(Adds estimates, details)
April 29 (Reuters) - U.S. refiner Phillips 66 reported a weaker-than-expected quarterly profit, hurt by lower margins for gasoline and other refined products.
"Crack spreads" - the difference between the cost of crude and refined products often used to estimate refining margins - have slumped as output exceeds demand, causing inventories to swell.
Earnings in Phillips 66's refining business plunged 84 percent in the first quarter.
Rival Marathon Petroleum Corp barely eked out a profit in the first quarter, hurt by weak crack spreads.
Phillips 66's consolidated earnings fell to $385 million, or 72 cents per share, in the quarter, from $987 million, or $1.79 per share, a year earlier.
Adjusted earnings were 67 cents per share, widely below the average analyst estimate of 87 cents, according to Thomson Reuters I/B/E/S.
The company's shares were down 1.3 percent at $86.67 in premarket trading on Friday.
Up to Thursday's close of $87.79, the stock had risen 7 percent so far this year.
(Reporting by Swetha Gopinath in Bengaluru; Editing by Maju Samuel)