Triangle Perimeter Calculator - Three-Side Sum and Semi-Perimeter

A triangle perimeter calculator is a tool that adds the three side lengths of any triangle and returns the total distance around the shape in a single step. Enter side a, side b, and side c in your chosen linear unit and the calculator returns the full perimeter, the semi-perimeter for Heron's formula, and a triangle inequality check.

• • Classroom and homework checks: Verify a triangle's perimeter on a worksheet, including 3-4-5 right triangles and equilateral reference problems, before submitting the answer.
• • Construction and DIY measurements: Sum the three edges of a triangular deck frame, garden bed, or shelf bracket to size trim, edging, and fabric.
• • Fencing and trim estimates: Estimate linear footage of fence, edging, or molding needed to enclose a triangular lot, plot, or fabric panel.
• • Heron's formula prep work: Get the semi-perimeter s = (a + b + c) / 2 ready to drop into Heron's formula for the triangle's area when only the three sides are known.

The calculator assumes a closed three-sided figure on a flat plane. Enter the three side lengths once, pick the unit from a dropdown, and the result panel returns the perimeter, semi-perimeter, and a quick triangle inequality check.

If your measurements come from a tape, ruler, or laser tool, the tool is unit-aware but does not convert. Pick one linear unit, enter all three sides in that unit, and the result carries that same label.

If your sides come in fractional inches, eighths, or sixteenths, the Perimeter of a Triangle with Fractions Calculator handles mixed numbers alongside straight decimals.

The calculator is built on the standard perimeter definition for a polygon: add the lengths of all sides. For a triangle, the result is the simple sum a + b + c, returned in real time as you type.

• P: Perimeter of the triangle, in the linear unit you selected (cm, m, in, ft, or yd).
• a: Length of side a, opposite vertex A.
• b: Length of side b, opposite vertex B.
• c: Length of side c, opposite vertex C.

The result panel also shows the semi-perimeter, the value you drop into Heron's formula for area. An 'Equilateral?' row confirms whether all three inputs match, and the triangle inequality row tells you whether the three lengths can close into a real triangle.

According to Wikipedia (Perimeter), the perimeter of any polygon is the sum of the lengths of its edges, so a triangle's perimeter is simply a + b + c.

According to Wolfram MathWorld, a triangle is a three-sided polygon whose perimeter equals the sum of its three side lengths, commonly written P = a + b + c.

Once you have the three side lengths of a right triangle, the Right Triangle Calculator can also work backwards from two sides and the right angle to find the missing hypotenuse or leg.

These four ideas cover the geometry the calculator relies on. They explain why a simple sum of three numbers can describe the boundary of every triangle.

These concepts come straight from the polygon definition of perimeter and the triangle inequality. They do not depend on the angle measurements of the triangle, so the perimeter of an obtuse triangle is no harder to find than the perimeter of a right one.

If you would rather start from one side and an angle, a dedicated triangle tool fits better.

When all three sides are equal, the Equilateral Triangle Area gives the area in a single step with the same one-input flow used here for the perimeter.

The calculator needs three numbers and a unit label. Use the steps below to read the perimeter, semi-perimeter, and triangle inequality check on the result panel.

1. 1 Pick one linear unit for every side: Choose centimeters, meters, inches, feet, or yards from the Unit dropdown. The calculator does not convert between units, so all three sides must use the same unit.
2. 2 Enter side a, side b, and side c: Type the three side lengths into the matching fields. Decimals are fine up to the precision of your measurement tool.
3. 3 Read the perimeter on the result panel: The result panel updates as you type and shows the total perimeter in the unit you selected, rounded to two decimal places.
4. 4 Copy the semi-perimeter for Heron's formula: If you also need the triangle's area, copy the semi-perimeter row s = (a + b + c) / 2 and use it inside Heron's formula for the area.
5. 5 Check the triangle inequality flag: The result panel shows a 'Triangle inequality' row. If it says Invalid, the three lengths cannot close into a real triangle, so recheck a measurement before you trust the perimeter.

A triangular garden bed is measured at 5.5 ft, 8.25 ft, and 10.75 ft. Enter the three lengths with Unit = Feet, and the calculator returns a perimeter of 24.50 ft, a semi-perimeter of 12.25 ft, and a valid triangle flag. That 24.50 ft is the edging strip you need for the border.

For the area of the same triangular garden bed, the Heron's Formula Calculator takes the semi-perimeter you just read and returns the area without any angle measurements.

The tool is small, but it solves a real measurement problem. These benefits show where it pays off in school, on the job, and at the planning desk.

• • Saves time on the three-side sum: You do not need to add three numbers by hand; the result panel updates as soon as you finish typing.
• • Catches bad inputs with the triangle inequality: A quick check of a + b > c, a + c > b, and b + c > a flags measurements that cannot form a real triangle, so you can correct a typo before it becomes an estimate mistake.
• • Pairs directly with Heron's formula: The semi-perimeter is half of the perimeter and is the value Heron's formula needs for the area, so you can copy the s row straight into an area tool without recomputing.
• • Works for any triangle type: Acute, right, obtuse, equilateral, isosceles, and scalene triangles all use the same a + b + c rule, so the calculator handles them with no extra steps or angle inputs.
• • Unit-flexible for school and field use: Centimeters, meters, inches, feet, and yards cover the linear units you will see on a worksheet, a tape measure, or a building plan.

These benefits apply whenever the three side lengths are already known or measurable. If you only have one side and a few angles, an angle-and-side triangle solver is a better starting point.

The tool pairs well with a length converter when the three sides are stored in mixed units. Convert everything to a single unit first, then add them here.

For non-triangle shapes like rectangles, circles, or regular polygons, the general Perimeter Calculator accepts several side inputs and returns a sum in the same unit-aware style.

The sum a + b + c is short, but the number you read off the panel can still be skewed by a few practical factors. Watch these before you commit a perimeter to a quote or cut list.

• • The calculator returns a numeric sum even when the three sides do not form a valid triangle. Always check the triangle inequality row before you trust the perimeter for any real-world estimate.
• • The tool does not convert between linear units. If your sides are stored in different units, run them through a length converter first.
• • Perimeter is a flat-plane measurement, so the result assumes a planar triangle. For 3D triangular surfaces such as roof panels or fabric, account for the slope or the diagonal of the face.

These factors rarely change the formula, but they do change the answer. An Invalid flag is a sign that one side was taken on the wrong line.

If you need the area, the semi-perimeter s feeds Heron's formula, so this tool is also a quick pre-step for a full side-only solution.

According to Wikipedia (Triangle), an equilateral triangle has all three sides equal so its perimeter is 3a, and any triangle must satisfy the triangle inequality a + b > c, a + c > b, and b + c > a.

When the three angles of a triangle are given instead of the three sides, the AAA Triangle Calculator works from two angles and a reference side to fill in every edge length before a perimeter tool is used.