We present a method for absolute calibration of received radio frequency in the ultra-low frequency and very low frequency range. This is achieved with the use of a radio frequency optically pumped magnetometer. We describe a method using an optically pumped sample where the RF broadening of the Cs magnetic resonance allows the magnitude of the received field to be calibrated against the ground-state gyromagnetic ratio of the Cs atoms. This frequency-based calibration avoids the geometric and electrostatic response functions that affect inductive sensors, such as fluxgates, search coils, and SQUID magnetometers. We demonstrate the calibration of magnetic measurement using oscillating magnetic fields in the 300 Hz–20 kHz range and a sensor noise floor of 15 fT Hz−1/2. This radio frequency sensor may be used as a widely tunable narrowband receiver for communication, ranging, or penetrative conductivity imaging.