dBm to mW converter

Convert optical power between dBm and milliwatts, turn a loss in dB into the share of power that remains, and add several powers together the right way — in milliwatts.

Power: dBm and milliwatts

Power

In microwatts: —µW
In watts: —W

Loss and the power that remains

Loss

Power remaining

Power lost: —%
Power falls by a factor of: —×

Sum of optical powers

Powers, dBm One value per line, or separated by spaces or semicolons — for example, the channels of a WDM system.

Values: —
Total power: —dBm
Total power: —mW

Formulas

Conversion	Formula
mW to dBm	P [dBm] = 10 · log₁₀ (P [mW] / 1 mW)
dBm to mW	P [mW] = 10^(P [dBm] / 10)
Loss in dB to the power that remains	P_out / P_in = 10^(−loss / 10)
Sum of powers	P_total [dBm] = 10 · log₁₀ Σ 10^(P_i / 10)

0 dBm is 1 mW. Every 3 dB halves or doubles the power, 10 dB is a factor of ten, and 1 dB is about 26 %.

Quick reference

dBm	mW	dBm	mW
+10	10	−10	0.1
+3	2.0	−13	0.05
0	1.0	−20	0.01
−3	0.5	−30	0.001
−7	0.2	−40	0.0001

Adding powers

Powers add in milliwatts, never in dBm: two DWDM channels at −3 dBm each total 0 dBm, not −6 dBm, and N equal channels add 10 · log₁₀ N dB to the power of one. A loss, on the other hand, is a ratio and is subtracted in dB: −8.2 dBm through 6.2 dB of loss arrives at −14.4 dBm (0.036 mW).

More in the documentation: Units and conversions, Formulas and worked calculations.