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Triangle Calculator

Find triangle sides, angles, area and perimeter from SSS, SAS or two angles and a side.

Your numbers

Make it your own
Updates as you type
Triangle area
6square units

Calculated from your inputs · rounded for display

Side a3
Side b4
Side c5
Angle A36.8699°
Angle B53.1301°
Angle C90°
Perimeter12

Use the same length unit for every side. SSS, SAS and ASA/AAS only; ambiguous SSA cases are intentionally not inferred.

Your calculation, step by step

No black boxes
  1. Input method: SSS. Side a is opposite A, b opposite B, c opposite C.
  2. Use the cosine law or sine law to determine missing sides.
  3. Area follows Heron’s formula, evaluated in a stable scaled form: 6 square units.

How to use this calculator

  1. Enter your own values in the labeled fields. Check the units and assumptions before comparing results. Only the fields needed for your selected calculation mode are shown.
  2. Your answer updates automatically. Review the calculation steps to see exactly how it was calculated.
  3. Copy, share, save, or download your result. Saved inputs stay on this device.

How the calculation works

Cosine law: c² = a² + b² − 2ab cos C; area from Heron’s formula

Sides a, b and c face angles A, B and C respectively. Enter all side lengths in the same unit. SSS validates the triangle inequality; SAS uses the angle between a and b; the two-angle mode derives the third angle before using the sine law. Ambiguous two-side nonincluded-angle cases are not inferred.

A worked example

A triangle with sides 3, 4 and 5 has area 6 square units and perimeter 12 units.

Reading your result

Use the main result together with its units and supporting figures. Display values are rounded; calculations keep their underlying precision. A change in inputs or assumptions can change the answer. Compare the step-by-step working with your intended use before relying on an estimate.

Frequently asked questions

Can I enter an impossible triangle?

The calculator rejects nonpositive sides and degenerate or impossible combinations.

What units are supported?

Any consistent length unit; area uses its square.

Sources & methodology

This calculator uses the mathematical identity and assumptions stated above. No current rates, prices, or external data are loaded. Conversion tools use international unit definitions.

Formula review: October 24, 2026. Read our methodology.