We obtain ADM and Komar surface integrals for the energy density, tension and angular momentum density of stationary p-brane solutions of Einsteinʼs 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.