  We obtain ADM and Komar surface integrals for the energy density, tension and
angular momentum density of stationary $p$-brane solutions of Einstein's
equations. We use them to derive a Smarr-type formula for the energy density
and thence a first law of black brane mechanics. The intensive variable
conjugate to the worldspace p-volume is an `effective' tension that equals the
ADM tension for uncharged branes, but vanishes for isotropic boost-invariant
charged branes.
