Procter & Gamble Co. and Kimberly-Clark Corp., the two largest U.S. makers of diapers, said they will raise prices by an average of 6% in mid-September to offset higher costs. Between them, the two companies make 80% of the disposable diapers sold in the U.S. Both will reduce the number of diapers included in each package by an average 13% while cutting prices just 7%. That works out to an effective 6% increase on Kimberly-Clark's top-selling Huggies and Procter & Gamble's Pampers and Luvs. Paper pulp prices have risen by more than a third in the last year, prompting manufacturers to raise prices earlier this year on bathroom tissue and paper towels.