Chemists' Ultimate Sunscreen Application Guide

chemists how to choose sunscreen guide
First Posted on June 5, 2023 in:insights & debunksskincare fundamentalssunscreentop reads

We’ve hit sunscreen season again! With summer in full swing, the most important yet basic fundamental for good sunscreen protection is simply applying sunscreen correctly to safeguard skin from sunburns, risks of skin cancer, and premature skin aging. It’s not enough to just remember to use sunscreen here and there. Proper application (and re-application!) is key to ensuring you’re getting the proper, advertised level of sun protection throughout the day. So whether you’re got grandiose travel adventures, you’re lounging by the pool, or busting out your backyard grill, let’s take a look at some top chemists’ sunscreen application tips to help your skin stay safe and healthy.

Before We Begin! Reminder of Golden Rule #1 Texture is King

It feels like the quest for the perfect sunscreen texture is quite the elusive one for both consumers and chemists’ alike.

We’ve probably all experienced that thick, greasy classic sunscreen lotion that make us dread the next time we have to apply sunscreen. Even though sunscreen technology has made quite some progress in the past few decades, the texture landscape can still be quite the minefield.

Why are sunscreen textures so challenging? First, here in the US we’re limited to a much smaller pool of approved sun filters, while the rest of the world has moved on to more elegant textures thanks to new and improved sun filters like the Tinosorbs. Yes, that means the US is still currently stuck with filters from the Stone Age. The most problematic part of this limitation is that here in the US, we only can rely on one chemical filter for UVA protection, and that’s avobenzone. And of course, avobenzone texure isn’t great. And don't get us started on its stability problems.

But wait! What about physical filters like zinc oxide that can also offer broadspectrum protection? In recent years, physical (aka. mineral sunscreens have enjoyed a major rise in popularity. That said, these are even messier in terms of texture. Mineral filters like zinc oxide and titanium dioxide are the main culprits for the dreaded clown-worthy white cast. In our experience, the mineral sunscreen field is even more prone to unusable duds than chemical sunscreen.

All in all, sunscreens are just inherently difficult to formulate. The two of us can tell you that working on a “Tear your hair out” sunscreen project is practically a rite of passage for many skincare chemists. These are incredibly finicky formulas. Even changing what seems to be a harmless, vanilla ingredient can greatly impact the final performance of the sunscreen.

This whole rant is really just a reminder that our sunscreen #1 rule is to just find a texture you’re willing to use. There is no better sunscreen protection than applying generous amounts of a sunscreen on the daily.

Golden Rule #2: More is More

A majority of people drastically under-apply sunscreen. In fact, studies have shown that most people apply just ⅓ to half of how much you need to get the advertised coverage. Why is that?

All sunscreens’ SPF values are measured the same way – applying 2 mg (milligram) of product per 1 cm2 of skin. Since we come in all shapes and sizes, most people have no idea what their actual skin surface area is… nor weigh their sunscreen out upon every application, you can imagine actual usage level is all over the place. We did some digging and with some stitched-together values, we found that:

  • The surface area of adults is about 18,000 cm2 (men) or 16,000 cm2 (women).
  • Fraction of body surface area that includes the face & neck area:

human approximate skin surface area

  • For females, the face and neck area is ~990cm2. 
  • The average sunscreen needed for face and neck per female and male is roughly: 1.76 g for females and 1.98 g for males.

Add on sunscreen formats (lotions, sticks, sprays) plus the added variability of texture, 2 g can feel very different between a creamy formula vs a spray formula. All these exacerbate the problem of sunscreen under-application.

So how does one figure out if they're using the right level of sunscreen? Thanks to the wonderful interwebs, there are two main methods used to gauge proper sunscreen usage: the 1/4 teaspoon rule and the two finger method. How good are these? We did the test!

The ¼ Teaspoon Rule: First up is the ¼ teaspoon rule. This guideline suggests that your face and neck needs about ¼ teaspoon worth of sunscreen to get the proper, measured, advertised level of UV protection.

sunscreen 1/4 teaspoon rule

Turns out, the ¼ teaspoon sunscreen guidelines is a pretty good one. Gloria approximated her face and neck surface area to calculate how much product she would actually need to reach 2mg/cm2 for her face dimensions, then we weighed out a bunch of different sunscreens with a ¼ teaspoon and weighed it. As you can see from the chart, this is actually a solid rule to go by regardless of what type of sunscreen you’re using.

The Two-Finger Method: We’re not always going to have a little measuring spoon on us each time we want to put on sunscreen, so how about the viral “Two Finger” method. Exactly as it sounds, this method claims that your face needs about two fingers worth of sunscreen. So how did this one fare?

sunscreen use two finger method

This one definitely comes with a LOT of variability. On this chart, the green line represents the calculated amount of sunscreen needed for face & neck, while the red line represents about how much the amount of sunscreen is needed for just the face. The orange bar represents Gloria applying the 2 finger rule to a range of sunscreen types, whereas the grey bar represents Victoria using the sunscreens. As you can see, this comes with a LOT of user and formula variability. For some formulas, the two-finger rule was way under for even just face application, while a few formulas provided enough for face AND neck. Though we only did each test once, there’s no doubt in our minds that if we re-did the test several times, there would also be a lot of variability between trials.

Even though this isn’t the most accurate method, the 2 finger method is still something. If nothing else, it’s a lot more convenient than the ¼ teaspoon method. So from our learnings, here are some of our suggestions from executing the 2 finger rule correctly:

  1. Coat your fingers! One of the biggest variables came from just how covered your fingers are with sunscreen. For products with a smaller nozzle, it’s possible to squeeze out a pretty skinny line of sunscreen. The reality is, you most likely need to cover your entire finger like an extra glazed donut for it to be enough.
  2. Consider adding that 3rd finger: If you’re like Gloria, and you consistently under-apply per the 2 finger rule, consider adding a third finger worth of sunscreen to get proper coverage.

But wait! What about all the variability from the different sunscreen formula types? This brings us to golden rule # 3….

Golden Rule #3: Choose Your Sunscreen Format Wisely

Nowadays, you can get your sunscreens in a range of formats. Each of the formats has its own quirks and application tendencies. We also noticed that packaging plays a role as well. Here’s a quick run-through on the pros and cons of each!

  • Lotions (Large Nozzle Size): Naturally dispenses enough product with even just two fingers. If there’s a texture you like in this format, this is probably the most reliable way.
  • Lotion (Small Nozzle Size): More prone to underapplication than ones with larger nozzles. If you remember to glaze your finger thoroughly or go up to that third finger, this is still a reliable format.
  • Fluid: With drippy textures, the 2 finger rule is probably a no-go. We would recommend at least three fingers worth, or even measuring with a ¼ teaspoon once and pouring into your palm to give yourself a visual gauge on how much you would need for your face & neck.
  • Mousse: A slightly less common format. In this category, we tested EVY technology and Vacation’s Whip sunscreen. We were pleasantly surprised by this category! Though this format completely negates the ¼ teaspoon method and the two finger method, we were pleasantly surprised that it isn’t difficult to get a proper amount of sunscreen. A dollop (golf-ball size) dispensed by both of these sunscreens in your palm provides enough weight for your face & neck area.
  • Spray: More challenging. Sprays are a pretty popular form of sunscreen for its ease of application. But, after testing the spritz application, it’s pretty difficult to get the amount needed for proper protection. The standard testing protocol will spray the designated amount onto your hand before rubbing on for that even coverage. However, if you spray directly onto your skin, be sure to spritz lavishly and rub in well. A+ for convenience factor, C- in terms of reliability.
  • Balm/Stick: Similar situation to spray but worse than spray. The way you’d naturally want to use a sunscreen balm, you’re most likely way way WAY under apply. The couple of sticks we’ve tried sadly didn’t come with textures that make us want to rub on an extra thick layer. We would say sticks just aren’t the most reliable for every day use.
  • Powder: Of all the formats out there, powder sunscreens are the worst. Mainly because powders are flighty and that means you would need an unnecessary amount of powder to get proper protection. When we tested this format, not only did our scale not detect the amount of the powder dispense, using the actual amount of powder needed on skin was so excessive that all of it didn’t absorb and just piled up on skin. We know there are many that want to be able to use this as a reapplication technique over makeup, but the amount would be pretty negligible that you could probably just save yourself a step. A better but more cumbersome method is finding a tinted moisturizer that you can reapply (makeup wipe needed).

Bonus! Pro-Sunscreen Tips on Reapplication and Wash-off

Reapplication is probably the worst part of sunscreen since it’s usually at a pretty inconvenient time point and you’re most likely sweaty, sandy, or both! The slightly cumbersome method for a cleaner reapplication is to have a makeup wipe handy. We felt this was incredibly necessary for tinted sunscreens because applying a second coat of a heavily pigmented formula was not fun. Not only do you look pretty silly, but we’re talking some pretty gross textures and ruined laundry as well.

As far as washing off sunscreen goes, we’ve tested many with our gentle Blank Slate Gel Cleanser and most did pretty well except for the tinted formulas. There’s no real concern that sunscreens need a special makeup remover. However, we do find that for Victoria’s oily skin, constant reapplication had a tendency to cause some skin congestion over time. We recommend a good weekly double cleanse to help skin stay congestion free from all the good sunscreen protection. 

Chemists’ Notes

Need a sunscreen recommendation? This year we decided to test 30 sunscreens and 30 days…and we’ll be adding to that as summer progresses. Checkout our 30 for 30 thread on our personal sunscreen experiences. Ultimately we found Asian sunscreen textures on average to have better textures with great wearability over time, but read on to find your next sunscreen love.


  • Golden Rule #1: The sunscreen with the best protection is the one you’re willing to apply generously, and reapply on the daily. Sunscreen texture trumps all. 
  • Golden Rule #2: A majoriy of us have a tendency to underapply. The ¼ teaspoon method is a good gauge but we can’t all carry spoons around. The two-finger method is not always suitable and we recommend instead using three-fingers worth of sunscreen.
  • Golden Rule #3: Sunscreen formats is a major factor in getting enough coverage. Lotions and fluids are still the more surefire methods compared to sprays and sticks. Forget about powders.


  • Bender, Arnold E. & David A. Bender. Body Surface Area. A Dictionary of Food and Nutrition. New York: Oxford University Press, 1995.

  • Liu, Youcheng & Stowe, Meredith & Bello, Dhimiter & Sparer, Judy & Gore, Rebecca & Cullen, Mark & Redlich, Carrie & Woskie, Susan. (2008). Skin Exposure to Aliphatic Polyisocyanates in the Auto Body Repair and Refinishing Industry: III. A Personal Exposure Algorithm. The Annals of occupational hygiene. 53. 33-40. 10.1093/annhyg/men070. 

  • Petersen, B. and Wulf, H.C. (2014), Application of sunscreen − theory and reality. Photodermatol. Photoimmunol. Photomed., 30: 96-101.


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