How do I find the missing side of a triangle?

Depends on what you know. For SSS (three sides): use Heron's formula for area, then law of cosines for angles. For SAS (two sides and included angle): use law of cosines for the third side. For AAS/ASA: use law of sines. Our triangle calculator handles all six cases automatically. The triangle calculator picks the right formula based on your inputs.

What is the Pythagorean theorem?

For a right triangle: a^2 + b^2 = c^2, where c is the hypotenuse. For 3-4-5 triangle: 9 + 16 = 25. Our triangle calculator uses this for right triangles. The triangle calculator also supports the law of sines and cosines for non-right triangles. Pythagorean theorem only applies to right triangles.

How do I calculate the area of a triangle?

Area = 1/2 x base x height (if you know base and height). Area = sqrt(s(s-a)(s-b)(s-c)) for Heron's formula (if you know three sides, s = semi-perimeter). Our triangle calculator picks the right method based on what you know. The triangle calculator also shows the area in multiple units (sq ft, sq m, etc).

What is the law of sines?

a/sin(A) = b/sin(B) = c/sin(C). Used when you know two angles and a side, or two sides and an angle opposite one of them. Our triangle calculator uses this for AAS and SSA cases. The law of sines has an ambiguous case (SSA can have 0, 1, or 2 triangles) that the triangle calculator handles.

Triangle Calculator: 3-4-5 = Right (90°) (Free, 2026)

Enter three known values

Type any three of: side a, side b, side c, angle A, angle B, angle C. The calculator solves the missing three values.

We use the Law of Sines and Cosines

For SSS or SAS triangles, we use Law of Cosines. For ASA, AAS, or SSA, we use Law of Sines. The calculator picks the right method automatically.

See area and perimeter

Get the area (using Herons formula for SSS, 0.5 x b x h for others) and perimeter. The calculator also shows the angles in degrees and radians.

Key things to know about our triangle calculator:

Triangle types: Equilateral (all 60 degrees), isosceles (two equal sides), scalene (no equal sides), right (one 90 degree angle), acute, and obtuse.
Real-world uses: Roof pitch, land surveying, navigation, construction, trigonometry problems, and engineering design all use triangle math.
Law of Sines: a/sin(A) = b/sin(B) = c/sin(C). Useful for AAS and ASA triangles. The Law of Cosines handles SSS and SAS: c^2 = a^2 + b^2 - 2ab cos(C).

For related calculations, try our square footage calculator.

Frequently asked questions

Everything you need to know about this calculator. Can't find what you're looking for? Email our team.

A triangle calculator is a free geometry tool that finds missing sides, angles, area, and perimeter of any triangle. Our triangle calculator uses the law of sines, law of cosines, and Pythagorean theorem depending on which values you know. The triangle calculator supports equilateral, isosceles, scalene, right, acute, and obtuse triangles.

How to find missing sides and angles of a triangle

Our triangle calculator lets you solve any triangle given two sides and an angle, two angles and a side, or three sides.

1
Pick the case you know
Choose SSS (three sides), SAS (two sides and included angle), ASA (two angles and included side), AAS (two angles and a non-included side), or SSA (two sides and a non-included angle). The triangle calculator accepts any of these.
2
Enter the known values
Type the known sides and angles. The triangle calculator auto-detects which case you have and picks the right formula. Angles in degrees or radians. The triangle calculator handles ambiguous cases (SSA).
3
Read the result
The result shows the missing sides and angles, plus the area and perimeter. The triangle calculator also shows whether the triangle is acute, right, or obtuse. The share button creates a link with your inputs pre-filled.

Triangle Calculator

Triangle Calculator

How our triangle calculator works

Enter three known values

We use the Law of Sines and Cosines

See area and perimeter

Key things to know about our triangle calculator:

Trigonometric Functions

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Instant results

Your data stays private

Learn as you calculate

Frequently asked questions

How to find missing sides and angles of a triangle

Pick the case you know

Enter the known values

Read the result

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