Will my iPhone 12 case fit the iPhone 13?

No, the iPhone 13 does not fit in older cases designed for the iPhone 12.

While the iPhone 13 dimensions are close to the previous model's, the device is 0.25 mm thicker. This alone may not be a deal breaker for an old case, however the iPhone 13 camera bump is almost a millimeter larger in size. For this reason, iPhone 12 cases with tight camera bump cutouts will not fit on iPhone 13. Looser fitting sleeve-type cases may work.

The iPhone 13 design represents a slight change in the next-generation form factor. iPhone 13 features a thinner notch at top and slight increase in thickness. Like the iPhone 12, Apple has released four iPhone 13 models, with an increase in the overall thickness by 0.25 mm. The rear-facing camera sees a slight size increase by one millimeter over its predecessor.

There have also been rumors floating around that Apple may go with an entirely notchless and portless design putting an end to the Lighting connector with iPhone 14 in 2022.