The key things to remember:

• dB is a ratio
• m in dBm stands for milliwatt
• W in dBW stands for Watt
• 0dB is 1x of anything
• 3dB is x2 of anything
• 10dB is x10 of anything
• dB can be added and subtracted freely and in any order

so…

• 20dB is x100 (x 10 x 10) of anything, 30dB is x1000 (x10 x10 x10) of anything
• 0dBW = 30dBm