What is the "lodestar" calculation used by courts to determine attorneys' fees?

"District courts must calculate awards for attorneys' fees using the `lodestar' method," Ferland, 244 F.3d at 1149 n. 4, and the amount of that fee must be determined on the facts of each case, Hensley v. Eckerhart, 461 U.S. 424, 429, 103 S.Ct. 1933, 76 L.Ed.2d 40 (1983). "The `lodestar' is calculated by multiplying the number of hours the prevailing party reasonably expended on the litigation by a reasonable hourly rate." Ferland, 244 F.3d at 1149 n. 4 (citation and internal quotation marks omitted). "Although in most cases, the lodestar figure is presumptively a reasonable fee award, the district court may, if circumstances warrant, adjust the lodestar to account for other factors which are not subsumed within it." Id.