Use this calculator when you want the clean free-space received-power estimate tied directly to distance and frequency.
Inputs
This topic also has a deeper guide and a printable reference pack, so you can move from the live answer into the method, assumptions, and worked examples without leaving the topic cluster.
These are the main values the calculator uses. Keep the units consistent and, where relevant, match the assumptions explained in the related guide.
Unit: dBm
Use the transmitter output power in dBm before path loss is applied.
Unit: dBi
Enter antenna gain relative to isotropic for the transmit side.
Unit: dBi
Enter the receive-side antenna gain in the same dBi convention.
Unit: km
Use the free-space path length in kilometres.
Unit: MHz
Enter the operating frequency in megahertz to match the free-space model.
Use this page when you want a received-power estimate from the geometry and frequency of a free-space path rather than from a separately prepared path-loss number.
The main result is received power in dBm. The supporting figure shows the free-space path-loss term used to reach that result.
If transmitter power and antenna gains are known, Friis shows how distance and frequency shape the free-space received level before other real-world losses are added.
This page assumes free-space propagation. It does not model clutter, feeder losses, fading, noise floor, or receiver performance beyond the clean receive estimate.
Combine transmitter power, antenna gains, path loss, and miscellaneous losses to estimate received power.
Estimate the line-of-sight path loss between two points from the distance and frequency you enter.
Use the Watts to dBm Calculator for quick watts to dbm estimates in RF, radar, and communications work.
Use the dBm to Watts Calculator for quick dbm to watts estimates in RF, radar, and communications work.