Go to “Settings” and tap “General”.

Tap “About”
Scroll down to “Capacity”

Now it’s easy, just follow the simple rule of thumb below to determine the Capacity of your iPhone:

8GB iPhone = Less than 8GB
16GB iPhone = Less than 16GB but greater than 8GB
32GB iPhone = Less than 32GB but greater than 16GB
64GB iPhone = Less than 64GB but greater than 32GB
128GB iPhone = Less than 128GB but greater than 64GB
256GB iPhone = Less than 256GB but greater than 128GB