Can you use round needles in place of double pointed needles of the same size?

Note: round needles are almost always called circular knitting needles.

In some cases, circular needles will work just as well as double pointed needles. But more often than not, only double pointed needles will work.

Why? With double pointed needles, one can knit with as few stitches as they want. This makes them ideal for knitting the tops of hats, which are decreased to a very few number of stitches, and other items where few stitches are knitted. However, if one is knitting with a large number of stitches, then circular needles will work fine, maybe even better, but circular needles have a "minimum stitch limit." Thus, they only work if the pattern works with lots of stitches.

So in order to figure out whether circular knitting needles will work for your pattern, read through the instructions, and determine the point at which there will be the least number of stitches on the double pointed needles. Try to estimate the lowest number of stitches that will work comfortably on the circular needles, and decide if the stitches in the pattern will fit on the circular needles.