Perimeter of a Triangle Calculator

The perimeter of a triangle is the total length of its three sides. If the side lengths are called a, b, and c, the formula is simply P = a + b + c. Unlike area formulas, perimeter does not depend on height or angles when all three side lengths are known. It is a linear measurement describing one complete trip around the triangle's boundary. Before adding the sides, it is important to confirm that the measurements can actually form a triangle. The triangle inequality says that the sum of any two sides must be greater than the remaining side. For lengths 3, 4, and 5, all three comparisons succeed, so the perimeter is 12. For lengths 2, 3, and 6, the first two total only 5, which is shorter than 6; those segments cannot close to form a triangle. The strict greater-than condition matters. If two sides add to exactly the third, the shape collapses into a straight line. This is called a degenerate triangle and has zero area, so the calculator does not treat it as a valid triangle. Every entered length must also be positive. Zero and negative values cannot describe physical side lengths. Any consistent unit works. Entering all sides in meters produces a perimeter in meters, while centimeters produce centimeters. If the source measurements use different units, convert them before entering the values. For example, convert 1.2 meters to 120 centimeters before combining it with sides measured in centimeters. Perimeter uses ordinary length units rather than square units. Special triangle types use the same addition rule. An equilateral triangle with side s has three equal sides, so its perimeter is 3s. An isosceles triangle with equal sides s and base b has perimeter 2s + b. A right triangle still uses a + b + c; the Pythagorean theorem is only needed when one side must first be derived from the other two. Triangle perimeter appears in fencing, framing, surveying, navigation, craft layouts, and geometry exercises. The calculator preserves decimal values and rounds only the displayed total for readability. Enter the measured or given sides, calculate the boundary length, and use the triangle validation message to catch impossible dimensions before relying on the answer. When a drawing is not to scale, always use its labeled measurements rather than estimating from appearance.