Today I answered this customer question and I hear questions like this often enough that I thought I'd share my response. How many feet of cord is ultimately going to depend on how big your tree is at the base and what number of lights you need to consider them "enough" is all a matter of personal taste and how tightly your wrap the tree. (This question was asked on our 100-foot cords with 15 inch spacing.) You'll need 80 bulbs for each 100-foot cord that has 15 inch spacing. Using our How Many Lights Should I Put on My Tree Calculator, we see that a nice starting range is 350-525 C9 bulbs. I would consider starting with 4-6 of these strings for your tree depending on how tightly you want to spiral the cords around the tree. You might want to add an additional 200 feet if your tree is "fat". You might consider loosely measuring the base of your tree and the measure some diameters of cross sections as far up as you can so you can roughly estimate how many linear feet you'll need to wrap the tree in as tight of a spiral as you'd like. Keep in mind that these cords have only one plug and with traditional C9 bulbs you'll need to run every 2 cords on their own breaker (not just outlet but their own breaker). You'll also need an extension cord for every 2 100-foot cords and you'll want to hide that near the trunk of the tree and install the cords out in a capitol T configuration from there. Sorry I don't have a more definite answer on exactly how many bulbs and cords you need but hopefully I've given you a place to start with things to consider as you plan your design.