You've got a digital photo. How big should you print it? In the old days, making prints was pretty easy. You'd take a roll of film to the chemist, and in an hour you'd get a bunch of 4x6in prints. If you especially liked one of them, you could order a 5x7in or an 8x10in print. That was pretty much all there was to it.

These days, it's not so simple. There's really no standard print size for digital photos - you can print at any size you want as long as you can fit the paper into your printer. And if you're using an online service such as Jessops or Snapfish, the sky is the limit, with posters as large as 20x30in a real possibility. But how do you know what size to choose? What dimensions will work well with the images that come out of your digital camera? Keep reading to find out.

It's all in the megapixels

The answer is surprisingly simple. The more pixels you have, the larger your print can be. For most home inkjet printers, you should assume that you'll get the best results when you print about 300dpi (dots per inch), so just take the dimensions of your photos and divide them by 300 along both the horizontal and the vertical. Whatever you get is the largest size that you should consider printing.

Consider this example: you have a photo that's 2,240x1,680 pixels. Do the maths, and you get an image size of 7.5x5.6in; in other words, with a little rounding, you can safely make 5x7in prints. Or how about a 3,872x2,592-pixel image? That's 12.9x8.6 inches, or, in terms we more commonly use, a standard 8x10.

Another way to think about this problem is by referring to the number of megapixels. The 3,872x2,592-pixel image I discussed earlier is from a Nikon D200, which makes 10Mp photos.

Here's a list that correlates megapixels to print size:

  • 2 megapixels = 1200 by 1600 pixels = 4 by 5 inches
  • 3 megapixels = 1536 by 2048 pixels = 5 by 7 inches
  • 6 megapixels = 2400 by 3000 pixels = 6.5 by 10 inches
  • 10 megapixels = 2592 by 3872 pixels = 8.5 by 13 inches
  • 12 megapixels = 4368 by 2912 pixels = 9.7 by 14.5 inches

Okay, it's not that simple

Now that I've given you a handy-dandy reference, let me add that it may not be as simple as I've led you to believe.

For starters, the list assumes that you're printing at 300dpi - and if you do, you'll generally get great results. But the reality is that you can print at lower resolutions and still get fantastic prints. This is especially true of larger prints that you'll hang on the wall. If you're making a 20x30in poster, most people will stand back and look at it from several feet away. And the farther away you view a photo, the lower the resolution needs to be to give you great results. Think of your digital photo like a television screen - the closer you sit to it, the more obvious the imperfections are. From across the room, almost any TV looks great.

The lesson here is that you shouldn't be afraid to experiment. Try printing your photos at sizes larger than what I've suggested, and see if you like the results.

In addition, techno-purists will argue that there's more to print size than pixels alone - and they're right.

Your camera's sensor size, for example, affects picture quality. A small point-and-shoot might take the same 8Mp images as a more costly digital SLR, but the SLR probably relies on a physically bigger sensor. That adds up to a higher quality image, and the resulting prints will be better. As a result, you might have better results printing your point-and-shoot photos a tad smaller.

Your camera's exposure settings can have an effect on print quality as well. Remember that a high ISO will add digital noise to the final print. An ISO 800 photo will look better printed at 5x7in than at 8x10, even if you use a 10Mp camera.