-free and non-comedogenic
- Formulated for acne-prone skin, which is where shaving irritation lives
Flaws
- 50 ml at a price that makes daily face-and-neck use a real decision
- SPF 46 is a body-and-face lotion price for a face-only bottle
The working behind the 9.1
- How long it lasts: 9.2
- 50 ml / 1.25 ml per face-and-neck dose (assumed) = about 40 applications. At five shaves a week, that is about 1.8 months. A 2-month supply scores 10 in this category, so 1.8 / 2 x 10 = 9.2.
- Ease of switching: 9.0
- Published active percentages, non-comedogenic and fragrance-free (++), formulated for the acne-prone skin shaving irritation produces (+), 50 ml at a price that discourages using enough of it (-).
- Refill economics: 8.0
- Nothing to refill. The only future cost is replacing the item itself.
- What the maker publishes: 10.0
- 4 of 4 specifications a buyer would expect are published: volumeMl, spf, keyIngredients, fragranceFree. 4 / 4 x 10 = 10.0.
Axes with no published input are dropped and the remaining weights re-normalized, so a product is not punished for a category where an axis does not apply. Specifications compiled 3 September 2026 from EltaMD UV Clear SPF 46 published actives.
2Using enough of it
La Roche-Posay Anthelios Melt-In Milk SPF 60
148 ml of SPF 60 for face and body, with published actives, fragrance-free, oil-free, non-comedogenic and water-resistant for 80 minutes. The reason it is here is volume: sunscreen only works in the quantity people actually apply, and a bottle three times the size is a bottle you use generously.
- Volume
- 148 ml
- SPF
- 60
- Fragrance
- Fragrance-free
- Refill
- Nothing to refill
- Warranty
- Not applicable
- How long it lasts
- 120 uses
Published key ingredients: Avobenzone 3%, Homosalate 10%, Octisalate 5%, Octocrylene 7%.
The arithmetic: 148 ml / 1.25 ml per face-and-neck dose (assumed) = about 120 applications.
What it gets right
- 148 ml for face and body — the volume that makes generous application affordable
- Published actives: avobenzone 3%, homosalate 10%, octisalate 5%, octocrylene 7%
- Fragrance-free, oil-free, non-comedogenic, water-resistant for 80 minutes
- SPF 60, the highest here
Flaws
- A chemical filter set rather than mineral, which some reactive faces prefer to avoid
- Body-lotion texture is heavier on the face than a dedicated facial sunscreen
The working behind the 9.3
- How long it lasts: 10.0
- 148 ml / 1.25 ml per face-and-neck dose (assumed) = about 120 applications. At five shaves a week, that is about 5.5 months. A 2-month supply scores 10 in this category, so 5.5 / 2 x 10 = 10.0.
- Ease of switching: 9.0
- 148 ml removes the rationing that makes most people under-apply sunscreen (++), published actives and fragrance-free (+), a body texture on the face and a chemical filter set (-).
- Refill economics: 8.0
- Nothing to refill. The only future cost is replacing the item itself.
- What the maker publishes: 10.0
- 4 of 4 specifications a buyer would expect are published: volumeMl, spf, keyIngredients, fragranceFree. 4 / 4 x 10 = 10.0.
Axes with no published input are dropped and the remaining weights re-normalized, so a product is not punished for a category where an axis does not apply. Specifications compiled 3 September 2026 from La Roche-Posay Anthelios Melt-In Milk published actives.
Prices last verified September 24, 2026. They move constantly; the figure on Amazon when you click is the real one.
Questions people actually ask
Is SPF 50 twice as good as SPF 25?
No. SPF is a ratio, not a percentage, and the protection curve flattens fast. SPF 15 filters roughly 93 per cent of UVB, SPF 30 about 97, SPF 50 about 98. Doubling the number adds about one point once you are past thirty.
What does broad spectrum actually mean?
That the product is tested to protect against UVA as well as UVB. SPF only measures UVB - the burning wavelength. UVA penetrates deeper, does not burn, and is the one associated with photoageing. A high SPF without broad spectrum is protecting you from the sunburn and not from the rest.
How much sunscreen should you actually use?
For a face and neck, roughly 1.25 ml - about a third of a teaspoon, or two finger lengths squeezed onto two fingers. That is the assumption behind our yield arithmetic and it is far more than most people apply. Under-application is the single biggest gap between the number on the bottle and the protection on your face.
Mineral or chemical filters?
Both work. Mineral filters - zinc oxide, titanium dioxide - sit on the surface and are the safer bet on reactive skin. Chemical filters absorb the light and generally feel lighter and leave no cast. What matters more is that whichever you choose is one you will reapply.
Do I need sunscreen in winter or indoors?
UVA passes through window glass and is present year round, so a daily habit is easier to keep than a seasonal judgment. If you are indoors away from windows all day it genuinely matters less - but the effort of deciding each morning exceeds the effort of just applying it.