Do I need my phone when I use my Apple Watch?

While you can leave your phone behind, we generally don't recommend doing so for two reasons:

• The Apple Watch battery is dramatically improved with your phone still in your pocket. The Apple Watch is smart and will use your phone's GPS instead of its own when it can, dramatically saving battery. With your phone in a pocket you can expect the Watch to easily last all day, whereas without the phone you'll likely see closer to 6 or 7 hours of battery before it dies.
• The GPS in the iPhone is also a bit more accurate, especially with altitude data. Keeping your phone with you means you get the most accurate data. During testing, we found that keeping the iPhone nearby ensured accurate metrics, and also had an easier time keeping reliable data.