Depending on how frequently you use the phone and what you typically do with it, you can automatically help conserve the remaining charge by enabling Power Saving mode.
If you prefer to take the manual approach, you can disable features that aren’t currently needed or change certain default settings so that less power is drawn. To monitor how various apps and the operating system use the battery, you can run the Battery app.
Show the Battery Percentage
Although the battery indicator in the status bar gives you a rough indication of the remaining charge, you can alter it to show the exact percentage remaining. Open Settings, tap Display, and enable Display Battery Percentage.