You know how big your photo is when you print it, but do you know how to convert that from inches to pixels so it fits on your website? Knowing this information is particularly helpful if your images are designed primarily for print, and you want to upload them to yourPhotoBiz site. If we have a print we know will be 10 inches wide by 8 inches tall when we print it at 300 DPI, how do we determine what size it will be on the screen? We can open the image in an image editing program and easily see the pixel dimensions, but what if we don't have access to an image editor or it isn't convenient to open it right then? Fortunately there's a quick and simple way to convert between print inches and pixel dimensions.
You've probably seen people in a football stadium do the "card stunt". Each person holds up a card of a particular color, and all of the cards together form a recognizable image. This is actually very similar to how your digital images work - images are made up of individual points of color called pixels. The difference between the crowd at that stadium holding up their cards and the image on your computer is the size of the color points.
Pixel dimensions indicate the number of points used to make the image, but they don't tell us how big those individual points are. DPI or Dots Per Inch is the measurement that tells us how big those individual points are in the image. The size of those individual points will affect the size of the image itself.
Let's say we have an image we took with a 12 megapixel SLR digital camera. We know if we print it at 300 DPI it'll be 10 inches wide by 8 inches tall. What we don't know is what the pixel dimensions are, and we want to find that out so we can also upload it to a gallery page. What we need to do is multiply the inches on each side by the DPI:
10 x 300 = 3000 8 x 300 = 2400
That means the pixel dimensions of our image are 3000x2400.
If we take the same size image at 150 DPI:
10 x 150 = 1500 8 x 150 = 1200
We can quickly figure out that this image's pixel dimensions are 1500x1200.
As you can see from the calculations we used, the DPI determines the size of the image in pixel dimensions. Our 300 DPI image needs more dots (or pixels) for the same size image because the individual dots are smaller – there are more of them per inch. Conversely, if we use larger dots, we need fewer of them which in turn makes the pixel dimensions smaller.
We can use this equation in reverse if we know the pixel dimensions of an image and what DPI we want to print it at, and want to see how big it will be when we print it: We just need to divide the number of pixels by the DPI.
1500 divided by 150 = 10 1200 divided by 150 = 8
And we can see that if our image is 1500 x 1200 at 150 DPI, it will print 10 x 8.
If we know the pixel dimensions of an image, and we know what DPI we want to display or print it at, this will show how large it will be once we print it.
Pro Tip: Tip: Your camera doesn't assign DPI to the image – it assigns pixel dimensions. The higher your megapixel rating, the larger your pixel dimensions. This translates to being able to print larger images at the DPI you want to use, because the DPI is determined by the output device. Click here for information on how to calculate your camera's image pixel dimensions from it's megapixel rating.