Calculators
Triangle Area Calculator
From base and height.
About the Triangle Area Calculator
The Triangle Calculator finds the area of any triangle from its base and height, its three sides, or two sides and the angle between them. With sides it also gives the perimeter, all three angles and the type of triangle, and draws it to scale.
What you can do with the Triangle Area Calculator
- Three ways in: Base and height, three sides, or two sides and an angle.
- Angles and type: All three angles and whether it's acute, right or obtuse.
- Perimeter: Worked out when you enter sides.
- Drawn to scale: A diagram of your exact triangle.
The height must be perpendicular
The height is the straight-line distance from the base to the opposite corner, at a right angle to the base, not the length of a sloping side. The dashed line in the diagram shows it.
If you only know the sides
Heron's formula calculates area from three sides: find s, half the perimeter, then area = √(s × (s − a) × (s − b) × (s − c)). A triangle with sides 3, 4 and 5 has an area of 6. Any two sides must add up to more than the third, or the sides can't meet.
Types of triangle
By angles, a triangle is acute (all under 90°), right-angled or obtuse (one over 90°). By sides it's equilateral (all equal), isosceles (two equal) or scalene (none equal). The calculator names both for you.
Related tools
Frequently Asked Questions
Can I use any side as the base?
Yes, as long as the height you enter is measured perpendicular to that side.
How do I find the area from three sides?
Choose "Three sides" and type them in, Heron's formula does the rest.
What's the area of a right-angled triangle?
Half of the two shorter sides multiplied together: sides 6 and 8 give an area of 24.
How do I find the area with two sides and the angle between them?
Choose "2 sides + angle" and enter both sides and the angle in degrees between them. The formula is area = ½ × a × b × sin(angle). With sides 7 and 9 and a 60° angle, the area is about 27.28 and the third side is about 8.19.
Why does it say my sides can't make a triangle?
Any two sides must add up to more than the third side, which is called the triangle inequality. Sides of 3, 4 and 50 fail because 3 + 4 is far less than 50. Check for a typing mistake, or a side measured in a different unit.
How do I find the area of an equilateral triangle?
Use area = (√3 ÷ 4) × side², or about 0.433 × side². A triangle with all sides 6 cm has an area of about 15.59 cm². In the tool, pick "Three sides" and enter the same number three times to get the area, perimeter and 60° angles.
How do I find the height of a triangle if I know the area?
Rearrange the formula: height = 2 × area ÷ base. A triangle with an area of 30 cm² on a 10 cm base has a height of 6 cm. The height must be measured at right angles to the base, not along a sloping side.
How do I find the area of a triangular plot of land?
Measure all three sides in feet or metres and choose "Three sides"; the tool uses Heron's formula. A plot with sides of 30, 40 and 50 ft has an area of 600 sq ft. For irregular plots, split the land into triangles and add their areas.
How can I tell if a triangle is acute, right or obtuse?
Look at its largest angle: under 90° is acute, exactly 90° is right-angled, and over 90° is obtuse. The three angles always add up to 180°. When you enter sides, the tool works out all three angles and tells you which type it is.
What should I enter for the angle?
Enter the angle in degrees between the two sides you typed, not an angle opposite one of them. It must be more than 0° and less than 180°; the tool warns you if it is 180° or more. For a right angle, enter 90.
Why is my answer different from the textbook's?
Usually the base and height don't match: the height must be the perpendicular distance from the base to the opposite corner, not a slanted side. Answers can also differ slightly if the book rounds square roots or uses rounded angle values, since this tool keeps full precision.
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