How come my battery drained so fast

That percentage is about right for an iPhone 7.

Batteries degrade over time with usage due to chemical aging, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Repair & Replacement - Apple Support

About Lithium Ion Batteries

iPhone Battery and Performance - Apple Support (IN)

petaia32 Said:

"How come my battery drained so fast: Last weeks it’s 95% and now my battery is 88% how?"

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Battery Health:

There is no way to determine why, nor when, a Battery Health degraded. It is all just a guestimation, on Apple's part. In fact, I've even seen mine increase at times.