Paired t-test
Find sample size:

If you know the mean difference and its standard deviation, use this form to find the number of subjects you need. 		Find effect size:

If you know the number of subjects and the standard deviation of the change, use this form to find how small a difference you can detect.
Mean difference:
Standard deviation: *
Click here for sample size:

You will need subjects

Number of subjects:
Standard deviation: *
Click here for effect size:

You can show a difference of size

* If you don't know the SD of the difference
For different power or significance level, change the fields below:
Alpha: Prob(reject H0 when H0 is true)
Power: Prob(reject H0 when H1 is true)
Choose another study design

Contact: sfh2@columbia.edu
Source: G.W. Snedecor & W.G. Cochran
Statistical Methods (7th ed., p. 104)