Identify the triangle’s shape and any known measurements
Use the given formula if the area and height are known: base = 2 × area ÷ height
Use the Pythagorean theorem if it is a right triangle and the other sides are known
Use trigonometry if one side and an angle are known
Use the triangle’s perimeter and the other side lengths if all side lengths are involved
Measure the side you want to treat as the base directly if working from a drawing or physical triangle
Choose any side as the base if no side is specified