Work out field area (irregular plot) instantly with clear inputs, formula shown and shareable results.
For a four-sided field measured only along its boundary, the cyclic-quadrilateral (Bretschneider/Brahmagupta) form gives the largest area those four sides can enclose: sqrt((sp-a)(sp-b)(sp-c)(sp-d)) where sp is the semi-perimeter. Real plots enclose slightly less, so treat this as an upper bound and split badly skewed fields into triangles.
Brahmagupta area
sp = (a+b+c+d)/2; Area = sqrt((sp-a)(sp-b)(sp-c)(sp-d))
The formula assumes the quadrilateral is cyclic, which maximises area for the given sides. Any other shape with the same sides encloses less, so a survey using diagonals or GPS will usually read lower.
Split it into triangles from one corner, measure each triangle's three sides and add the Heron areas, or walk the boundary with a GPS app and use the polygon area it reports.