How much is 2 milligrams per centimeter squared?
Sunscreen is tested at 2 mg/cm2, an amount that is incredibly divorced from the typical consumer. Most people use much less than this amount and subsequently get much lower protection. If you use half of this amount, you will receive substantially less than half of its protection.
Makeup Alley has a fabulous chart that helps outline the amount of protection you actually get from your sunscreen if you skimp on the application:
|labeled SPF (at 2 mg/cm2)||
SPF at 1.5 mg/cm2
SPF at 1 mg/cm2
This raises an important and relevant question: how much sunscreen, in real terms, do you actually need to get the recommended SPF? To investigate, I used some paper, some math, and a kitchen scale.
|My non-fancy scale.|
FutureDerm has kindly addressed part of this question already. On their blog, the calculated the amount of sunscreen needed to cover your face. They determined that covering your face would require about 0.04oz of product. Although this is very helpful, it doesn’t tell the entire story. In the summer, most of us find ourselves showing a lot of skin.
|This is how much sunscreen you need for your face alone.|
To calculate how much sunscreen I actually need, I first needed to figure out the surface area of my body. Since I very rarely find myself frolicking naked, I did this while wearing a bikini. I took strips of paper and taped them all over myself. It certainly wasn’t perfect, but it gave me a good approximation. The skin I had left showing was about 1.508 meters squared (although there is certainly some fuzziness around that number).
If you don’t have a patient boyfriend to help you tape paper to your butt, there is an alternative method worth considering– body surface area is commonly used to calculate chemotherapy drug doses, so some “rules of thumb” have emerged. One is the DuBois and DuBois Body Surface Area Formula. It’s hardly perfect, especially for individuals who are not within the “normal” BMI range, but it may be close enough for your purposes. According to the formula, BSA=[weight (kg) x height (cm)/3600](1/2). I plugged in my numbers and found a BSA of 1.467 meters squared. This is almost certainly an underestimation, as my original calculations excluded areas that were already covered, such as my scalp, my boobs, and my butt, however it’s definitely in the same ballpark as my experimental data.
The next step, of course, is to calculate the amount of sunscreen needed per square meter. 1 centimeter squared is equivalent to 0.0001 meters squared. Thus, 2 mg/cm2 equates to 20,000 mg per square meter. 20,000mg is about 0.705oz. You can calculate how much sunscreen you need by multiplying 0.705oz by the number you get on the DuBois and DuBois BSA Formula, but I will go ahead and use my experimental data, since I have it.
By these calculations, I need 1.063oz of sunscreen to cover my bikini-clad body.
|I am not wearing enough sunscreen, you guys.|
HOLY SHIT THAT IS A LOT OF SUNSCREEN.
Anyways, I am pretty sure the moral of this story is never go outside. But if you do go outside, be sure to put on more sunscreen than you could possibly imagine that you might need.