Hi Miriam, and welcome to the Lenspiration forums!
DPI stands for “dots per inch”. For digital files that you’re viewing on a screen it’s not a very useful number, (it’s mostly used for printing) but it’s good to know what it means so you can understand it when people toss the term around. 🙂
Digital photos are made up of millions of little colored dots called “pixels”. A 16 megapixel camera will produce a photo with approximately 16 million pixels in it, arranged in a grid that when viewed from a distance makes a colorful photo appear. When you print a photo, the computer takes those colored pixels and converts them into little colored dots of ink that are sprayed onto the page from the printer nozzle. In order to get a good-looking print you want to have enough dots close together that blend well enough to make a nice high-quality printout. That’s where DPI comes in. You want to have at least 250-300 dots per inch in your print in order to make it really good even when viewed up close.
So the question actually becomes, “How do I know if my photo has sufficient DPI to look good in a print?” The answer completely depends on what size you plan to print. 🙂 A 4×6″ photo at 300 DPI will require a photo resolution of at least 1200×1800 pixels (multiply 300 x print size in inches to get the pixel dimensions).
A calendar will likely be printed at 8×10″ or 8.5×11″ (check the contest guidelines), so if they require 250 DPI, then you’ll want to multiply by 250 to find the minimum photo dimensions. In this case, you’ll want at least 2000×2500 or 2125×2750 respectively. If your photo is 4608 x 3456 then you’ve got plenty of pixels to spare!
Note that lots of photo software shows DPI in the info/settings for a photo as 72 or 96 or 180 DPI. That’s a meaningless number because without knowing the print size, you can’t know what the actual DPI is going to be. Just take the pixel dimensions and the desired print size and calculate it yourself for optimum results. 🙂
BTW, the reason most photos show up as 72 or 96 DPI is because many computer screens (used to) display approximately 72/96 pixels per inch. As you might imagine, that’s nowhere near as high quality as a good print! If you have a newer “high resolution” display, such as a 2K/4K monitor, or modern Apple products with Retina displays, then they often have a much higher PPI, sometimes even more than 300! You’ll notice that such displays often show lots of detail in your high resolution photos that just weren’t visible on older screens that spread the pixels so thin.