For a one sample z-test, Cohen’s *d* is a measure of the effect size of the test. It is calculated by using the following formula:

**Cohen’s d** : (x – µ) / σ

where x is the sample mean, µ is the population mean, and σ is the population standard deviation.

To find Cohen’s *d* for a given sample mean, population mean, and population standard deviation, simply enter the appropriate values in the boxes below and then click the “Calculate” button.

Sample mean (x) | |

Population mean (µ) | |

Population standard deviation (σ) |

Rules of thumb for interpreting Cohen’s *d* :

d = 0.2 : small effect

d = 0.5 : medium effect

d = 0.8 : large effect