Here's the easy answer: just check in your Settings! Go to Settings>General>About and it'll be there, easy peasy.
But what if you want to tell just by looking? We've got some pro tips after more than a decade in the biz. Oh, and make sure to take a look at those color options. Lots of iPhone sizes have exclusive colors, so sometimes it's a dead giveaway.
iPhone 7, 8, and SE
The iPhone 7, 8, and SE have the camera and flash aligned horizontally (that means right-to-left, for you non-directioners out there.) Lucky for you, they all use the same shell, so a case for one is a case for all of them!
iPhone 7/8 Plus
You can tell between the standard and Plus models by the shape of the camera! If it's a circle, you've got a base model. If it's an oval, you've got a Plus. Or (shocker) you could just check how big they are. Easy peasy!
All the X-series models have vertical cameras (up-and-down, people.) They're the only iPhone models with this camera shape, so they're easy to spot. And don't worry about finding the dirrerences between X and XS: they're in the same shell. The only difference is their electronic guts!
Basically, if you take the camera configuration for the 7/8/SE (just the one circular camera and the flash) and flip it to be vertical, you've got yourself an XR. This is the first of the modern iPhones with lots of fun bright colors, too!
iPhone X/XS Max
Similar cameras and colors to the X/XS models, but this one's just bigger! If it looks a little chunkier than you'd expect, it's probably a Max.
For the first time, iPhones have separate, protruding cameras. Look at this bad boy from the side, and you'll see two bumps jutting out. The cameras here are two perfect circles right on top of each other, like a happy snowman that just wants you to say "cheese!" And don't forget, the iPhone 11 still has rounded edges.
iPhone 11 Pro
The biggest difference between the 11 and the 11 Pro is that third camera. It's the first time Apple configured their cameras like that, and we're guesing they loved it because they haven't stopped yet. The 11 Pro models are the only three-camera iPhones with rounded edges, though, so they're much easier to spot than the newer ones.
iPhone 11 Pro Max
This is what differentiates the strong from the weak. (or the Pro from the Pro Max, if you're boring about it.) If you see three cameras and rounded edges, stop and ask yourself, "Does that phone look so gigantic that it must have been made for gorillas? Could a human being even use that thing with one hand?" And the answer will come to you.
iPhone 12 mini
Just like the 11, but more boxy! Two little cameras, right on top of each other, with squared edges. It's a particularly small phone, too, so if it fits snugly in someone's palm, you've got your answer.
iPhone 12/12 Pro
The 12 and 12 Pro are the same size! Think Goldilox: Not crazy small, but not crazy big. The big differences are the cameras (who'da thunk?) and the color options.
iPhone 12 Pro Max
Another culprit of Gorilla Hands Syndrome. This thing requires two hands to use, like a stick shift. So remember: giant phone, square edges, three cameras: iPhone 12 Pro Max. (or 13... but we'll get to that.)
iPhone 13 mini
The iPhone 13 Mini has cute little off-kilter cameras, and it's the first time Apple's done something like that. The 13 line as a whole has much bigger cameras that protrude further out of the back of the phone, so they wouldn't be able to fit one on top of the other like on the 11's and 12's. It's a tiny case with diagonal cameras that's easy to spot and adds a little sunshine to your day!
The iPhone 13 is the only other model that has diagonally-placed cameras. The biggest difference is size, which you definitely saw coming. The 13 is to the 13 Mini what the 12 is to the 12 Mini. That's just math.
iPhone 13 Pro
This one's a little tricky. The 12 Pro and 13 Pro are almost exactly the same phone to the untrained eye, and it's difficult to tell the difference between them unless you have both right in front of you. It's helpful to remember that the 13 Pro has significantly larger cameras (and a larger square the cameras sit on.) If you're lucky enough to be looking at a blue model, though, you've got it in the bag. The 13 Pro's Sierra Blue is much lighter than the 12's Pacific Blue. It's like night and day (or light blue and dark blue.)
iPhone 13 Pro Max
Just rinse and repeat what you learned for the 13 Pro, and make it the size of a T-rex footprint, and you've got the 13 Pro Max. Rawr, as the kids say.
And that's all of them (so far)! If you're still having trouble and want some hands-on examples, check out our YouTube Video going through the subtle differences of each side.
If you end up ordering the wrong case size, don't panic! Smartish Support is here to save the day, and we'll be happy to swap it out so you can get your Smartish Smartphone decked out just the way you want it.