How to use the Triangle Area Calculator
Start by choosing the measurements you have. The Triangle Area Calculator accepts a base and perpendicular Height, three side lengths, or three coordinate points. Select a Unit, enter the values, and select Calculate. The result includes the formula steps; Three sides also shows Perimeter and Semi-perimeter.
- Choose Base & height, Three sides, or Coordinates.
- Enter the fields shown for that method: Base and Height, Side a through Side c, or x1 through y3.
- Select a Unit for every length or coordinate.
- Select Calculate.
- Read Result and Formula & steps. For Three sides, check Perimeter and Semi-perimeter too.
- Select Copy result or Download result (.txt). Select Reset to clear the values and result.
Triangle area from base and height
Choose Base & height when you know one side and the perpendicular distance to the opposite vertex. Base and Height must each be greater than zero. The calculator applies A = (1/2) × base × height.
Base: 10 m
Height: 6 m
Area: 30 m²Triangle area from three sides
Choose Three sides when Side a, Side b, and Side c are known. The calculator first finds the semi-perimeter with s = (a + b + c) / 2, then uses Heron's formula, A = sqrt(s(s-a)(s-b)(s-c)). The Result also includes Perimeter.
Each side must be shorter than the sum of the other two. A value equal to or greater than that sum is rejected, so a zero-area or impossible triangle does not produce a result.
Triangle area from coordinates
Choose Coordinates for three vertex points. Enter x1, y1, x2, y2, x3, and y3; coordinates may be positive, negative, or zero. The shoelace formula uses the points in the order entered: A = |x1(y2-y3) + x2(y3-y1) + x3(y1-y2)| / 2.
Use one Unit for all six values. The area uses the squared unit, such as m² or ft². If you change the method, a value, or Unit after calculating, the old result is hidden until you select Calculate again.
When triangle inputs are rejected
Blank and non-numeric entries produce an error. Base, Height, and the three side lengths must be greater than zero. Coordinate values may be zero or negative. An absolute input value above 1e12 is rejected, and three collinear points are rejected because their area is zero.
The arithmetic and result text are generated locally in the browser from the entered values. Copy result sends the displayed text to the clipboard. Download result (.txt) creates a text file in the browser.
Choosing a method for measured values
Base and Height fits a geometry exercise with a perpendicular measurement. Coordinates fit vertices recorded on a 2D grid. Three sides fits a measured object when no perpendicular Height was recorded.
For a missing side of a right triangle, use the Pythagorean Theorem Calculator instead.
When another math calculator fits
Use the Polygon Area Calculator for shapes with more than three sides or for a regular-polygon calculation. Use the Geometry Calculator for slope, distance, midpoint, circle measurements, or common solid measurements. Other equation and number tools are listed under Math Calculators for Equations, Geometry & More.
Frequently asked questions
How do I calculate triangle area from base and height?
Enter positive values in Base and Height, then select Calculate. The calculator applies A = (1/2) × base × height and shows the result in the squared Unit.
Can I calculate triangle area from three sides?
Yes. Choose Three sides, enter Side a, Side b, and Side c, and the calculator uses Heron's formula after checking the triangle inequality.
How do I find triangle area from coordinates?
Choose Coordinates and enter x1, y1, x2, y2, x3, and y3. The calculator applies the shoelace formula and rejects the input if the three points are collinear.
Why are my triangle measurements rejected?
Blank, non-numeric, non-positive length values, oversized values, invalid side combinations, and collinear coordinate points are rejected. Coordinate values may be zero or negative, while lengths must be greater than zero.