A lookahead carry unit (LCU) is a logical unit in digital circuit design used to decrease calculation time in adder units and used in conjunction with carry look-ahead adders (CLAs).

4-bit adder

A single 4-bit CLA is shown below:

16-bit adder

By combining four 4-bit CLAs, a 16-bit adder can be created but additional logic is needed in the form of an LCU. The LCU accepts the group propagate (P_G) and group generate (G_G) from each of the four CLAs. P_G and G_G have the following expressions for each CLA adder: The LCU then generates the carry input for each CLA. Assume that P_i is P_G and G_i is G_G from the ith CLA then the output carry bits are Substituting C_{4} into C_{8}, then C_{8} into C_{12}, then C_{12} into C_{16} yields the expanded equations: C_{4} corresponds to the carry input into the second CLA; C_{8} to the third CLA; C_{12} to the fourth CLA; and C_{16} to overflow carry bit. In addition, the LCU can calculate its own propagate and generate:

64-bit adder

By combining 4 CLAs and an LCU together creates a 16-bit adder. Four of these units can be combined to form a 64-bit adder. An additional (second-level) LCU is needed that accepts the propagate (P_{LCU}) and generate (G_{LCU}) from each LCU and the four carry outputs generated by the second-level LCU are fed into the first-level LCUs.