Find a missing side of a triangle using the Law of Cosines, Law of Sines, Pythagorean Theorem, or subtract known sides from the perimeter. Select a method, enter known values, and get the unknown s...
The **Triangle Side Calculator** helps you find any missing side of a triangle when you know enough other measurements. It supports four methods:
1. **Law of Cosines** — Use when you know two sides and the included angle (SAS). The formula c² = a² + b² − 2ab·cos C gives the third side directly. 2. **Law of Sines** — Use when you know one side and two angles (or one side, one opposite angle, and one other angle). The ratio a/sin A = b/sin B lets you solve for the unknown side. 3. **Pythagorean Theorem** — The simplest method, applicable only to right triangles. a² + b² = c² where c is the hypotenuse. 4. **Perimeter Method** — If you know the perimeter and two sides, the third side is simply P − a − b.
Each method covers a different set of known information, making this calculator a versatile one-stop tool for solving triangles. Whether you are a student working through geometry homework, an engineer verifying structural dimensions, or a surveyor computing distances, this calculator handles the arithmetic and shows detailed intermediate steps so you understand every result.
Load any of the eight presets to see real examples, then adjust the inputs to match your problem.
Finding one missing side is not a single-type problem. Sometimes you know two sides and an included angle, sometimes you know two angles and one side, and sometimes the problem is really just a perimeter subtraction or a right-triangle shortcut. This calculator is useful because it keeps those cases separate and applies the correct method automatically for the data you actually have.
That reduces a common source of mistakes: choosing the wrong theorem. Instead of forcing every problem into one formula, the tool makes the method part of the setup, then shows the completed triangle so you can see whether the result is reasonable in context.
Law of Cosines: c = √(a² + b² − 2ab·cos C). Law of Sines: b = a·sin B / sin A. Pythagorean: c = √(a² + b²). Perimeter: c = P − a − b.
Result: Missing side c ≈ 6.46
Known: a = 7, b = 10, angle C = 40°. Using the Law of Cosines: c = √(49 + 100 − 2·7·10·cos 40°) = √(149 − 107.25) ≈ √41.75 ≈ 6.461.
Most triangle-side errors come from method selection, not arithmetic. If you know two sides and the included angle, the Law of Cosines is the direct path. If you know an angle-side pair and another angle, the Law of Sines is usually better. Right triangles invite the Pythagorean theorem, while perimeter-based problems are often simple subtraction dressed up in geometric language.
A missing-side result is more useful when you can immediately see the full triangle it creates. Once the unknown side is found, you can test the triangle inequality, inspect the angle sizes, and compare the perimeter and area. That follow-through helps confirm that the answer is geometrically sensible, not just numerically possible.
When a worksheet mixes SAS, ASA, right-triangle, and perimeter questions together, slow down before substituting numbers. Identify what is known, note whether an angle is included or opposite, and decide which theorem fits the information. That one decision usually determines whether the rest of the work is smooth or messy.
Use the Pythagorean Theorem for right triangles, Law of Cosines for SAS, Law of Sines for ASA or AAS, and the Perimeter method if you know P and two sides. Use this as a practical reminder before finalizing the result.
When using the Law of Sines with SSA (two sides and a non-included angle), there can be 0, 1, or 2 valid triangles. This calculator shows both solutions when they exist.
No. Three angles determine the shape but not the size. You need at least one side length.
Results use IEEE 754 double-precision floating point, giving roughly 15 significant digits. The display rounds to your chosen precision.
All methods work for obtuse triangles. The Law of Cosines handles angles greater than 90° seamlessly because cos C is negative.
No. For non-right triangles, use the Law of Cosines or Law of Sines instead.