Quick answer
The area of a triangle is A = ½ × base × height. A base of 10 and height of 4 give an area of 20.
Enter the base and the perpendicular height. The calculator returns the area.
Related tools: Right Triangle Area Calculator: Area of a right triangle from its legs. Area Calculator: Area of common two-dimensional shapes. Triangle Calculator: Area, perimeter, angles and type of a triangle from its three sides.
How to use the Triangle Area Calculator
- Enter base (above 0).
- Enter height (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Base | A number | > 0 |
| Height | A number | > 0 |
Formula and method
Formula
A = ½ b h
IEEE-754 double precision; shown to at most 6 decimal places.
Half the product of base and perpendicular height.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Base = 10; Height = 4.
| Result | Value |
|---|---|
| Area | 20 |
Example 2
Inputs: Base = 3.5; Height = 2.
| Result | Value |
|---|---|
| Area | 3.5 |
The first line is the headline result (area). IEEE-754 double precision; shown to at most 6 decimal places.
Half a parallelogram
Two identical triangles form a parallelogram of 10 × 4 = 40, so each is 20. The height is perpendicular to the base, not along a slanted side.
Other methods
With three sides use Heron's formula; with two sides and the included angle use ½ab sin C; with coordinates use the shoelace formula. All values must be positive.
Choosing the right formula
Base × height ÷ 2 works for any triangle when the height is perpendicular to the base: 10 × 4 ÷ 2 = 20. For a right triangle with legs 6 and 8 the legs are base and height, so the area is 24.
Keep units consistent. A base of 36 in and a height of 24 in give 432 in², which is 3 ft² because 1 ft² = 144 in².
How the result changes with base
Holding every other input at the first worked example, this table changes only base. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Base | Area |
|---|---|
| 5 | 10 |
| 10 | 20 |
| 20 | 40 |
| 40 | 80 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Base: entering "0" shows "Base must be greater than 0." beside that field and no result.
Choosing between the Triangle Area Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Right Triangle Area Calculator](/math/right-triangle-area/) | leg a and leg b | Area |
| [Area Calculator](/math/area-calculator/) | shape and a | Area |
| [Triangle Calculator](/math/triangle-calculator/) | side a, side b and side c | Area |
| [45-45-90 Triangle Calculator](/math/45-45-90-triangle-calculator/) | leg length | Leg |
| [30-60-90 Triangle Calculator](/math/30-60-90-triangle-calculator/) | short leg length | Short leg |
Background: geometry measurements
Geometry results carry units that depend on the quantity: a perimeter or length is in linear units such as cm, an area or surface area is in square units (cm²) and a volume is in cubic units (cm³). Use one length unit for every input, because the calculator multiplies the numbers as given.
Flat (2D) shapes have an area and a perimeter; solids (3D) have a surface area and a volume. Two familiar checks: a circle of radius r has area πr² and circumference 2πr, and a sphere has volume (4/3)πr³ and surface area 4πr².
Assumptions and limitations
- Positive values
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for base to confirm this.
Frequently asked questions
What is the area of a triangle with base 6 and height 5?
Base 6 and height 5 give an area of ½ × 6 × 5 = 15.
Does the shape of the triangle matter?
Only base and height.
What if I know three sides?
Use the Triangle calculator.
What is the area of a right triangle with legs 6 and 8?
The legs are perpendicular, so the area is 6 × 8 ÷ 2 = 24.
How do I convert a triangle area from square inches to square feet?
Divide by 144. An area of 432 in² is 3 ft².
What does the Triangle Area Calculator calculate?
Area of a triangle from base and height. It returns Area.
What formula does the Triangle Area Calculator use?
A = ½ b h. Half the product of base and perpendicular height.
What do I need to enter in the triangle Area Calculator?
You enter Base and Height.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Positive values.
What is an example of a triangle Area result?
With base 10 and height 4, the area is 20.
Related calculators
- Right Triangle Area Calculator — Area of a right triangle from its legs.
- Area Calculator — Area of common two-dimensional shapes.
- Triangle Calculator — Area, perimeter, angles and type of a triangle from its three sides.
- 45-45-90 Triangle Calculator — Sides of a 45°-45°-90° triangle from one leg.
- 30-60-90 Triangle Calculator — Sides of a 30°-60°-90° triangle from the short leg.
- Right Triangle Calculator — Solve a right triangle from its two legs: hypotenuse, angles, area and perimeter.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.