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When ever we say Allowable bearing capacity, It can be either Gross allowable bearing capacity or net allowable bearing capacity. And it is the most important thing to decide when to add over burden, when to subtract and when not to. More over keep this equation before you for equilibrium q(all.gross) > P/A +Y.Df where q(all.gross) = gross bearing capacity Y.Df = Backfill load P/A = Applied pressure q(all.net) = q(all.gross)-Y.df = (P/A +Y.Df) - Y.Df hence for equilibrium q(all.net) > P/A where q(all.net) = net allowable bearing capacity Y.Df = Backfill load P/A = Applied pressure So gross bearing capacity is capacity available to bear pressure of existing soil as well as applied loads and net bearing capacity is capacity available to bear just applied pressures because pressure caused by existing soil has already been deducted. Case when NOT to deduct over burden Lets say in my geotechnical report i have been provided by NET bearing capacity of 1 tsf at a depth of 5 ft. Now it means at 5 ft depth, soil is capable to bear pressure of 1 tsf IN EXCESS of existing soil. Because it is NET allowable bearing pressure and pressure of existing soil has already been subtracted/balanced from bearing capacity. Now only extra available bearing capacity is reported as net. The soil can bear 1 tsf of EXTERNAL/APPLIED pressure which includes load of above structure as well as load of footing. Load of footing is also external and applied. In this case, you need not to deduct over burden because over burden is already deducted and you are provided with NET bearing capacity. But If you have bearing capacity at 5 ft depth but in return you have plan to backfill upto 7 ft, then obviously extra 2 ft soil is applied pressure. which you need to either add in applied pressure or you should deduct it from the bearing capacity. Case when to deduct over burden But if my geotech report says me that at 5 ft depth, i have 1.25 tsf GROSS allowable bearing capacity. Now it means my soil is capable to bear 1.25 tsf pressure including pressure caused by existing soil. Because it is gross allowable bearing pressure and i have not subtracted pressure of existing soil. The soil can bear 1.25 tsf of total pressure which includes EXTERNAL/APPLIED pressure caused by load of above structure including load of footing, and pressure caused by existing soil . In this case, for the sake of sizing of footing, you need to to deduct weight of over burden soil so that so may find out net bearing capacity available to resist applied load of structure and footing. Note that, after deducting the pressure caused by existing soil i.e. Y.Df = 110*5 = 550 psf = 0.25 tsf, we again get the same 1tsf bearing capacity available for external/applied loads. Case when to ADD over burden Now Let say you are given with any of capacity. Let say Net allowable capacity of 1tsf at 5 ft depth. But you know that you are not going to backfill the foundation (in case of basement etc), then NOT ONLY this 1 tsf is available for applied pressure but also that 0.25tsf available which was going to be consumed by existing soil backfill. Now you can add this 0.25tsf to your net bearing capacity to get bearing capacity, available for external loads and you need to find out size of foundation according to this available bearing capacity.