Choose the most suitable formula based on the information you have available. If you know the length of the base and the vertical height of the triangle you can find the area by multiplying half the base by the height. If you do not know or are unable to measure the height, you can use an alternative formula.
Multiply the length of two sides of the triangle by the sin of the angle in between. Divide the result by two to get the area of the triangle. For example if the two sides are 7 and 12 and the angle in-between is 30 degrees, the area of the triangle would be 1/2 x 7 x 12 x sin30 = 21. If you do not have a measurement for the angle, the area can be found if you have the length of all three sides of the triangle.
Add together the length of the three sides, a, b and c, and divide the result by two in order to find the semi-perimeter of the triangle, s. The area can then be found using Heron's Formula. This states that the area of a triangle is a square root of s x (s-a) x (s-b) x (s-c). For example, if the triangle has sides of 6, 8 and 10, the semi-perimeter would be 12 and the area would be 24.