Rule of the Floating Subtrahend Law and Legal Definition

Rule of the floating subtrahend is a common law doctrine of tort law that a plaintiff whose damage was not caused entirely by the defendant must prove the amount of damage that is not attributable to the defendant (the subtrahend) or else recover nothing.

The following is an example of a case law defining the Rule of the floating subtrahend:

Where a plaintiff has shown the gross amount of expense or loss, but where defendant is not liable (by substantive law) for all of the loss, or where it appears that certain credits or deductions should be made against the total expense some courts are strict require the plaintiff to prove affirmatively the amount that should be subtracted, before he can recover anything on account of the loss or expense in question. This is known as the rule of the floating subtrahend. [Crescent Towing & Salvage Co. v. M/V Chios Beauty, 2008 U.S. Dist. LEXIS 62247, 40-41 (E.D. La. 2008)]