How big can you print phone photos?

Phones and digital cameras these days have a lot of megapixels and the amount of megapixels is often used as a selling point. I will discuss how megapixels don't matter as much as the marketers want us to believe and you can get great quality prints with lower resolution photos.

If you don't want to read all this below here's the summary in one paragraph:
Your phone in your pocket takes amazing photos and you can print them big, really big. The bigger you print them, the softer the photo will look but the further you will have to stand away from it so you won't notice that it's blurry.

To cover the basics first:

a digital photo is made up of lots of tiny dots, these dots are called pixels.
A 'megapixel' means one million pixels and the resolution of a 1 megapixel photo is 1000 x 1000 pixels (or little dots).
The average human eye can see around 300 dots/pixels per inch at a distance of 30 centimetres
the resolution of cameras are increasing rapidly over the years about our eyes will continue to only see 300 dots per inch (dpi) at 30 centimetres

When you look at a large TV with a full HD video the quality looks decent although a full HD video is 1920x1080 pixels which is equivalent to only a 2 megapixel photo. It looks ok because you have to stand far enough away from the TV so it's comfortable to look at and you can't notice that it's a low resolution image unless you walk up close.

A photo from an iPhone, GoPro or other 12 megapixel.
Lots of smartphones, iPhones, GoPros and point and shoot cameras take a 12 megapixel photo. This means the resolution of these photos are around 4000 x 3000 pixels (4000 x 3000 equals 12,000,000 pixels or 12 megapixels). This would mean you can print a 12 megapixel photo at any of these sizes:

8x10 inches: 400 dpi / much better than we can see
11x14 inches: 285 dpi / about the most detail we can see if we hold the photo 30cm away
16x24 inches: 166 dpi / still very good quality
poster size: 111 dpi / if you looked very close, you'd see dots but would look decent at a metre away.
A 12 metre billboard: 8 dpi / it would look low quality if you were close

This means if we print a 12 megapixel photo as big as a billboard and stand 30 centimetres away from it you will see eight dots in every inch. In reality we would never stand this close to a billboard and if we look at it from a normal distance it would look normal because the bigger you print a photo the farther away you'd have to stand to see it.

When megapixels do matter.

The reason though I like to have my photos very high resolution is because I like the fact that you can see a gigantic photo of a beach scene on the wall and then walk up super close to it and see tiny fine details and possibly even your own house