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****This post is authored by John Su.
In many, MANY posts that I have authored, the number 2.0 mg/cm2 —the amount of sunscreen necessary to achieve the labeled SPF rating, has been thrown around more times than I can count. Naturally, people ask, “How much is that exactly?” Well, today’s your lucky day!
First, we need to convert 2.0 mg/cm2 into units that we can understand, namely ounces and inches. (Keep in mind that I’m will not be obeying significant figures or rounding rules)
2.0 mg (2.54 cm)2 (12.0 in)2 1.0 g 1.0 oz ~0.0655 oz
————– X ———– X ———- X ———- X ——— = ————–
1.0 cm2 (1.0 in)2 (1.0 ft)2 1000 mg 28.35 g 1.0 ft2
As you can see 2.0 mg/cm2 comes out to ~0.0655 oz/1 ft2.
Now, assuming that you have a facial sunscreen, and a separate one that you use on the rest of the body including the neck and ears (which I do), we need to know how the total surface area of your face.
The average human face is ~0.60 ft2. In order to calculate this amount:
- Find a measuring tape or a flexible ruler.
- Take the end of your tool and place it on the front of your left ear, without blocking the opening.
- Holding the tool as close to your face as possible, drag it across your face until it reaches the opening of the right ear.
- Record that length (X).
- Then again, take the end of your tool and place it at the edge of your hairline directly above your nose.
- Holding the tool as close to your face as possible, drag it across your face until it reaches the chin.
- Record that length (Y).
- Finally, you can add an inch to either lengths if you want to factor in how prominent your features are.
So for me, my X = 12 inches, and my Y = 8 inches. Considering that my face is widest at the measuring point for X, which means that measurement was an overestimation; and that my face isn’t very flat, which means that my Y measurement was an underestimation; means that I don’t have to adjust my numbers because the two characteristics effectively cancel each other out.
Therefore, my facial sunscreen needs to cover 12.0 in X 8.0 in = 96 in2. To convert from in2 to ft2, we need to do the following:
96.0 in2 1 ft2 ~0.667 ft2
——— X ——– = ————
1.0 (12 in)2 1.0
As you can see, my sunscreen needs to cover ~0.667 (or two-thirds) ft2.
Therefore, to find out how many ounces of facial sunscreen to use, I need to multiple that amount with ~0.0655 oz/1 ft2.
0.667 ft2 0.0655 oz ~0.0437 oz
———– X ———– = ————-
1.0 1.0 ft2 1.0
The math comes out to be ~0.0437 oz. To a bit more accurate, let’s convert that into mL.
0.0437 oz 28.35 g 1.0 mL ~1.2439 mL
———— X ——— X ——– = ————-
1.0 1.0 oz 1.0 g 1.0
Because 1 g = ~1 mL, that comes out to ~1.2439 mL. This works perfectly for my example.
Yesterday, I received a sample of a moisturizer (pictured above) from my Sephora Chic Week order that contained slightly less than that amount (1.15 mL).
So I proceeded to empty the entire sample into the palm of my hand (pictured left). Then, I put an approximately equal amount of sunscreen in my other palm (pictured right).
THAT’S how much sunscreen I need (and what most people need) for the face alone, though I admit that my face is a bit larger than normal! I know, it’s a lot, but tough! MAKE IT WORK! Haha. One of the readers (Sarah) suggested to apply half that amount of sunscreen in two layers, meaning that you apply 1.0 mg/cm2 to your face, wait for it to dry, and then apply the second 1.0 mg/cm2 layer of sunscreen! It’s a good idea that may allow for a more cosmetically acceptable finish. Give it a try.
Also, I would recommend getting a sample that contains about how much sunscreen you should be using per day (around 0.04 oz), and do what I did with these two pictures. Remember how much sunscreen you used in your version of this experiment. That way, YOU will actually know about how much you need to apply on a regular basis. You can then use that amount to gauge and compare how much you actually use, and try your best to work up to that amount!
Also, if it’s available to you, consider buying a separate 1/4 teaspoon to measure your daily facial application amount of sunscreen. 1/4 teaspoon is approximately the amount that most people need to apply to achieve 2.0 mg/cm2. Let’s see how my face’s surface area matches up to this claim.
0.0437 oz 6.0 tsp ~0.2622 tsp
———— X ———– = ————–
1.0 1.0 oz 1.0
Yep, that’s about one quarter (1/4 = .25) of a teaspoon! So yeah, consider buying a separate 1/4 teaspoon from any grocery or drugstore. I’d like to thank reader (Maggie) for suggesting this!
But going back to the application amount, that’s why it’s so difficult for me to recommend more expensive sunscreens. For example, the Algenist Ultra Lightweight UV Defense Fluid SPF 50 provides great UVA and UVB protection with 17.1% zinc oxide, 2% titanium dioxide, and 7.5% octinoxate. But at $38.00 for 1.0 oz, it’d be used up really quickly.
How quickly? Well for me, assuming that I apply it once per day, here’s the math:
1.0 oz 1.0 day ~22.88 days
———— X —————– = —————–
1.0 0.0437 oz 1.0
That’s about three weeks! There are 356.24 days in a year, which means that on facial sunscreen alone, I’d be spending over $600 per year! That’s insane! Okay, I can bring that number down a bit by considering the fact that I’m not outside every day, and that there are sales throughout the year. Plus, I could stock up during these sales. But still, I’d estimate the final number to be well over $500.
But to each his own, right? I can’t see myself spending that much on sunscreen, but eh. *Shrugs* Anyways, I hope this was an enlightening post on what exactly 2.0 mg/cm2 means.