How to calculate effect size when I have the mean difference, CI of that mean difference and n? Any advice without using any coding software would be appreciated - thanks!

Assuming a 95% confidence interval using the normal distribution, where $CI_u$ is the upper confidence interval, and $\Delta$ is the difference, then:
$SE=\frac{CI_{u} - \Delta}{1.96}$
We could then calculate the $t-$statistic as:
$t = \frac{\Delta}{SE}$
Then, following this definition of Cohen's D we could calculate:
$D = \frac{t}{\sqrt{N}}$
