Depending on what you are seeking I have two different answers for you.
If you are simply after the area of concrete pad that exists along the ground measure length in nearest whole inches and then at 90 Degrees to the first measurement or perpendicular measure the width in nearest whole inches. Multiply each of these numbers and then divide the result by 144 (Because you used inches in the beginning you need to divide each measurement by 12 to convert back to feet, and because you had 2 measurements you do this twice resulting in 12x12 or 144) . This will give you an answer in units of feet squared or SQ FT. This method avoids problems with fractional feet and inches and the needed extra conversion steps.
This answer would be good enough for a property description of say a patio's square feet.
Second More Complex Answer
If for some reason you wanted to find smaller the concrete pad area as seen from above and knew the dimensions sloped along the ground you will have to use a little trigonometry. The explanation is lengthy for here but a good method to use would be the "pothagarean theorem" Imagine looking at the patio from the side elevation, slicing a typical section of the pad as it lays on the ground. This sloped piece of concrete will be the longest side of the right triangle. For the total amount of fall (you said 6 inches per foot, so multiply 6 inches or 0.5 if using feet by the total length to get the total fall). Convert to feet of fall if you have not already and this number will be the smallest side or end of the triangle using the pothagarean method. With 2 known sides (longest and shortest sides) of the right triangle you can solve for the shorter imaginary horizontal 0 degree sloped side. When you have this imaginary length multiply it by the width of the actual patio and you will now have the imaginary horizontal area.
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