How SPF Is Tested (and Why the Number on the Bottle Can Be Off)

Same formula, two labs, two different numbers. Here’s how SPF is actually tested, and why we labeled Cloudwear at 30.

Written by Dr. Aimee Paik, MD, board-certified dermatologist and VIZE co-founder

In short

SPF measures one thing: how well a sunscreen protects against UVB, the rays that burn. That number comes from a test on ten people's skin, run once, and nothing requires a brand to test again. Skin varies, and labs vary, so the same formula can come back with different numbers. That's how a label can end up overstating what a sunscreen actually delivers, without anyone intending it.

What SPF actually measures

SPF stands for sun protection factor, and it measures one specific thing: how well a sunscreen protects you from UVB, the rays that cause sunburn and most skin cancers. Technically, some shorter-wavelength UVA also contributes to burning and is captured in SPF testing. But SPF numbers do not capture the longer UVA rays that penetrate deeper into the skin and are linked to premature aging. We'll get to those. For now, hold onto this: SPF is a sunburn number, not a whole-protection number.

How SPF is tested (on actual people)

SPF is tested on real human skin, not in a machine. It's called an in vivo test, and in the US it's the only version the FDA accepts.

A lab applies a measured amount of sunscreen, a thick layer of 2 milligrams per square centimeter, which is more than most of us ever actually use, to a patch of skin on the back. After it dries, they expose that skin to increasing doses of UV light until it just barely reddens. Then they compare that to how fast bare skin reddens. The ratio between the two is your SPF. If protected skin takes 30 times longer to burn, that's SPF 30.

The FDA requires each sunscreen to be tested this way once, by a third-party lab, on 10 people, with the results averaged. Because everyone's skin reacts a little differently, that averaging matters.

Where SPF testing gets complicated

During the test, two people can get different SPF results from the same sunscreen, simply because their skin responds differently. That part is normal, and the testing rules already account for it. The labeled SPF isn't the average of those ten results, it's a lower figure calculated to sit underneath them. 

Ten people is the whole sample. That's all the FDA requires to put a SPF number on a bottle and sell it in the US, and nothing in the regulation asks for a second test or a larger panel. This is why discrepancies can happen. 

Because sunscreen takes years to develop and is really expensive to formulate and test, it’s hard for brands to find real incentive to invest in more than one SPF test when it is not required. A second test could come back higher, but that doesn't help you much, because you have to use the lower result. If it comes back lower, you still have to go with the lower SPF result or go back to the formulation phase to try and achieve a higher rating. 

There's a second thing the system doesn't require, and it matters just as much. Someone has to read the results. Knowing what a normal spread looks like, noticing a result that doesn't sit right, understanding what the formula should be capable of in the first place. That takes formulation expertise and technical understanding. Without it, a lab result is just a number you accept. At VIZE, a chemist and a board-certified dermatologist review every result for exactly this reason.

For our first hybrid sunscreen, Cloudwear Daily SPF 30 PA+++, the first lab measured it at SPF 46. We sent the same formula to a second lab and it came back at SPF 30. Same product, two different results, with exactly the variability we just described. 

SPF is only part of the story (meet UVA and PA+++)

Remember that SPF only measures UVB. But what about UVA? The ultraviolet light waves that make up as much as 95% of the UV that reaches your skin. The SPF number on the bottle tells you nothing about the protection from UVA rays.

UVA reaches all the way down to the dermis, the layer where your collagen lives. Over time it breaks that collagen down, which is what shows up as loss of firmness and fine lines. It also switches on the pigment-producing cells behind hyperpigmentation. Age spots? Those are sun spots. And UVA can do all of it without ever causing a sunburn. So there can be no redness, but the damage still accumulates over time. 

"Broad-spectrum" means a sunscreen that protects against both UVA and UVB, which is great. But broad-spectrum alone doesn't tell you how much UVA protection you're getting. For that, there's a separate grade called PA, running from PA+ (some) up to PA++++ (the most). Cloudwear is PA+++, which is high UVA protection. So the full picture on our label is SPF 30 for UVB and PA+++ for UVA.

The VIZE testing protocol 

If we had to sum up how we formulate a sunscreen in one word, it would be meticulous.

Every VIZE formula is SPF tested at a minimum of two different independent labs and reviewed by a chemist and board-certified dermatologist with technical expertise. Whenever the numbers disagree, we use the lower one. Instead of relying solely on a pass/fail broad-spectrum qualification, we perform additional testing to determine the level of UVA protection, resulting in a PA+ rating. We perform photostability testing to ensure the UVA protection is reliable with sun exposure. We invest in clinical testing so every claim has data behind it, from sensitive skin to hydration. Each one is verified by our regulatory expert before it reaches the label.

We built VIZE to be a sunscreen you can trust completely. Sunscreen is one of the most technically challenging categories in skincare. It requires precise chemistry, where the raw materials, formulation, and testing all matter. Every decision behind Cloudwear was made to hold the product to an exceptionally high standard of verification.

What this actually means for you

A sunscreen's number is only useful if two things are true:

• The testing behind it was done right.

• You actually wear enough of it.

When you're shopping for an SPF, see if the company has this information clearly accessible on its website: 

• How many SPF tests did they run? And at how many labs?

• Did they test UVA separately? Does it have a PA rating? 

• Who reviewed the results?

And remember, the most carefully tested sunscreen only works on the days you wear it, and only at the amount it was tested with. Most people apply only a quarter to half that much, and because protection drops steeply with under-application, real-world SPF often lands far below the number on the bottle. Use 2 finger lengths (at least a quarter teaspoon) for your face and neck, every morning.