How can I estimate how much sunscreen I need to pack?

I am working on this calculation still. There is no way I am going to apply 30 ml of sunscreen every 2 hours, 6-7 times a day, every day for 35 days. It's not that kind of holiday.

So far the factors I've identified are:

• how much you use at a time. Various articles say 30 ml for your whole body, while at the same time saying most people don't use enough. I'm going to define "enough" as "I don't get a sunburn" and I know I don't use one third of an 88 ml tube every time I put sunscreen on. Some articles say most people use only a third of what they should, which would be 10 ml. I bet 20 ml is a reasonable guess for how much I use - half that if I'm just doing face, neck, and arms because I'm wearing pants and a short sleeved shirt.
• how frequently you re-apply. Again all the articles are saying every two hours, more if you swim, but I don't know anyone who does that much. I do every four hours, which means that on a canoeing day I might do it four times, assuming that I don't put clothes on for part of the day. In practice I usually wear clothes at least part of every day.
• how many days of the trip are outdoors. I don't need sunscreen on the plane, and might need very little if I'm wearing long pants, a long sleeved shirt and a hat while going in and out of buildings.

I think it's useful to declare say a "beach day" means I will use 20 ml at a time and do that 4 times, while an "indoors day" means none and a "partly sunny" day might mean 1 use of 20 ml and two top-ups of 10 ml on the face and arms. Then I can categorize each of my days and do the math. My first pass leads to an estimate of 20 tubes (88ml each) which is not realistic - I may have to look into shipping this stuff to myself in advance - but I can fine tune it now that I've thought about the parameters.

Use 1oz (30ml) Per Application

Since you are looking for heuristics, searching around on the web yields various forum threads (see here, here and here), a as well as a dedicated page from the American Academy of Dermatology (AAD) website. According to these, the right amount of sunscreen per application is 1oz (30ml), given a person with an average height and weight. Your mileage may, and will, vary hence consider adding an extra 0.5oz (15ml) to your calculations. Quoting from the AAD website (emphasis mine):

3. Use enough sunscreen. Most adults need at least one ounce of sunscreen, about the amount you can hold in your palm, to fully cover all exposed areas of your body.

This other website reiterates this concept, and adds some details regarding what the average adult size might be:

Use your body size to determine the total amount of sunscreen that you will need. Let's assume you are an average adult. You are 5'4" (163cms) tall, you weigh 150 lbs (68kgs) and your waist size is 32" (82cms). To cover your entire body, you'll need one full ounce of sunscreen. An adult who is 6'5" and who weighs 225 pounds and has a 36 inch waist needs 1.5 fluid ounces. This is based on an FDA calculation that was developed in 1998 using world-wide standards for SPF testing. It takes into consideration the amount needed to cover the entire body surface minus a bathing suit (bottom).

Calculate Your Needs

Given the 1.5oz (45ml) starting point, you now how to think about how many times per day you think you will apply the sunscreen. The AAD suggests re-applying sunscreen every two hours:

5. Reapply sunscreen at least every two hours to remain protected, or immediately after swimming or excessively sweating.

With these two metrics in mind (application quantity and rate) you should be able to formulate an estimate of your needs for this 5-week trip in sunnyland, of which I am sure we on TSE are all very jealous. After computing your estimate, consider giving yourself a safety margin by adding 10-20% more.

Final Thoughts

A pure guesstimate on my side would look like this:

• 1.5oz every 2 hours
• Say you spend an average 12 hours under the sun = 6 applications per day
• There are 35 days in 5 weeks so, 6 applications x 1.5oz x 7 days x 5 weeks = 315oz
• Add a safety margin of 10-20% to make it an even 350-400oz

You are going to love carrying around that much sunscreen. All jokes aside, you might not be spending 12 hours under the sun every single day. In any case it is up to you to calculate how much you need.

For more information on sunscreen, see the AAD FAQ page on the topic.