This paper presents a method for weighing biomolecules, single cells, and single nanoparticles in fluid using suspended microchannel resonators. These resonators are highly sensitive to mass changes and can measure masses as small as sub-femtograms (1 fg = 10^-15 g) in water. The key to this sensitivity is the low mass of the resonator (100 ng) and high quality factor (15,000), which allows for a mass resolution six orders of magnitude better than commercial quartz crystal microbalances. The resonators are fabricated in silicon-on-insulator wafers and are vacuum-packaged to minimize viscous damping from the fluid. The resonators can measure mass changes in two modes: one where the mass of molecules or particles adsorbed to the surface is measured, and another where particles flow through the resonator, and their mass and position are determined by the frequency shift. The frequency shift is calculated using the equation f = 1/(2π)√(k/(m* + αΔm)), where k is the spring constant, m* is the effective mass, and α is a constant that depends on the localization of the added mass. The resonators have been used to measure the binding of proteins to antibodies on the surface, the mass of individual bacteria and nanoparticles, and the mass of single particles in transit. The method allows for high-resolution mass measurements with sub-femtogram sensitivity and has potential applications in mass-based flow cytometry, pathogen detection, and colloidal particle sizing. The resonators are also suitable for applications requiring minimal reagent consumption and high capture efficiency. Future developments could enable the detection of specific cells in a manner similar to flow cytometry, with potential applications in disease monitoring and pathogen detection.This paper presents a method for weighing biomolecules, single cells, and single nanoparticles in fluid using suspended microchannel resonators. These resonators are highly sensitive to mass changes and can measure masses as small as sub-femtograms (1 fg = 10^-15 g) in water. The key to this sensitivity is the low mass of the resonator (100 ng) and high quality factor (15,000), which allows for a mass resolution six orders of magnitude better than commercial quartz crystal microbalances. The resonators are fabricated in silicon-on-insulator wafers and are vacuum-packaged to minimize viscous damping from the fluid. The resonators can measure mass changes in two modes: one where the mass of molecules or particles adsorbed to the surface is measured, and another where particles flow through the resonator, and their mass and position are determined by the frequency shift. The frequency shift is calculated using the equation f = 1/(2π)√(k/(m* + αΔm)), where k is the spring constant, m* is the effective mass, and α is a constant that depends on the localization of the added mass. The resonators have been used to measure the binding of proteins to antibodies on the surface, the mass of individual bacteria and nanoparticles, and the mass of single particles in transit. The method allows for high-resolution mass measurements with sub-femtogram sensitivity and has potential applications in mass-based flow cytometry, pathogen detection, and colloidal particle sizing. The resonators are also suitable for applications requiring minimal reagent consumption and high capture efficiency. Future developments could enable the detection of specific cells in a manner similar to flow cytometry, with potential applications in disease monitoring and pathogen detection.