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Heron's Formula Calculator

Calculate the area of a triangle using Heron's formula from three side lengths. Get the semi-perimeter, area, perimeter, inradius, circumradius, all three altitudes, interior angles, and triangle type with step-by-step formulas and an interactive triangle diagram.

Heron's Formula Calculator
Examples:
LIVE TRIANGLE PREVIEW

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About Heron's Formula Calculator

The Heron's Formula Calculator computes the area of any triangle when you know all three side lengths. Enter sides \(a\), \(b\), and \(c\), and instantly get the area using Heron's formula \(A = \sqrt{s(s-a)(s-b)(s-c)}\) where \(s = \frac{a+b+c}{2}\) is the semi-perimeter. The calculator also provides the perimeter, all three altitudes, interior angles, inradius, circumradius, and triangle classification with step-by-step formulas and an interactive diagram.

What Is Heron's Formula?

Heron's formula (sometimes called Hero's formula) is named after Hero of Alexandria, a Greek mathematician who lived in the 1st century AD. It allows you to calculate the area of a triangle using only the three side lengths — no angles or heights needed. The formula is:

$$A = \sqrt{s(s-a)(s-b)(s-c)}$$

where \(s = \frac{a+b+c}{2}\) is the semi-perimeter (half the perimeter). This elegant formula works for all types of triangles — equilateral, isosceles, scalene, acute, right, and obtuse.

Real-World Applications

🏗
Construction
Calculate roofing, flooring, and lot areas from measured sides
🗺
Surveying
Find land area from boundary measurements
🎨
Design
Compute triangular panel areas for fabrication
📐
Education
Verify geometry homework and exam solutions
🌍
Navigation
Triangulation area in GPS coordinate mapping
🎮
Game Dev
Triangle mesh area calculations in 3D modeling

Key Formulas

PropertyFormulaDescription
Semi-perimeter\(s = \frac{a+b+c}{2}\)Half the triangle's perimeter
Area (Heron's)\(A = \sqrt{s(s-a)(s-b)(s-c)}\)Area from three side lengths
Altitude\(h_a = \frac{2A}{a}\)Height perpendicular to side \(a\)
Inradius\(r = \frac{A}{s}\)Radius of inscribed circle
Circumradius\(R = \frac{abc}{4A}\)Radius of circumscribed circle
Angle (Law of Cosines)\(\angle A = \arccos\left(\frac{b^2+c^2-a^2}{2bc}\right)\)Interior angle opposite side \(a\)

How to Use the Heron's Formula Calculator

  1. Enter side lengths: Type the three side lengths (a, b, c) of your triangle. You can use decimal values or click a quick example button to auto-fill sample values.
  2. Preview the triangle: As you type, the live triangle preview updates in real-time, showing the actual shape and proportions along with a quick area estimate.
  3. Click Calculate Area: Press the button to compute all results. The calculator validates the triangle inequality automatically.
  4. Review results: See the area, perimeter, semi-perimeter, all three altitudes, interior angles, inradius, circumradius, and triangle classification. Use the diagram toggles to show or hide heights, angles, the incircle, and circumcircle.

Triangle Inequality Theorem

Not every combination of three positive numbers can form a triangle. The triangle inequality theorem requires that the sum of any two sides must be greater than the third side: \(a + b > c\), \(a + c > b\), and \(b + c > a\). If any of these conditions is not met, the three lengths cannot form a valid triangle. This calculator automatically checks this condition and displays an error message if the sides are invalid.

Triangle Types

Triangles can be classified by their sides and angles. By sides: an equilateral triangle has all three sides equal, an isosceles triangle has exactly two sides equal, and a scalene triangle has all three sides different. By angles: an acute triangle has all angles less than 90°, a right triangle has one angle exactly 90°, and an obtuse triangle has one angle greater than 90°. This calculator determines both classifications automatically.

Inradius and Circumradius

The inradius (\(r\)) is the radius of the inscribed circle — the largest circle that fits inside the triangle, tangent to all three sides. It is calculated as \(r = A/s\). The circumradius (\(R\)) is the radius of the circumscribed circle — the circle passing through all three vertices. It is calculated as \(R = abc/(4A)\). These two radii are related by Euler's formula: the distance between the incenter and circumcenter is \(\sqrt{R^2 - 2Rr}\).

FAQ

What is Heron's formula?
Heron's formula calculates the area of a triangle when you know all three side lengths. The formula is A = √(s(s−a)(s−b)(s−c)), where s = (a+b+c)/2 is the semi-perimeter. It works for any triangle — acute, obtuse, or right — without needing to know an angle or height.
When should I use Heron's formula instead of base × height?
Use Heron's formula when you know all three side lengths but do not know the height. The base-times-height formula (A = ½ × base × height) requires knowing an altitude, which is often not directly given in a problem. Heron's formula only needs the three sides.
What is the triangle inequality theorem?
The triangle inequality theorem states that the sum of any two sides of a triangle must be greater than the third side. If a+b ≤ c, a+c ≤ b, or b+c ≤ a, the three lengths cannot form a valid triangle. This calculator checks this condition automatically and alerts you if the sides are invalid.
What are the inradius and circumradius of a triangle?
The inradius (r) is the radius of the largest circle that fits inside the triangle (inscribed circle), calculated as r = A/s where A is area and s is semi-perimeter. The circumradius (R) is the radius of the circle passing through all three vertices (circumscribed circle), calculated as R = abc/(4A).
Can Heron's formula work for all types of triangles?
Yes. Heron's formula works for equilateral, isosceles, and scalene triangles, as well as acute, right, and obtuse triangles. It is a universal formula for any valid triangle as long as you know the three side lengths.

Reference this content, page, or tool as:

"Heron's Formula Calculator" at https://MiniWebtool.com/heron-s-formula-calculator/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: 2026-04-04

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