How to calculate sample size using spss

#1
I want to calculate sample size of 150 water bacteria identified in river. I cannot microscopically study all so please tell me the appropriate sample size with confidence level of 95% and 5% marginal error acceptable.

What is the step wise process of calculating sample size in spss
 
#2
Your question isn't very clear. Are you asking what sample size you would need for your analysis?

It completely depends on the analysis in question, the desired effect size, and margin of error.

Edit: Also I believe to conduct a power-analysis in SPSS you need to download (maybe buy?) a bolt-on package.
 
#3
Is there a particular reason you need to use SPSS?

G*Power is a free software system which can be used to predict sample sizes needed to detect differences based on specified inputs.
http://www.gpower.hhu.de/en.html

But as Evelyn13 said, the answer depends on the analysis. If what you are measuring has only a very small difference in the population means, then your sample size has to be large in order to detect it. If the difference in population means is massive, then your sample needn't be very large (assuming fixed alpha and beta). To estimate an effect size you either need a pilot study or refer to previous research (again all depending on what it is you're doing).
 
#4
As i want to perorm independent t test using g power. with effect size of 0.5. and my total number of bacteria identified for water containation study are 150. i want to identify the sample size i should choose for microscopic testing of bacteria specimens collected from river and comparing it with bacteria specimens collected from households.
 
#5
As i want to perorm independent t test using g power. with effect size of 0.5. and my total number of bacteria identified for water containation study are 150. i want to identify the sample size i should choose for microscopic testing of bacteria specimens collected from river and comparing it with bacteria specimens collected from households.
I'm still not quite sure what you're asking. If you know the effect size then download G*Power and use it to calculate the sample size you need. Can you provide any more specific information? You have 150 bacteria, but what is your t-test on? Are you comparing two different populations of bacteria? How have you achieved 0.5 as an effect size?