Apple Fitness is designed for Apple Watch users. If you don't have an Apple Watch, you can kind of use the app ... a little bit. But not really, here's why.

Apple Fitness+ Needs an Apple Watch to Sign Up

To sign up for Apple Fitness+, you need an Apple Watch paired with your iPhone. It's impossible to sign up for Fitness+ without an Apple Watch. However, once you're signed up, you don't strictly need an Apple Watch to do a workout.

You Don't Need an Apple Watch to Start a Workout

If your Apple Watch is out of battery, left at your parents' house, or otherwise not connected, when you go to start a workout on your iPhone or iPad (but not an Apple TV), you'll see a pop up explaining that without it, you won't be able to see your heart rate and calorie burn on the screen.

Tap "Work Out Without Watch" to start the workout anyway.

Fitness+ Is Designed for the Apple Watch

Fitness+ is clearly not really designed to be used without an Apple Watch. It tracks your workout metrics and shows them on screen. For example, this makes it easy to check your heart rate. You can see how your effort compares against other people in the same workout. Apple Fitness is all about this integration.

But, if you don't have your Apple Watch on you and you want to start a workout, Apple Fitness+ won't stop you from getting your sweat on.

If you don't have an Apple Watch and you want to get started with Apple Fitness+, consider buying an Apple Watch. You can get an Apple Watch Series 3 for less than $200, and it works with Apple Fitness+

In theory, if you really wanted to sign up to Fitness+ but don't own an Apple Watch, you could borrow one, pair it with your iPhone, sign up, then reset the watch and give it back. However, that seems like a lot of work to go to so you can pay Apple $10/month for exercise videos without the extra features.