Use a calculator, conversion table (pdf) or equation to convert the power units.  Here is a short conversion table:

dBm	Watts
0	.001
15	.032
24	.250
27	.500
30	1.000
36	4.000

The equation is: 

dBm = 10 * (log (1000 * P))

P = Power in Watts

1000mW = 1 Watt

Note, doubling the wattage increases dBm by 3.  Here is a more comprehensive tutorial on Using and Understanding Decibels.