Mass sensing has so far rather developed along the resolution axis, reaching atomic-scale detection, than into the direction of high speed. This letter reports on a novel self-calibrating technique that makes high-speed inertial mass sensors capable of instant high-resolution detection and weighing. The sensing nanoelectromechanical resonator is embedded into a phase-locked loop and the sensor-inherent nonlinear phase-frequency relation is exploited for autocalibration.