The last thing you want is to realize that you don't have enough lights — and end up with a patchy or lopsided look. Ash recommends using 100 lights for every foot of your tree's height.
"A good tip to check if you have used enough lights is to step back and look at your tree with your eyes squinted, this way you will be able to spot where the darker gaps are and where more lights need to be placed," says Ash.
You should also measure your tree before buying your lights — you need to know the height and the width of the trunk. As a guideline, a single wrap around a tree trunk can easily use up 30-40 lights, so if you're doing a full wraparound design, plan to get (a lot) more than you think you need.
As a rough guideline, multiply the height of your tree by the number of thicker branches you plan to wrap by the trunk circumference. You've got a rough estimate of how many lights you'll need.