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Triangle Calculator

Use this triangle calculator to calculate a geometry relationship for triangle. The page returns area plus supporting values for perimeter, semiperimeter, and angle A; check the assumptions before reusing the result.

Primary answer
Area
Inputs to verify
Side a, Side b, and Side c
Use type
Use as a direct calculation check.
Keyword intent
triangle calculator

Calculator

Triangle Calculator

Calculates area from side a, side b, side c. Defaults are filled in so you can review a working example before changing inputs.

units

Use the same length unit for all three sides.

units

Use the same length unit for all three sides.

units

Use the same length unit for all three sides.

Result

Result reflects the current submitted inputs.

  • Risk A
  • Reviewed 2026-05-26
  • 2 sources
Area6 square units
Perimeter12 units
Semiperimeter6 units
Angle A36.8698976458 degrees
Angle B53.1301023542 degrees
Angle C90 degrees
Triangle typeright scalene

Breakdown

Formula
Heron's formula and law of cosines
Side scope
SSS only
  • The three side lengths use the same unit.
  • Only SSS triangles are supported in this packet.
  • Angles are returned in degrees.
  • Degenerate triangles are rejected.

Accuracy notes

Risk level
A
Reviewed
2026-05-26
Sources
2
Primary result
Area

Formula logic is kept in a pure calculator module with fixtures, source notes, and page-visible assumptions.

Categories

What the result means

Area answers the page's main triangle question. Triangle area from Heron's formula. Read the main measurement first, then verify whether secondary outputs use the same unit basis. Use perimeter, semiperimeter, and angle A to explain why area moved when an input changed. Write the unit beside the result so area, volume, and linear measurements are not confused. Check unit consistency, diameter versus radius, squared or cubed units, and rounding precision before treating the result as final.

AreaTriangle area from Heron's formula.
PerimeterDisplayed as decimal in units.
SemiperimeterDisplayed as decimal in units.
Angle AInterior angle opposite side a.

Use the result this way

  1. Start with Area, then use supporting outputs only to explain the primary answer.
  2. Verify side a, side b, and side c before copying the result.
  3. Keep units consistent with the labels shown in the form and stay within the documented minimum and maximum ranges.
  4. Write the unit beside the result so area, volume, and linear measurements are not confused.

User job

How to use this calculator

Use Triangle Calculator when you need area, then use perimeter and semiperimeter to check the context for quick number work, classwork, spreadsheet checks, and explaining a calculation to someone else.

Best for

  • Checking the core numeric relationship
  • Comparing the main result with supporting outputs
  • Reviewing a default example before entering your own side a and side b.

Check before relying

  • Confirm sign, decimal, percent, and rounding assumptions before copying the number.
  • The three side lengths use the same unit.
  • Only SSS triangles are supported in this packet.
  • Source context: OpenStax, Rice University, reviewed 2026-05-26.

Next useful step

Formula

Validate the triangle inequality, compute area with Heron's formula, and compute angles with the law of cosines. Key assumptions: The three side lengths use the same unit. Only SSS triangles are supported in this packet. Angles are returned in degrees.

  • Validate the triangle inequality, compute area with Heron's formula, and compute angles with the law of cosines.
  • The three side lengths use the same unit.
  • Only SSS triangles are supported in this packet.
  • Primary source context: OpenStax, Rice University.

Inputs

Enter side a, side b, and side c for number checks, homework, spreadsheet review, and quick comparisons. Before calculating, keep units consistent with the labels shown in the form and stay within the documented minimum and maximum ranges. Side a: Use the same length unit for all three sides. Side b: Use the same length unit for all three sides. Side c: Use the same length unit for all three sides.

Side aUse the same length unit for all three sides.
Side bUse the same length unit for all three sides.
Side cUse the same length unit for all three sides.

Example

Using the default inputs, Triangle Calculator returns area of 6 square units. Adjust side a, side b, and side c to match your own scenario.

FAQ

How is area calculated here?

Validate the triangle inequality, compute area with Heron's formula, and compute angles with the law of cosines. The first assumption to check is: The three side lengths use the same unit.

What does Area mean for triangle?

Read the main measurement first, then verify whether secondary outputs use the same unit basis. Secondary values such as perimeter, semiperimeter, and angle A are there to explain the primary answer, not to replace it.

What should I enter for Side a?

Use the same length unit for all three sides. Use units for this field. Keep units consistent with the labels shown in the form and stay within the documented minimum and maximum ranges.

How does Side b change area?

Use the same length unit for all three sides. Changing it can alter area because the formula uses the submitted inputs together. Also compare unit consistency, diameter versus radius, squared or cubed units, and rounding precision.

Why does the triangle example show 6 square units for area?

The default inputs produce 6 square units for area. Treat that as a format and scale check, then replace every default value with your own inputs.

Why does rounding matter for area?

Rounding affects the displayed answer and can compound if you reuse the number. Keep more precision for intermediate work when the next step depends on it.

Sources

Last reviewed: 2026-05-26

  • Reviewed 2026-05-26
    Algebra and Trigonometry 2e, Section 10.2: Non-right Triangles: Law of CosinesOpenStax, Rice University. Law of cosines for deriving interior angles from three side lengths and Heron's formula discussion.
    Scope
    English-language trigonometry source for solving SSS/oblique triangles.
    Supports
    Law of cosines for deriving interior angles from three side lengths and Heron's formula discussion.
    Limits
    This packet supports only SSS triangles and does not solve ambiguous SSA or angle-side cases.
  • Reviewed 2026-05-26
    Algebra and Trigonometry, Chapter 10 Key EquationsOpenStax, Rice University. Heron's formula, semiperimeter definition, and law of cosines equations.
    Scope
    Reference table of triangle area and law-of-cosines equations.
    Supports
    Heron's formula, semiperimeter definition, and law of cosines equations.
    Limits
    Equation reference only; validation and UI scope are defined in this package.