dBm / mW Converter
Convert optical and RF power between dBm, milliwatts, watts and dBW, with a reference scale of the levels you actually see on transceivers and radios.
Result
Reference scale
| dBm | Power | Where you see it |
|---|---|---|
| +30 dBm | 1 W | High-power PON OLT / RF amplifier output |
| +13 dBm | 20 mW | Typical WiFi radio transmit ceiling (regulatory) |
| 0 dBm | 1 mW | Reference point - the definition of dBm |
| -3 dBm | 0.5 mW | Half of 1 mW - every 3 dB halves the power |
| -8 dBm | 158 µW | Healthy 10G SFP+ receive level |
| -14 dBm | 39.8 µW | Comfortable single-mode receive level |
| -23 dBm | 5 µW | GPON ONU warning threshold |
| -28 dBm | 1.6 µW | GPON class B+ ONU sensitivity limit |
| -40 dBm | 0.1 µW | Below almost every optical receiver sensitivity |
The maths
- Formula (dBm to mW)
- P(mW) = 10 ^ (dBm / 10)
- Formula (mW to dBm)
- dBm = 10 × log10(P(mW))
- +3 dB
- double the power
- -3 dB
- half the power
- +10 dB
- ten times the power
About dBm / mW Converter
dBm is power on a logarithmic scale referenced to one milliwatt. It exists because optical and RF budgets are made of gains and losses that multiply - and on a log scale, multiplication becomes addition you can do in your head.
Adding and subtracting decibels correctly
Decibels add when they describe a chain of gains and losses, which is the whole convenience of the scale. They do not add when combining two independent power sources: two transmitters at 10 dBm produce 13 dBm together, not 20 dBm, because the powers sum in linear units (10 mW + 10 mW = 20 mW = 13 dBm). Convert to milliwatts, add, and convert back whenever you are combining rather than cascading.
The numbers worth memorising
0 dBm is exactly 1 mW. +3 dB doubles power, -3 dB halves it, +10 dB multiplies by ten. From those three facts you can estimate almost anything: -13 dBm is 10 dB below -3 dBm, so it is a tenth of 0.5 mW, or 50 µW. Note that dB is a ratio and dBm is an absolute level: a 3 dB splitter loss is a ratio, while -20 dBm at a receiver is a level.
Common use cases
- Reading SFP diagnostic output and deciding whether a receive level is healthy.
- Converting a radio datasheet figure in watts to the dBm your link budget uses.
- Checking how much margin a 3 dB splitter or a dirty connector actually costs.
Edge cases and gotchas
- dBm and dBW differ by exactly 30 dB - mixing them up misstates power by a factor of 1000.
- Zero power cannot be expressed in dBm; a transceiver reporting "-40 dBm" or "-inf" usually means no light at all.