Irregular Plot Area Calculator (4 sides + diagonal)
Find the area of a plot with four unequal sides by measuring the four sides and one diagonal. The surveyor’s method.
Quick answer
Sides 50, 40, 55, 42 ft, diagonal 65 ft: 2,145.87 sq ft.
How it is calculated
- The diagonal splits the plot into two triangles: A-B-diagonal and C-D-diagonal.
- Each triangle’s area by Heron’s formula: √(s(s−a)(s−b)(s−c)), with s = half the perimeter.
- Add both triangles to get the plot area.
Worked example
Sides A = 50, B = 40, C = 55, D = 42 ft and diagonal 65 ft. Area 2,145.87 sq ft = 238.43 gaj = 199.36 m².
| Side A | 50 ft |
| Side B | 40 ft |
| Side C | 55 ft |
| Side D | 42 ft |
| Diagonal (D–A corner to B–C corner) | 65 ft |
| Area | 2,145.87 sq ft |
| Square gaj | 238.43 gaj |
| Square metre | 199.36 m² |
Formula
Heron's formula, triangle method used by surveyors
ar = heron(a,b,dg)+heron(c,d,dg) Area = ar Square gaj = ar/9 Square metre = ar/10.7639
Good to know
- Measure the diagonal from the corner where D meets A to the corner where B meets C.
- Averaging opposite sides gives a wrong answer for irregular plots.
Questions people ask
How do I find the area of an irregular plot?
Measure one diagonal, split it into two triangles and add them with Heron’s formula.
Why is the diagonal needed?
Four sides alone do not fix the shape; the diagonal does.
The result shows “—”?
The sides and diagonal cannot form triangles; recheck the measurements.
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Updated 13 Nov 2026 · Desktop N Calculator v1.2.0