Extinction, scattering, and absorption cross sections and efficiencies were experimentally measured for chemically clean silver nanoparticles in water for 16 different sizes ranging from 29 to 136 nm. The measured efficiencies indicate that particles interact with light 4-10 times stronger than the geometric cross section suggests. Absorption and scattering components of the plasmon resonance were separately measured across the visible spectral range. A method, termed standard subtraction, is suggested for the simple and reliable determination of particle concentrations independent of their size, shape, and aggregation state.