Numerous websites now allow for easy calculation of the power of a statistical hypothesis test, given certain sample size(s); or calculation of sample size(s) required to reach a certain level of power. Below you will find a few free sites that will prove helpful.
We have avoided listing any commercial sites. Some of those sites are convenient to use, and their “calculators” are usually free, but the sites also try to market their services for analytic or consulting work. If you want to check on some of them, just enter “power calculations for statistics” into any Internet search engine.
These first two sites are simple to use for the most common statistical tests.http://stat.ubc.ca/~rollin/stats/ssize/index.html
This site, from the Campbell Collaboration, has calculators for effect size in numerous designs. www.campbellcollaboration.org/effect-size-calculato.html
The following site, at the Center for Bioinformatics and Molecular Biostatistics, University of California-San Francisco, lists many sites available for calculating power: www.biostat.ucsf.edu/sampsize.html
is a free, downloadable module that allows many types of power calculations. www.psycho.uni-duesseldorf.de/abteilungen/aap/gpower3/