Sunscreen Math: Can You Add Up SPF? - Beautiful With Brains

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Last Updated on March 20, 2025 by Giorgia Guazzarotti

If you use a moisturiser with SPF 20, a sunscreen with SPF 30, and a foundation with SPF 15, what SPF do you get?

30 + 20 + 15 = 30.

Wait, what? Shouldn’t it be SPF 65? Not quite.

SPF math isn’t like regular math. You can’t just add up all the numbers and expect superhero-level sun protection. Here’s why:

Table Of Contents

• Does Layering Sunscreens Increase Sun Protection?
• How SPF Layering Works in Real Life
• Is Layering Sunscreens Useless?
• The Problem With Layering Sunscreen
• How To Layer Sunscreens The Right Way
• The Bottom Line

Does Layering Sunscreens Increase Sun Protection?

Short answer: No. But there’s more to the story.

Layering sunscreens will NOT give you added SPF protection.

It’s a common misconception, and honestly, it makes sense why people believe it. We’re used to thinking that more layers = more protection. If you wear two sweaters, you stay warmer. If you apply two coats of nail polish, you get a more opaque color. But SPF doesn’t work like that.

In the words of Mona Gohara, a dermatologist and an associate clinical professor of dermatology at Yale University, “SPF isn’t an equation. Your sun protection is only as strong as your highest SPF.”

In the example above, the highest SPF is that of your sunscreen, SPF 30. That’s what you’re getting. No matter what else you apply, that number won’t go up.

Why? Sunscreen works by limiting the number of UV rays that hit your skin.

• SPF 30 blocks 97% of UVB rays (the ones that cause sunburn).
• SPF 50 blocks 98% (not a huge difference, right?).
• SPF 15 only blocks 93%, which means a lot more UV rays are still getting through.
• Even SPF 100 isn’t perfect. It still lets 1% of rays through.

Notice how the difference between SPF 30 and SPF 50 is only 1%? That’s why sunscreen layering doesn’t give you a magic boost in SPF—it’s all about how much actually gets absorbed by your skin.

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How SPF Layering Works in Real Life

Let’s see what this means in practical terms using the example below.

Your foundation has SPF 15. That allows 7% of UV rays to reach (and harm) the skin. But you’ve learned a moisturiser with SPF 20 underneath. That allows only 5% of UV rays to reach the skin. 2% of the rays that made it through your foundation are now neutralised by your moisturiser.

But you have a third layer on. Your sunscreen with SPF 30 only lets 3% rays through, neutralising a further 2% that made it past your foundation and moisturiser with SPF.

The higher the SPF, the fewer % of UV rays reach the skin. Unfortunately, no sunscreen blocks 100% of them. Even SPF 100 still lets 1% through.

If applied correctly, that is…

Related: Take A Number: What Level Of SPF Is Right For You?

How effective is your sunscreen? Sign up to the newsletter below to receive the “Sunscreen Audit” Worksheet and find out if your sunscreen is really up to the job:

Is Layering Sunscreens Useless?

Layering products with SPF will not add up SPF, but it may increase your overall coverage.

This is where layering can be helpful. Even though it doesn’t boost your SPF level, it can help you cover areas you might have missed the first time around.

Let’s say you’re in a rush and applying sunscreen quickly. It’s easy to miss a spot or two. Those spots have no protection and are fair game for UV rays.

But if you slather on a second SPF product, you have more chances of covering every little nook and cranny.

One study found that “the combined effect of two sunscreen applications gave on average 2.3 times better protection from UVR than a single sunscreen application.”

I personally don’t do this because I am very diligent with sunscreen application, but if you think you need it, go ahead.

WARNING! If you don’t apply the recommended amount (1/4 of a teaspoon for the face alone), you may get better coverage, but you won’t get the SPF stated on the label.

Related: Are You Applying Sunscreen The Right Way?

The Problem With Layering Sunscreen

If you decide to layer sunscreens to get that extra bit of sun protection, be careful.

Not all UV filters work well together. Some can deactivate each other and compromise the level of protection you’re getting.

This is where it gets tricky. Sunscreen formulas are carefully designed to balance different UV filters in a way that makes them stable and effective. But if you start mixing different brands and types of sunscreens, you could be messing with that balance without even realizing it.

In particular, beware of mixing Avobenzone with Octinoxate, Zinc Oxide, and Titanium Dioxide.

If you’ve just checked the label of your fave sunscreen and noticed it contains a few of these (for example, Avobenzone and Octinoxate), don’t throw it in the bin yet!

Cosmetic chemists know all kinds of tricks to make UV filters get along. For example, by coating them so they give you the best protection without getting too close to each other.

It’s when you use separate products that were never meant to be mixed together that you may get in trouble.

Another issue? Different sunscreens have different textures and finishes. If you layer a thick, occlusive sunscreen under a lightweight gel formula, it might pill or rub off. Worse, if your first layer is too greasy, the second layer might not adhere properly, meaning you’re getting less protection than you thought.

How To Layer Sunscreens The Right Way

I’m not a fan of using different sunscreens. It may help, but unless you know more about the formulations and how UV filters interact with one another, you may reduce – rather than increase – sun protection. Ouch!

But if you still want to do it, here’s how to do it right:

1. Reapply the same sunscreen twice: You still get the same level of SPF, cover every nook and cranny, and run no risk of deactivating UV filters.
2. Use two sunscreens with the same (or similar) UV filters: You’ll still get the highest SPF level of the two, better coverage, and a much lower risk of anything deactivating UV filters.

And most importantly, apply enough.

• 1/4 teaspoon of sunscreen for your face.
• 1 shot glass of sunscreen for your body.
• Reapply every 2 hours.

It doesn’t matter how expensive or fancy your sunscreen is, if you’re not applying enough, you’re not getting the full SPF on the label. Period.

The Bottom Line

SPF works by limiting the amount of UV rays that actually reach your skin. Layering sunscreens and products with SPF won’t give you a higher SPF. But it may give you better coverage.