Design your fence and draw its path on a piece of paper. Use your measuring tape to make sure that you have the exact dimensions of your property, any buildings or obstacles and the fence that you want to build.
Determine how many fence posts you will need. A fence post will be required at the start and end of the fence, at every corner and on both sides of any gate. On top of that, a post will be required every 6 to 10 feet to give the fence stability. Mark the posts down on your paper and then add them up. Keep in mind that fence posts require a larger dimensional size than other wood fence pieces, and 1/3 of the post will be buried; so, if you want a 4-foot-high fence, you need 6-foot-long pieces of wood.
Add up the number of fence rails you will need. These fence rails will run between the posts and support the fence material, if applicable. These fence pieces are generally made from 2-by-4s, but they can be any size that you require. Most fences will have two or three rails between every two posts, although higher fences can sometimes use more. Multiply the number of rails by the number of spaces between posts, not counting any gates, to figure out how many pieces of wood you need.
Determine how much wood you will need to build the fence material for one section of fence between two fence posts. This fence material will vary wildly depending on the type of fence you are making but will normally consist of pickets, battens or large wood panels that will be attached to the rails. Measure the distance between the posts to determine how much wood you will need to construct the fence material, then multiply that amount by the number of spaces between the posts. Remember that most dimensional lumber can be ripped in half to make small pickets, meaning that if you want 4-foot pickets that are 1 1/2 inches wide, an 8-foot long 2-by-4 can make four of them.
Add in the wood it will take to build your gate. The amount will vary based on the type of gate but will normally consist of a constructed wood frame with fence material attached to the front.