How to Make a Perfect Right Angle on a Fence

When laying out a fence, it is important that the lines are crisp and straight and the angles perfectly aligned. For example, if you are enclosing an area with four lengths of fence, the corners need to be exactly 90 degrees for the segments to line up correctly. The "3-4-5" method is a simple technique used by carpenters to create a right angle. It is based on the ratio of the three sides of a right triangle.


Drive a wood stake in the ground in the location of the first fence post.


Tie two lengths of string to the stake and mark one at 3 feet from the stake and the other at 4 feet from the stake.


Pull the strings taut at a roughly 90 degree angle to each other in the direction where the two fence segments will be built. It is helpful to have a second person help with this.


Measure the distance between the 3- and 4-foot marks on the strings. Move the strings closer or farther apart as needed to make the distance between the two points exactly 5 feet. Drive a stake into the ground at the point where the 3- and 4-foot marks are 5 feet apart. As long as the 3-4-5 ratio is maintained, the points can be rotated either way to better align with the planned location of the fence lines.


Install the first corner post in the location of the stake. Tie a string between the post and the two remaining stakes and check to make sure they still form a 3-4-5 triangle with the corner post.


Attach two rolls of string to the corner post and extend them in the direction of the two adjacent corner posts. Mark the intended length of the two adjacent fence segments on the lines of string. Hold each string at this point and align it with the lines that go between the corner post and the other two stakes. When they are perfectly aligned, mark the point for these two corner posts with a stake and install the posts in these exact locations.


Repeat the procedure to mark the locations of additional posts.