Calculate received wireless signal strength from transmit power, distance and path loss.
dBm is a logarithmic power unit referenced to 1 milliwatt: dBm = 10·log10(power_in_mW). This logarithmic scale compresses the enormous dynamic range of wireless signal power (from nanowatts to watts) into a compact, easy-to-work-with number, and — since it's logarithmic — gains and losses along a signal path can simply be added or subtracted in dB rather than multiplied or divided, which is why link budgets are always computed in dBm/dB.
dBm to milliwatts
mW = 10^(dBm / 10)
Milliwatts to dBm
dBm = 10 × log10(mW)
Wireless signal power spans many orders of magnitude, from picowatts of noise to watts of transmit power. The logarithmic dBm scale compresses this into small, manageable numbers, and critically, lets path losses and gains be added/subtracted rather than multiplied/divided.
0 dBm equals exactly 1 milliwatt (1 mW), since dBm is defined relative to that 1 mW reference.
Because received wireless signals are almost always weaker than 1 mW after traveling through air and obstructions, so their dBm value is negative — e.g. -50 dBm (strong) is about 10 microwatts, while -90 dBm (weak) is about 1 nanowatt.
You can't add dBm values directly — convert each to mW (or W) first, add the linear values, then convert the sum back to dBm. Only gains/losses expressed in dB (not absolute power in dBm) can be added or subtracted directly.