PPI and Print Size

How the pixels per inch in your digital photo affect the print size

By now you should be aware that the number of megapixels captured by your digital camera affects the size of the prints that you can make. If this is still unclear, read more about megapixels, then come back here once you’re done.

While a simple chart can let you know how many megapixels you need when you want to purchase a camera, you need to understand more about dots per inch and print quality once you’re ready to print some photographs.

Megapixels and Resolution

We won’t go into great detail about megapixels again, but will take a moment to refresh. A digital photograph is made up of millions of tiny dots called pixels.

Many digital camera specifications mention the number of horizontal and vertical pixels that make up the digital photograph. For example, one camera might produce photos that are 2272 pixels wide and 1704 pixels tall (2272 x 1704). Another camera will produce an image that is 4492 x 3328.

You can find out the number of megapixels by multiplying the horizontal and vertical pixels. In the first example, the camera captures about 3.9 megapixels (2272 x 1704 = 3,871,488). In the second example, the camera captures about 15 megapixels (4492 x 3328 = 14,949,376).

An image with a lot of pixels is also called high resolution. An image with a low number of pixels is called low resolution. This is why the terms megapixels and resolution are sometime used to mean the same thing.

Pixels Per Inch (PPI)

Now that you know about the relationship between megapixels and the number of horizontal and vertical pixels in the image, we can relate these numbers to the display and print size of the photo.

Let’s go back to our 3.9 megapixel camera that produces file sizes that are 2272 pixels wide and 1704 pixels tall. We have some measurements for the size of the image, but what do they really mean? Is there anyone you know of who measures size in pixels? How do you convert this to a more common measurement?

You can tell how big your image is going to be by using pixels per inch or PPI. Divide the number of pixels in the photo by the PPI to determine the size of the printed photo in inches.

There are some common PPI numbers that you can choose from.

Monitor PPI

Your computer monitor displays images at 72 pixels per inch. This means that our 3.8 megapixel image is going to measure about 32 inches by 24 inches when viewed on a monitor. That is pretty big!

You can determine the display size of the image by dividing the horizontal and vertical pixels by 72. In this case, 2272 / 72 = 31.6 and 1704 / 72 = 23.7.

Use the 72ppi standard when you want to post an image to the Internet (since most people will view the photo on a monitor).

Print at Home PPI

When you print at home, you can actually tell your printer to use any number of pixels per inch. For example, you can print your image at 100ppi if you want to. You can also print at 250ppi. What’s the difference?

The difference is that a printer needs to use more pixels per inch to produce a high-quality image than your monitor does. If you print a photo at 100ppi, it is not going to look like a professional print. You will be able to see grain and fuzziness, the actual pixels that make up the digital photograph.

If you print at 250ppi, the photo is going to look more like a print made by a professional service. More pixels per inch = higher quality printed photograph. But have you figured out the problem when printing with lots of pixels per inch?

When you increase the pixels per inch, you reduce the size of the printed photo. Let’s say you print your 3.9 megapixel photo at 100ppi. This isn’t high-quality, but you can print a photo that measures 22.7 by 17 inches (2272 / 100 = 22.7 and 1704 / 100 = 17).

Now you print the same photo at 250ppi. You get a great looking photo, but the print size is 9.1 by 6.8 inches (2272 / 250 = 9.1 and 1704 / 250 = 6.8). Big difference.

Professional Printing PPI

The professional print standard is 300ppi. This is used by professional photographers and any time a photo is printed in a magazine or other print publication.

Megapixels and Print Size

Now we can finally clarify the relationship between the number of megapixels and print size, and why capturing more megapixels lets you create larger prints.

Let’s say that you really want high-quality prints of your digital photographs, but you just want to print them at home. In this case, 200ppi should be enough to get the quality you are looking for.

This table shows the relationship between camera megapixels, horizontal and vertical pixels in the image, and the size of a print at 200 pixels per inch.

MegapixelsImage Size (pixels)Printed Size (inches) *
2.01224 x 16326.1 x 8.2
3.01536 x 20487.7 x 10.2
4.01704 x 22728.5 x 11.4
5.01944 x 25929.7 x 13.0
6.02048 x 307210.2 x 15.4
8.02236 x 350411.2 x 17.5

* The printed size is determined by dividing the horizontal and vertical pixels by the pixels per inch.

When your camera has more megapixels, you get large size prints even if you print with lots of pixels per inch. You can always print using less pixels per inch to increase the size of your photographs, but image quality will suffer.

So What the Heck is DPI?

DPI means dots per inch, and has to do with just how much ink your printer lays down when it prints a photograph. Printers with higher DPI settings can produce more detailed photographic prints.

DPI and PPI are used interchangeably a lot of the time, but are not the same thing. For example, you can print an image on your 300dpi printer at 72ppi, 100ppi, or 300ppi. Changing the ppi affects the size of the printed photograph. You can’t change the DPI, since this is set for whatever printer you are using.

For a great description of DPI, check out the definition at Wikipedia.

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