How to calculate an irregular 4-sided plot
An irregular four-sided plot does not have to be a rectangle, square, trapezoid, or parallelogram. To calculate its area accurately, measure all four boundary sides and one diagonal connecting opposite corners.
- Measure all four boundary sides.
- Measure one diagonal across the plot.
- Enter all measurements in the same unit.
- The calculator divides the quadrilateral into two triangles.
- Each triangle is calculated using Heron's formula and the two areas are added together.
Formula
The irregular four-sided plot is divided into two triangles using diagonal AC. The area of each triangle is calculated using Heron's formula.
Step 1: Calculate the semi-perimeter
s = (a + b + c) / 2
Step 2: Calculate each triangle's area
Triangle Area = √[s(s − a)(s − b)(s − c)]
Step 3: Add both triangle areas
Total Plot Area = Area of Triangle ABC + Area of Triangle ACD
Here, the diagonal AC is shared by both triangles. The first triangle uses sides AB, BC, and AC. The second triangle uses sides AC, CD, and DA.
Why a diagonal is required
Four side lengths alone are generally not enough to determine the exact area of an irregular quadrilateral. The measured diagonal fixes the two triangles used for the calculation.
Example calculation
Suppose the four sides are 3 m, 4 m, 5 m, and 6 m, and the measured diagonal is 5 m. The plot is divided into triangles with sides 3-4-5 and 5-5-6. Their areas are 6 m² and 12 m², giving a total plot area of 18 m².
Important measurement note
Use reliable boundary measurements and a properly measured diagonal. This calculator is intended for a simple, non-self-intersecting four-sided plot. For legal boundaries, land records, registration, or disputed property measurements, use measurements from a qualified survey professional.
Frequently asked questions
Can I calculate the area with only four sides?
No. For a general irregular quadrilateral, four side lengths alone do not uniquely determine the area. One measured diagonal is also required.
Which diagonal should I measure?
Measure the diagonal connecting the two opposite corners used to divide the plot into the two triangles represented by the four side inputs.
Can I use feet instead of metres?
Yes. Enter all four sides and the diagonal using the same length unit, then select the desired area unit for the result.