Given a spacelike 2-surface Σ in a spacetime N and a constant future timelike unit vector T0 in , we derive upper and lower estimates of Wang–Yau quasilocal energy E(Σ, X, T0) for a given isometric embedding X of Σ into a flat 3-slice in . The quantity E(Σ, X, T0) itself depends on the choice of X; however, the infimum of E(Σ, X, T0) over T0 does not. In particular, when Σ bounds a compact domain Ω in a time symmetric 3-slice in N and has nonnegative Brown–York quasilocal mass , our estimates show that equals . We also study the spatial limit of , where Sr is a large coordinate sphere in a fixed end of an asymptotically flat initial data set (M, g, p) and Xr is an isometric embedding of Sr into . We show that if (M, g, p) has future timelike ADM energy–momentum, then equals the ADM mass of (M, g, p).