If you've noticed that your face moisturizer has an SPF rating, you've likely wondered whether it counts as your daily sunscreen. (This sure would be convenient, wouldn't it?)

The answer: It depends.

"I usually call any sunscreen that's in your moisturizer — or makeup — a bonus sunscreen," says Annie Christenson, a medical aesthetician at Houston Methodist. "This is because, when these products do contain sunscreen, it's generally a fairly low SPF. Sometimes as low as SPF 15."

SPF, which stands for sun protection factor, is a measure of how well a sunscreen filters out ultraviolet (UV) rays. The higher the number, the greater the protection. The American Academy of Dermatology recommends using sunscreen with an SPF of at least 30. No sunscreen can block 100% of UV rays, but SPF 30 sunscreen blocks around 97% of them.

Protecting your skin from the sun is important to reduce your lifetime risk of developing skin cancer. And this is a skin cancer prevention measure that rings true year-round and in any weather condition, not just on sunny, summer days.

It's why experts recommend wearing broad-spectrum sunscreen with an SPF of 30 or higher every single day, regardless of whether you plan to be out in the sun or not.

"I actually recommend higher than that — SPF 50 or greater, since sunscreen also helps prevent premature aging of skin," adds Christenson. "Most moisturizers don't have SPF ratings that go up that high, which is why my preference is to apply a dedicated sunscreen every day in addition to using a moisturizer."

You might not be so focused on the anti-aging properties of sunscreen, though. In any event, Christenson says your moisturizer with an SPF rating can sometimes double as your sunscreen — but sometimes is the operative word.

5 times your face moisturizer can't double as your sunscreen

There are certain times when moisturizer isn't an effective replacement. Here's when you'll still want to apply actual sunscreen.

1. Your face moisturizer doesn't have an SPF rating

First thing's first, not all face moisturizers contain sunscreen.

If they do, it's almost always listed somewhere on the label — many are designed for this dual purpose, in fact. If the label doesn't list an SPF, make no assumptions.

"If you use one of the more advanced, effective moisturizers that include antioxidants, peptides and things of that nature, it's generally not going to contain sunscreen," says Christenson. "If you don't see the SPF rating on the label, you'll want to apply sunscreen on top of your moisturizer."

2. The SPF rating of the sunscreen in your moisturizer is less than 30

Don't just look for the letters "SPF" or the word "sunscreen" and think they mean you're good to go. If the SPF rating is anything less than 30, it can't replace your daily sunscreen.

"Some moisturizers that contain sunscreen only have an SPF of 15, and this definitely isn't high enough," says Christenson. "Yes, it's still protective, but it's more of bonus protection. It's not enough to be the only thing you use."

3. You don't see the words "broad spectrum"

Remember that term broad spectrum. Here's the definition: Broad spectrum means the sunscreen protects against both UVA and UVB rays released by the sun.

• UVA rays, also sometimes called aging rays, primarily lead to premature skin aging but can also contribute to the development of skin cancer over time
• UVB rays are the primary cause of sunburn and skin cancer

Since both types of rays contribute to skin cancer, be sure the sunscreen in your moisturizer is broad spectrum. This should be listed on the label.

4. It's been a while since you put your moisturizer on

As much as it'd be great to only have to apply sunscreen once a day and be done, we know the reality is that we're going to be exposed to the sun several hours after the initial application. All sunscreens need to be reapplied according to the instructions on the packaging.

It's why the moisturizer you put on in the morning can't always double as your sunscreen all the way into the evening.

"How long is that drive home from work?" asks Christenson. "You get more sun while sitting in traffic than you might think. That's a time you might want to reapply some more sunscreen."

You should definitely reapply sunscreen if you plan to be outdoors, like going for a walk or run after work or sitting on a patio for dinner. This is where a dedicated sunscreen comes into play, even when your moisturizer contains SPF 30, since you're probably not going to re-moisturize until bedtime.

"If you wear makeup, this is when you'll want a sunscreen product you can easily reapply over it," says Christenson. "There are many great options these days."

Powder sunscreens are sold in wands that you can keep in your purse or handbag without worrying about it leaking. Compact sunscreens can come in a cream form, adds Christenson. These can easily be stippled on over makeup.

5. If you're going to be out in the sun

If your day consists of very little time outdoors — say, you have a limited commute or you're working from home — a moisturizer containing SPF 30 might be all you need to effectively protect your skin.

But any time you plan to spend extended time in the sun, Christenson recommends applying a sunscreen with SPF 35 or higher, preferably SPF 50+. This includes if you're going to the beach or a pool, walking, running, hiking or playing an outdoor sport, as well as if you're shopping at a farmer's market or outdoor mall.

"The higher the SPF, the more protection you're going to get from skin cancer-causing sunburns as well as those aging UV rays," says Christenson. "In fact, the best anti-aging product money can buy is a $12-$15 over-the-counter sunscreen that's broad-spectrum SPF 50."

(Related: When Should You Start Using Anti-Aging Skin Care Products?)

Christenson makes an important point: You don't always know what your day might end up looking like when putting your moisturizer on in the morning.

"This is why I really just recommend applying an SPF 50 sunscreen in addition to your moisturizer each day and reapplying according to the label's instructions," says Christenson. "That way, if something comes up and you're out in the sun more than expected, you're all set."

Should you apply sunscreen before or after your moisturizer?