# Estimating Sample Size with no Standard Deviation

I am conducting a one-sided, independent two-sample t-test for a gender study and I can guesstimate my effect size from previous studies. I want 95% significance, 80% power. I am looking to calculate appropriate sample size and realise I need the standard deviation to do this.

Is there any way of approximating the standard deviation from the effect size? My lecturer said doubling the effect size provides a rough approximation of an appropriate standard deviation. Is this correct? Why is this the case??

I ask this clarification, because many effect sizes are measured in standard deviation units. In this case, you can treat everything as $z$-scores, and assume the standard deviation is one.