Square feet are widely used to measure area in the United States and a few other countries. While an area defined by a triangle can be calculated in a number of ways, the Heron's Theorem (formula) allows you a straightforward computation of the triangle’s area. All you need to know are the lengths of all three of the triangle’s sides.

Measure or obtain elsewhere the lengths of all three sides of the triangle.

Convert the triangle side lengths into feet if the original measurements are in other units. For instance, if the sides are measured in inches, divide the measurements by 12. If they are given in meters, multiply the values by 3.28. For example, if the triangle's sides are 92.5, 123 and 167 inches, they will be converted to 7.71 (92.5 divided by 12), 10.25 (123 divided by 12) and 13.92 (167 divided by 12) feet.

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Add up the lengths of all three of the triangle's sides and then divide the sum by two to calculate the triangle's semiperimeter. In the example above, the semiperimeter can be obtained by the following equation: (7.71 + 10.25 + 13.92)/2 = 15.94 feet.

Subtract the length of the first side from the semiperimeter. In this example, it is 15.94 - 7.71 = 8.23 feet.

Subtract the length of the second side from the semiperimeter. In this example, it is 15.94 - 10.25 = 5.69 feet.

Subtract the length of the third side from the semiperimeter. In this example, it is 15.94 - 13.92 = 2.02 feet.

Multiply the triangle semiperimeter by each value obtained in Steps 4 to 6. In the example, the equation would be: 15.94 x 8.23 x 5.69 x 2.02 = 1507.83

Take the square root of the product from Step 7 to calculate the triangle's area. In the example, the area of the triangle is the square root of 1507.83, or 38.83 square feet. Note that this result as well as those in Steps 2 to 7 are rounded to the second decimal point.