Ask LH: How Do I Figure Out How Many MegaPixels Are Necessary For Printing A Photo At A Specific Size?

Dear Lifehacker, I have a camera that's capable of more megapixels than I can fathom, and I know I don't need photos at such a high resolution, but I do want to be able to at least print full-page photos. How can I determine how many megapixels are necessary to print a photo at a given size? Sincerely, MegaConfused

Dear MC,

This is easy to do but a bit harder to explain. First, we need to translate the megapixel rating of your camera to actual pixels. A megapixel is one million pixels, so you can easily find out the number of pixels your camera can capture by multiplying your megapixel rating by one million, but that's just one step of many in a tedious series of calculations to find out the information you can gather by simply taking a picture. Go through every setting on your camera and take a photo at each size. If your camera gives you the actual width by height resolutions (e.g. 6000x4000, for example), you don't need to do this. If it just gives you a megapixel rating, you do. Once you've taken that picture, load it up in your favourite photo-editing software and find out the height and width of each photo in pixels. This should be in the information panel of any photo editing software, whether you're using iPhoto, Picasa, or Photoshop (although you can access it most quickly in Photoshop by pressing Command/Control + Shift + I).

Once you've noted the dimensions in pixels, you need to convert them to inches. Converting pixels to inches requires a little more information, and that's the dots per inch (DPI) resolution in which you'll be printing the photo. Images are traditionally displayed on screen at 72 DPI. While you can get away with 200 DPI when printing a photo, most inkjet printers can handle print resolutions of up to 300 DPI. If you want a really sharp photo, that's the DPI number you want. Whatever number you choose, just divide both the height and the width by that DPI number and you'll end up with your resolution in inches. Alternatively, if you don't like math, you can just use this calculator.

As you might suspect, the calculation also goes in the other direction. If you want to print full page photos, which come out to 8.5 x 11 inches, you can multiply those numbers by the DPI number you're aiming for. In the case of 8.5 x 11 inches, you'd need a photo that's 2550 x 3300 pixels, or about 8.4 megapixels.

Cheers, Lifehacker

So,
Assuming we're starting with a sharp image, and not putting the output under a microscope.

2 megapixels will do your standard 6"x4"s and stay passable up to a half page.

4 megapixels will do a page, unless you're trying for coffee table quality.

8 megapixels and up gets you into poster territory

Unless your 8 megapixels came from a tiny, noisy sensor in low light. Those photos will look horrible however you print them.

To print at a commercial quality 300dpi ( or pixels per inch ) is recommended . 220dpi for newspaper or your inkjet printer .

This is for a photo you look at in your hand .

The easiest way to think about it is 100px equals 1cm

So a 800x600 photo can be printed at a great quality at 8cm x 6cm.

A standard photo is 10cm by 15cm so 1000pixels by 1500px is the go

Has anyone else noticed that there inkjet printer can't reproduce 2^32 different colors in one dot the way a photo pixel does?