  We investigate the impact of Hilbert-space truncation upon the entanglement
of an initially maximally entangled $m\times m$ bipartite quantum state, after
propagation under an entanglement-preserving $n \times n$ ($n\geq m$) unitary.
Truncation -- physically enforced, e.g., by a detector's finite cross section
-- projects the state onto an $s \times s$-dimensional subspace ($3\leq s \leq
n$). For a random local unitary evolution, we obtain a simple analytical
formula that expresses the truncation-induced entanglement loss as a function
of $n$, $m$ and $s$.
