How to calculate square footage with 4 unequal sides
To calculate the square footage of a shape with 4 unequal sides, measure one diagonal to divide the irregular quadrilateral into two triangles, calculate each triangle's area, then add them. The Uneven Sides Area Calculator applies Heron's formula when you enter the three side lengths of each triangle.
Label the four sides a, b, c, and d in order around the perimeter. Measure diagonal AC (or BD). Triangle ABC uses sides a, b, and the diagonal. Triangle ADC uses sides c, d, and the same diagonal. Add the two triangle areas for the lot total in square feet (ft²).
Why four side lengths are not enough
A quadrilateral with four fixed side lengths can still flex. Different angles produce different areas while the sides stay the same length. You also need one diagonal or the angles to lock the shape.
Field surveyors therefore record a diagonal or coordinates, not only the four boundary lengths. Entering only four sides into an area formula would invent a result the measurements cannot support.
Example: a four-sided lot with unequal sides
A lot has sides 40 ft, 30 ft, 35 ft, and 25 ft, with a measured diagonal of 45 ft between the first and third corners. Triangle 1 uses sides 40, 30, and 45. Triangle 2 uses sides 35, 25, and 45. Heron's formula on each triangle, then addition, gives the lot area.
Heron's formula: with sides x, y, and z, the semi-perimeter is s = (x + y + z) ÷ 2, and the area is √[s(s − x)(s − y)(s − z)]. Run each triangle separately in the calculator, then add the results.
What is a 4-sided shape with unequal sides called?
A four-sided shape with unequal sides is an irregular quadrilateral. If one pair of sides is parallel, the shape is a trapezoid and the trapezoid calculator applies.
For L-shaped rooms and other composite floor plans made of rectangles, use the irregular shape calculator instead of this four-side method.
Convert to square meters
To convert a four-sided lot's square footage to square meters, multiply by 0.0929. Surveyors often report both units on the same plan.
Multiply square feet by 0.0929 to get square meters (m²).
Use the square yards calculator for any area.
For square yards of land cover, divide square feet by 9, or open the square yards calculator.
Uneven sides area calculator summary
The Uneven Sides Area Calculator finds the square footage of a shape with 4 unequal sides by splitting the irregular quadrilateral along a diagonal into two triangles and adding Heron's formula results. Four side lengths alone are not enough.
If one pair of sides is parallel, use the trapezoid tool. For L-shaped rooms, use the irregular shape method hub instead.