In this paper we introduce the notion of the fractional weak discrepancy of a poset, building on previous work on weak discrepancy in J.G. Gimbel and A.N. Trenk, On the weakness of an ordered set, SIAM J. Discrete Math. 11 (1998) 655–663; P.J. Tanenbaum, A.N. Trenk, P.C. Fishburn, Linear discrepancy and weak discrepancy of partially ordered sets, ORDER 18 (2001) 201–225; A.N. Trenk, On k-weak orders: recognition and a tolerance result, Discrete Math. 181 (1998) 223–237. The fractional weak discrepancy wd F ( P ) of a poset P = ( V , ≺ ) is the minimum nonnegative k for which there exists a function f : V → R satisfying (1) if a ≺ b then f ( a ) + 1 ⩽ f ( b ) and (2) if a ∥ b then | f ( a ) - f ( b ) | ⩽ k . We formulate the fractional weak discrepancy problem as a linear program and show how its solution can also be used to calculate the (integral) weak discrepancy. We interpret the dual linear program as a circulation problem in a related directed graph and use this to give a structural characterization of the fractional weak discrepancy of a poset.