  We evaluate the entanglement entropy of a single connected region in excited
states of one-dimensional massive free theories with finite numbers of
particles, in the limit of large volume and region length. For this purpose, we
use finite-volume form factor expansions of branch-point twist field two-point
functions. We find that the additive contribution to the entanglement due to
the presence of particles has a simple "qubit" interpretation, and is largely
independent of momenta: it only depends on the numbers of groups of particles
with equal momenta. We conjecture that at large momenta, the same result holds
for any volume and region lengths, including at small scales. We provide
accurate numerical verifications.
