Right Triangle Calculator

Grid mode accepts three coordinate points, with the right angle at C. It calculates side lengths from coordinate differences and rejects points that do not form a right angle at C. The grid preserves the entered orientation and labels each point.

Choose the measurements you know. Lowercase letters name sides; the matching uppercase letter names the opposite angle. All side lengths use the selected unit, and all angles use degrees.

For a right triangle, c is the hypotenuse and C is 90°. Enter both legs, a leg and the hypotenuse, or the hypotenuse and acute angle A. A 3-4-5 triangle has area 6 and acute angles approximately 36.8699° and 53.1301°.

A + B + C = 180°; area = ab sin(C) / 2
c² = a² + b² − 2ab cos(C); a/sin(A) = b/sin(B) = c/sin(C)

For three sides, area follows Heron’s formula: √[s(s−a)(s−b)(s−c)], where s is half the perimeter. A triangle with sides 5, 5 and 6 has area 12 and equal angles opposite the two sides of length 5.

A 30°-60°-90° triangle has side proportions 1:√3:2. A 45°-45°-90° triangle has proportions 1:1:√2. Enter the corresponding angle to calculate a scaled version.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Length unit

cm

Known measurements

Two legs

Side a

3 cm

Side b

4 cm

Primary result: Area: 6 cm².

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

Requires a non-degenerate flat triangle. The longest side must be shorter than the sum of the other two. Nearly degenerate triangles are sensitive to measurement error. The drawing is a schematic, not a full-size template or an answer to a missing worksheet image.