Carry select adder is used in many data processing processors to perform fast arithmetic operations. Generally, it consists of Ripple Carry Adders and a multiplexer. It uses multiple narrow adders to create fast wide adders. This adder comprises different blocks with either uniform or variable block size. The main idea is to pre-compute the sum and carry for each block with carry-in =0 as well as carry-in=1. Therefore, two adders are required for each block except first one. After the two results are calculated, the correct sum, as well as the correct carry-out, is then selected with the multiplexer once the correct carry-in is known. This adder speeds up addition at the cost of additional hardware. This adder design is used when speed is more important than the area consumption.