Summarize this article:

If you have or are planning to add a backsplash, account for its dimensions in your final measurement.  Measure the length of the backsplash for each section of countertop. Add the lengths together. Multiply the total length of the backsplash by 4 inches (the height of the backsplash). Divide the product by 140 to calculate the total square footage of your backsplash. Add this number to the total square footage of your countertops. Often countertops run along two perpendicular walls, forming an L-shape.  When measuring this layout, you must first verify that your corner is a 90° angle. Measure and mark 3 feet from the corner along one wall. Measure and mark 4 feet from the corner along the other wall. Measure the diagonal distance between the two points. If the distance is 5 feet, then your corner is square. Divide the counter into two sections. To determine the length of section A, measure from one end of the counter to the wall. To determine the width of section A, measure from the front edge of the counter to the wall. To determine the length of section B, measure from the opposite end of the counter to the wall. Subtract the width of section A to get the length of section B. To determine the width of section B, measure from the front edge of the counter to the wall.  Calculate the area (or square inches) of each section by multiplying the length times the width of each section (Length x Width = Area). Calculate the total square inches by adding together the areas of each section. Calculate the square footage by dividing the total square inches by 144 (Total Square Inches ➗ 144 = Total Square Footage). If your countertops are irregularly shaped, divide the countertop into square sections. These sections may overlap and or include negative or blank space. Once you have sectioned off the countertop into squares, measure the length and width of each section. For each section, multiply the length by the width.  Calculate the area (or square inches) of each section by multiplying the length times the width of each section (Length x Width = Area). Calculate the total square inches by adding together the areas of each section. Calculate the square footage by dividing the total square inches by 144 (Total Square Inches ➗ 144 = Total Square Footage).

Summary:
Measure the length of the backsplash and calculate the square footage. Measure an L-shaped counter. Measure irregular countertops.