Free tool
Effect size calculator (Cohen's d)
From two groups' means, SDs and sample sizes: Hedges' g with its 95% confidence interval and its size.
Cohen's d and Hedges' g from two groups
You get the APA 7 sentence to paste, with the effect size and its confidence interval where one applies.
The full reporting rules, with a worked example, are in the guide on how to report Cohen's d in APA. Have the SPSS output instead? Paste it into the SPSS to APA converter.
Questions
What is the difference between Cohen's d and Hedges' g?
Both are the difference between the means divided by the pooled SD. Hedges' g adds a small-sample correction, so it is the better estimate in small studies; above about 20 per group the two differ by less than 2%.
What counts as small, medium or large?
Cohen's conventions are 0.2 small, 0.5 medium and 0.8 large. They are a last resort: compare your effect with those in your field where you can.
Why report a confidence interval for the effect size?
APA 7 asks for it, and it shows how precise the estimate is. A large effect with an interval that reaches near zero is weak evidence.
Can I use it to plan a study?
Yes: an effect size from a comparable study is what the sample size calculator needs.