I am often sent images at 72 DPI from customers who have hired me to design a print product but the printer requires all images be at least 300 dpi. It is possible to increase the dpi of an image but that same image will be reduced in size by the same percentage.

Take this image for example. It is 5″ high, 72 dpi.

To increase the dots per inch:

1. Take the height of the image: 5″
2. Multiply by the current number of dots per inch: 72
3. Divide by the number of dots per inch that you need the image to be: 300

Final calculation:
5 x 72 / 300 = 1.2

Converting this 5″ image at 72 to a 300 dpi image will result in an image that is 1.2″ high.