Free tool
dBm, mW & dB Converter
Convert optical and RF power between dBm and milliwatts, and work out gain or loss in dB between two power levels.
Why decibels are worth getting comfortable with
dBm is an absolute power level referenced to one milliwatt, so 0 dBm is exactly 1 mW. dB on its own is a ratio, which is why a cable has a loss in dB and a laser has an output in dBm. Mixing the two up is the most common error in optical troubleshooting.
The useful shortcuts: every 3 dB doubles or halves the power, and every 10 dB multiplies or divides it by ten. So minus 3 dB is half the light, minus 10 dB is a tenth, and minus 20 dB is one hundredth. Once those are in your head you can estimate a link in your head on site.
The margin is what matters in practice. Received power above the receiver sensitivity means the link works, and the gap between them is your margin for ageing, dirt and the repair splice somebody will add next year. Under about 3 dB of margin, the link is working today and will fail eventually.