The rule is, 10 times the power - will give you a 10db increase. Here is a handy summary table which also lists the change in voltage and speaker excursion for each change in level:

Increases in Power / Voltage / Decibels:

1.26 x power (watts) = 1.12 x voltage/excursion = +1dB 1.59 x power (watts) = 1.26 x voltage/excursion = +2dB 2.00 x power (watts) = 1.41 x voltage/excursion = +3dB 2.52 x power (watts) = 1.59 x voltage/excursion = +4dB 3.18 x power (watts) = 1.78 x voltage/excursion = +5dB 4.00 x power (watts) = 2.00 x voltage/excursion = +6dB 5.04 x power (watts) = 2.24 x voltage/excursion = +7dB 6.35 x power (watts) = 2.52 x voltage/excursion = +8dB 8.00 x power (watts) = 2.83 x voltage/excursion = +9dB 10.0 x power (watts) = 3.16 x voltage/excursion = +10dB

Decreases in Power / Voltage / Decibels:

0.79 x power (watts) = 0.89 x voltage/excursion = –1dB 0.63 x power (watts) = 0.79 x voltage/excursion = –2dB 0.50 x power (watts) = 0.71 x voltage/excursion = –3dB 0.40 x power (watts) = 0.63 x voltage/excursion = –4dB 0.31 x power (watts) = 0.56 x voltage/excursion = –5dB 0.25 x power (watts) = 0.50 x voltage/excursion = –6dB 0.20 x power (watts) = 0.45 x voltage/excursion = –7dB 0.16 x power (watts) = 0.40 x voltage/excursion = –8dB 0.13 x power (watts) = 0.35 x voltage/excursion = –9dB 0.10 x power (watts) = 0.32 x voltage/excursion = –10dB

As mentioned above, a +10dB increase in SPL is considered "twice as loud" and to do so power will need to increase by ten times.