How do I know my iPhone's battery capacity?

Open Settings ➔ Battery ➔ Battery Health and let us know which percentage you see for Maximum Capacity. Also note what is written under Peak Performance. If your battery's Maximum Capacity shows a value of 80% or less, then it has gone through its lifecycle and therefore should be replaced, according to Apple.

Axel F.

Settings -> battery -> battery health

if its below 80% you should probably get it replaced