Sunscreen calculator: how much do you need?

For the face alone, about a quarter teaspoon, or 1.25 ml. Face, neck and ears take about half a teaspoon, and a whole body in swimwear about 1 fl oz, or 30 ml, roughly a shot glass. Those amounts come from the density sunscreen is tested at. The calculator above turns them into bottles, days and a carry-on check.

Almost every sunscreen question I am asked is about which product. The question that decides how much protection you actually get is how much of it you use, and that is a question with a number attached.

How much sunscreen should I use on my face?

The SPF on the front of a bottle is measured with the product applied at 2 milligrams per square centimetre of skin. For an average adult face, that works out to about a quarter teaspoon. Add your neck and ears and it is about half a teaspoon. A common rule of thumb is a line of product along the length of your index and middle fingers for face and neck.

Most people use far less. Petersen and Wulf's review of application studies found that real-world use typically lands at a quarter to a half of the tested amount, and protection falls away faster than that shortfall suggests. The easiest correction is to apply in two passes: one layer, a minute to settle, then a second.

How long does a bottle of sunscreen last?

Much less time than most people expect once the amount is right. A 50 ml face sunscreen used once a day lasts about 40 days. A 200 ml bottle used on the whole body at the beach, applied twice a day, lasts a little over three days for one person. That is why a week away with the family usually needs several bottles rather than one.

Outdoors, reapply at least every two hours, and straight after swimming or sweating. Put the number of applications into the calculator and it does the arithmetic, including how many bottles of your size to pack.

How much sunscreen can I bring on a plane?

In US carry-on luggage, sunscreen counts as a liquid. Each container can hold at most 3.4 fl oz, or 100 ml, and all of your liquids have to fit in one quart-size bag. Larger bottles belong in checked luggage, which has no such limit. Rules at airports outside the US can differ, so check before you fly.

How the estimate is worked out

The amounts assume an average adult and the 2 milligram test density, with body areas estimated from the standard proportions used to describe skin surface. The swimwear figure lines up with the American Academy of Dermatology's guidance of about 1 ounce for exposed skin. If your frame is larger, or you are unsure, round up: over-applying sunscreen costs you a little product, and under-applying costs you protection.

If a bottle has been in your bag since last summer, the expiry calculator will tell you whether it is still worth using. And if the recent headlines about sunscreen filters have you second-guessing the bottle itself, I went through what the FDA actually changed.

Frequently asked

How much sunscreen should I use on my face?

About a quarter teaspoon, roughly 1.25 ml, for the face alone, and about half a teaspoon if you include your neck and ears. That matches the density SPF is tested at. Most people apply a quarter to a half of it, so if in doubt, add a second layer after a minute.

How long does a 50 ml bottle of sunscreen last?

Used once a day on the face alone, a 50 ml bottle gives about 40 applications, a little under six weeks. Covering face, neck and ears halves that to about 20 days. On a beach day with full-body application twice a day, 50 ml lasts less than one day.

How much sunscreen can I bring on a plane?

In US carry-on luggage, sunscreen counts as a liquid, so each container can hold at most 3.4 fl oz, or 100 ml, and all of them must fit in one quart-size bag. Larger bottles go in checked luggage, which has no such limit. Rules outside the US can differ, so check before you fly.