Using the Triangle Calculator
Check the triangle inequality before trusting the input: every pair of sides must add up to more than the remaining side. A very narrow triangle can be valid, but measurements close to that boundary are sensitive to rounding.
Worked example
Use the example inputs below to reproduce this result. These are the form's starting values, not recommended targets. Changing your inputs updates your result above; this worked example stays fixed for comparison. Results are rounded for display.
Example inputs
- Side a
- 3
- Side b
- 4
- Side c
- 5
Example result
- Area
- 6
- Perimeter
- 12
- Angle A (degrees)
- 36.869898
- Angle B (degrees)
- 53.130102
- Angle C (degrees)
- 90
Method and assumptions
Heron’s formula gives area; the cosine rule gives angles. Side lengths must use the same unit.
Match the measurements to the shape
Sketch the shape and label each input before calculating. A radius, diameter, perpendicular height and sloping edge can have similar-looking measurements while playing different roles in a formula. Use one consistent length unit. Area will then be in its square units and volume in its cubic units, rather than the original length unit.
Common question
Why are some sets of positive sides rejected?
Positive lengths alone do not guarantee a triangle. If one side equals or exceeds the sum of the other two, the sides cannot form a nondegenerate enclosed triangle.
Guide updated .