(Reuters) - Delta Air Lines Inc <DAL.N> said its revenue for August was reduced by about $100 million due to an outage that led to the cancellation of 2,300 flights over three days.

Passenger revenue per available seat mile (PRASM), a key measure of an airline's performance that measures sales against flight capacity, declined 9.5 percent in August from a year earlier, its biggest drop in nine months. [nPn9p2d6Ra]

Excluding the impact of the outage, Delta's unit revenue was below expectations, J.P.Morgan's Jamie Baker wrote in a note.

Baker said he expects Delta to cut its third-quarter operating margin to at least 17-19 percent from its current forecast of 19-21 percent.

The No. 2 U.S. airline by passenger traffic said last month a power outage hit its computer systems, leading to the flight cancellations. [nL3N1AR3TH]

Delta in July forecast a drop of 4-6 percent in PRASM for the third quarter ending September. [nL1N1A00B2]

(Reporting by Arunima Banerjee in Bengaluru; Editing by Maju Samuel)