Free Space Path Loss Calculator

Free space path loss describes the reduction in radio power density as an electromagnetic wave spreads away from an ideal point source. It is not caused by air absorbing the signal. Instead, the same transmitted energy occupies an increasingly large spherical area. This calculator evaluates ideal path loss from separation and frequency, then subtracts that loss from transmitted power to estimate received power when both antenna gains and all other losses are zero. For distance in meters and frequency in hertz, the equation is twenty times the base-ten logarithm of distance plus twenty times the base-ten logarithm of frequency minus 147.55. The result is expressed in decibels. Doubling distance adds about 6.02 dB of loss, and doubling frequency adds the same amount when antenna gains are held constant. Received power in this simplified budget equals transmitted power in dBm minus path loss in dB. Because decibel quantities are logarithmic, subtraction represents division of linear power. A complete radio link budget includes transmitter antenna gain, receiver antenna gain, feed-line loss, connector loss, polarization mismatch, atmospheric absorption, rain fade, obstruction loss, and other margins. Directional antennas can offset some spreading loss through gain, but they require alignment. Real terrestrial links also encounter reflection, diffraction, scattering, foliage, buildings, terrain, and multipath fading. Free space path loss is therefore a baseline, not a guarantee of field performance. The Friis transmission relationship underlying this calculation assumes far-field propagation and unobstructed line of sight. Use positive distance and frequency values in the units shown. Frequency conversions are important: one megahertz is one million hertz, and one gigahertz is one billion hertz. Compare estimated received power with receiver sensitivity while reserving adequate fade margin for reliability. Satellite, microwave, cellular, Wi-Fi, Bluetooth, and sensor links can all use the same baseline equation, but each application needs appropriate antenna and environmental terms. Near-field coupling, guided transmission lines, and indoor empirical path-loss models require different methods. For a production wireless design, verify regulatory power limits and validate the final budget with propagation modeling and measurements.