How Do I Find The Linear Feet For A Counter Top?

Measure it using a ruler or tape measure graduated in feet.
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It depends on what you want to know. If you are buying material to cover it, you may want to measure along the wall (or wherever the longest dimension is) or down the center--depending on how you plan to cut the material at the corners. If you are buying edge molding, you will want to measure around the edges where the molding goes. The same would be true of backsplash material.