Result updated. Area: 6.
Triangle result
Main result from the values entered.
Area
A triangle with sides 3, 4 and 5 has area 6, perimeter 12 and angles 36.87°, 53.13° and 90°.
Perimeter
12
Triangle type
Scalene right triangle
Semi-perimeter
6
Angle A
36.87°
Angle B
53.13°
Angle C
90°
Show formulas
Use the same unit for all side lengths.
How this triangle calculator works
Enter three side lengths. The calculator checks that they can form a triangle, then calculates perimeter, area and angles from those sides.
Heron's formula and angles
Area uses Heron's formula with the semi-perimeter. Angles are estimated with the law of cosines, which can find each angle from the three side lengths.
What this calculator does not include
This is a three-sides-only triangle calculator. It does not solve ambiguous triangle cases, draw a precise diagram or replace specialist geometry software.
Key terms and assumptionsShort notes about side-side-side triangles, Heron's formula, angle estimates and unit behaviour.
- Three sides only
- This version calculates from three side lengths and does not solve other triangle cases.
- Triangle inequality
- The side lengths must satisfy the triangle inequality to form a triangle.
- Heron's formula
- Area is calculated from the semi-perimeter and three side lengths.
- Angle estimates
- Angles are estimated with the law of cosines and rounded for display.
- Unit-agnostic
- Use the same unit for all side lengths. Area is in square units of the input unit.
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FAQs
What can this triangle calculator solve?
This version solves perimeter, area and angles from three side lengths.
What is Heron's formula?
Heron's formula calculates triangle area from the three side lengths and semi-perimeter.
Can I enter only two sides?
Not in this version. Use the Pythagorean Theorem Calculator for right triangles with one missing side.
How does it find the angles?
It uses the law of cosines to estimate each angle from the three side lengths.
What if my sides do not form a triangle?
The calculator shows a clear triangle inequality warning.
Are units required?
No. Use the same unit for all side lengths; area will be in square units of that input unit.
Can I copy the result?
Yes. Copy result copies a compact triangle summary.