How Big a Photo Can I Print with 8 Megapixels?

I was on a train and took a photo with my iPhone 5 that I love of a bridge. I want to print it, but I don't want it to be all pixelated. Realistically, assuming it has enough light, how big a print can I make with it without sacrificing quality, and is there anything I can do to make it bigger?

All you can really do is take the image file to a lab and ask them how large a print you can make

There are other things to consider when making large prints:

* is the image is sharp focus?
* is there any blurring caused by camera or subject movement?
* did you capture the image at the cameras highest resolution
* is the image properly exposed.

If all four of those criteria are met, the the largest possible print can be made.

I've done 16x24 with 10mp image.

But the problem you are going to have isn't the number of pixels. If you took the image with an iphone, there just isn't enough detail in the image. I wouldn't go bigger than 8x10 personally. 11x14 will be really pushing it, unless you are going to only look at the picture from some distance.

I have an 8 megapixel image that was blown up to 3x5 feet and on display in the Long Island Aquarium. That was taken on an SLR. It looked fine unless you walked up to it with a magnifying glass. If printed professionally, you don't get pixelation; you get softness. Now your 8mp image is from a smart phone: tiny sensor, tiny optics. While the iPhone 5 camera is better than some, you're not going to be able to blow it up large and get a quality result. You might be safe going to 8x10 if the technical quality is good. An 8x10 is not expensive. Why don't you try it?

_IF_ everything else is ok:
- exposure
- in focus
- no movement blur (likely if you were on a train)
- no reflections and/or dirt on the window between your camera and the bridge (or even on the lens)

then there are people who say that 3.5 MP are good enough for everything. The reason is quite simple: usually, you watch the picture so that you can see the whole picture at once, i.e. The larger the print, the larger your viewing distance.
The problems start, however, when the theoretical resoluton of the sensor (8 MP in your case) is less than either the resolution of the lens or whatever the image processing in the camera does to the image (compression, sharpening, whatever).