confidence levels

confidence levels:

90% : 1,64
95% : 1,96
98% : 2,33
99% : 2,58

99.9% : ???

here's one:

sample size: 16
standard deviation: 5
average: unkown
confidence interval: 4.35 <-> 9,65

question: what confidence level was used? or rather how do I calculate that?