Apple - How do I show a percentage battery indicator on the iPhone?

iOS 4 and later natively provide a setting to toggle display of remaining battery percentage in the status bar of iPhone/iPad/iPod touch.

• iOS 9 and later: go to Settings app → Battery → Battery Percentage.
• iOS 4 – iOS 8: The option is made available under Settings app → General → Usage → Battery Percentage

Apple Knowledge base article Show the battery percentage on your iPhone or iPad.

iPhone X cannot display the same in the status bar due to presence of the notch. To view remaining battery percentage on iPhone X, simply swipe-down from top-right corner of the device to open Control Centre to view the reading on-screen. A brief video with the action can be viewed in the CNET article iPhone X: How to view your battery percentage.

iPhone 3Gs

Instructions: "Settings > General > Usage – and toggle the Battery Percentage setting to On."

iPhone 3G

Jailbreak then install SBSettings. Select Extras & Options → Numeric Battery

If you have the 3GS model:

Settings -> General -> Usage -> Battery Percentage -> ON

If you have a jailbroken iPhone:

Install SBSettings in Cydia

or

Install asBattery, also in Cydia, using this repo:

http://apt.iphone-storage.de