There is no mathematical formula that you can use to measure the area of an irregular surface. However, you can divide the surface into regular forms like squares, circles, triangles (forms with known formulas for its area) and then calculate the area of the whole surface by summing the areas of the smaller pieces.
In this matter, you can also use a tool like METRIC PE Plus. This product uses the same technique as above, but does this automatically, to calculate the area of a surface. However, you will need to upload a picture / image of the surface in order to use the application.