Main perimeter
Enter your data and press Calculate
Triangle perimeter calculator
Compute triangle perimeter by adding the 3 sides. Includes triangle inequality validation and unit conversion.
Triangle perimeter calculator
Pick the data you know: three sides, two sides plus the included angle, or an equilateral side.
Returns main perimeter, semiperimeter, normalized sides and Heron area with instant conversions to mm, cm, m, in and ft.
Inputs
e.g. 5.4
Comma or dot accepted.
e.g. 7
e.g. 6.2
e.g. 5
e.g. 4.3
1–179
The third side is found with the law of cosines.
e.g. 3.8
Equilateral: the three sides are equal.
The main perimeter will follow this unit.
0–10
Applies rounding to every result.
Results
Side a
—
Side b
—
Side c
—
Included angle
—
Semiperimeter (s)
—
Area (Heron)
—
Perimeter conversions
mm
—
cm
—
m
—
in
—
ft
—
How to calculate a triangle perimeter
The perimeter is the sum of the three sides: P = a + b + c. When you know two sides and the included angle, apply the law of cosines to find the remaining side: \( c = \sqrt{a^2 + b^2 - 2ab\cos C} \). For an equilateral triangle, multiply the side by three: \( P = 3a \).
This tool normalizes your unit, computes semiperimeter and Heron area automatically, and shows instant perimeter conversions to mm, cm, meters, inches and feet. Use the decimals control to match the precision you need.
Quick example
With sides 5.4 cm, 7 cm and 6.2 cm, the perimeter is \( P = 18.6\ \text{cm} \). The semiperimeter is \( s = 9.3\ \text{cm} \) and Heron’s formula gives an area close to \( 16.6\ \text{cm}^2 \). Using the angle option you could input two sides plus the angle to get the same result.
What input methods are available?
You can enter three sides, two sides with the included angle, or a single side for an equilateral triangle. The calculator fills the missing side and validates the angle.
Which formulas are used?
The law of cosines finds the missing side when an angle is provided. Heron’s formula delivers the area and the perimeter is the direct sum of the three sides.
Which units are supported?
Meters, centimeters, millimeters, inches and feet are available. Choose one input unit to see the main perimeter in that unit plus the other conversions.
