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This online calculator which calculates area of a polygon given lengths of polygon sides and diagonals which split polygon to nonoverlapping triangles.
As shown on picture, to calculate area of the polygon ABCDE, you can sum areas of triangles ABD, BCD and ADE. To do this you need lengths of diagonals: BD and AD, but this is all you need. The thing is that you can calculate area of any triangle given only its side lengths.
This is useful for apartments renovation, for example, cause at home, it is easier to measure lengths than to measure angles.
So, to calculate area of the polygon, you just measure side lengths, put them into table below, then split polygon to nonoverlapping triangles, measure needed diagonals, put them into table, and you're done.
Calculator computes total area and also outputs all triangles it is used for calculation. "Error" field gives you a hint which vertex still aren't part of any triangle.
The input table is filled with data for the picture, to clear it simply press "Clear table" button.