  We present functional-type a posteriori error estimates in isogeometric
analysis. These estimates, derived on functional grounds, provide guaranteed
and sharp upper bounds of the exact error in the energy norm. {Moreover, since
these estimates do not contain any unknown/generic constants, they are fully
computable, and thus provide quantitative information on the error.} By
exploiting the properties of non-uniform rational B-splines, we present
efficient computation of these error estimates. The numerical realization and
the quality of the computed error distribution are addressed. The potential and
the limitations of the proposed approach are illustrated using several
computational examples.
