It depends on the the physical property you're measuring.
For power/intensity ratio, 10dB is 10x; for amplitude ratio, 20dB is 10x.
Loudness is typically measured by "sound pressure level" (dB SPL), which uses the latter (amplitude ratio).
Also, it's worth pointing out that while sound perception (like almost all the perceptions people have, Stevens's power law [1]) is logarithmic, its base isn't exactly 10 or 100. [2] claims that every 20dB (10x) in SPL, the perceived loudness is 4x.
