  Functional error estimates are well-established tools for a posteriori error
estimation and related adaptive mesh-refinement for the finite element method
(FEM). The present work proposes a first functional error estimate for the
boundary element method (BEM). One key feature is that the derived error
estimates are independent of the BEM discretization and provide guaranteed
lower and upper bounds for the unknown error. In particular, our analysis
covers Galerkin BEM and the collocation method, what makes the approach of
particular interest for scientific computations and engineering applications.
Numerical experiments for the Laplace problem confirm the theoretical results.
