The online calculator below calculates the area of a rectangle, given coordinates of its vertices. In fact, the calculation is quite generic, so it can also calculate the area of parallelogram, square, rhombus, trapezoid, kite, etc., that is, the area of any convex quadrilateral.
Enter the set of points into the calculator below - one point per line, with x and y coordinates separated by a semicolon. The calculator builds a convex hull, using Jarvis march, then calculates four sides of the hull and one diagonal between the hull's first and third points. Then the calculation is trivial. Here, for example, a quadrilateral is split by diagonal into two triangles, and the area of each triangle then calculated using Hero's formula.