Using some simple math calculations and taking careful measurements are the keys to getting the right amount of edging for a circular garden bed. Block edging isn't as easy as flexible wood or plastic edging, but it will make for a sturdy, secure border for your garden beds and can easily be figured out.
Plan ahead to make sure you know how big you want your circular garden bed to be. Try marking out the circle with simple garden stakes and string if you are creating a whole new bed. If not, you can still use stakes and string to create a temporary border guide that will help you measure the bed. Put the stakes as close to the bed edges as you want the block pavers to be.
Measure the circumference of the garden bed, or in other words, the distance around the outside of the circle. If you have a measuring tape long enough for the job, that's easiest to use. Wrap it around the outside of the stakes and string you are marking the bed with, and write down the length in a notebook for later reference. If you don't have a measuring tape long enough, you can use a garden hose. Mark the distance on the hose with a pencil. Then, straighten the garden hose out and measure it with a yardstick.
Choose edging blocks or pavers for your bed. Different types of blocks have different dimensions, but the important dimension for your planning is their length. Take note of the length of the blocks you've chosen, and write it down.
Divide circumference by block length to find out how many blocks you will need. Convert all measurements to the same unit before calculating. For example, if the circumference is 9 feet long, and each block is 3 inches long, you would convert the circumference to 108 inches, and divide that by three. Your result would be 36 blocks to make it all the way around the circle. If you want to have several layers of blocks, multiply the one-layer number by the number of layers you want.
Round up in cases where you have uneven numbers. Then, buy a few extra blocks. This will make sure that you have enough in case some crack or break while you are making the bed and to make up for the rounding you've done.