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.