Triangle perimeter calculator

Calculate a triangle’s perimeter from three sides and check whether the lengths can form a valid triangle.

Result

Three segments must close

The drawing respects the entered side lengths. A side that is too long prevents closure: an error replaces the impossible triangle.

How do you calculate triangle perimeter?

Add the three side lengths. First check that each side is shorter than the other two combined; otherwise the sides cannot close into a triangle.

P = a + b + c

How to use the calculator

Enter measurements with a decimal point or comma: results update immediately. Input units apply to every length; changing them converts the measurements. Changing result units leaves the shape unchanged. The control below the drawing reveals or hides its visual explanation. Copy results includes valid measurements and the result. Reset and Try this example restore the initial values and centimetre units.

The first formula shows the relationship; the second substitutes measurements converted to the result unit. The ≈ sign indicates rounding, including in the substituted values.

A triangular frame

A frame with sides 3, 4 and 5 cm needs 12 cm of edging. Its semiperimeter is 6 cm: half the boundary, not another side length.

From measurements to the result

Input: $a = 3 cm; b = 4 cm; c = 5 cm$
Result: $12 cm$

Calculation steps

Add the first two sides.
$3 + 4 = 7$
Complete the calculation using the tool’s formula.
$P = 7 + 5 = 12$

Units and interpretation

A boundary is a length, measured in cm, m or another linear unit. One metre equals 100 centimetres.

Zero and flat triangles are not allowed. Nearly degenerate sides can exceed reliable numerical representation.

Precision and numerical limits

Lengths range from 0 to 10¹² in the selected unit; triangle sides must be strictly positive. Results use two decimal places, with scientific notation for very small or large values. Calculations use browser precision: unrepresentable positive values produce an error. A rejected unit change keeps the previous data. Calculation takes place in your browser.

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Triangle perimeter calculator

Result

Three segments must close

How do you calculate triangle perimeter?

How to use the calculator

A triangular frame

From measurements to the result

Calculation steps

Units and interpretation

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