It takes a minute, but you would need to compute it using the formula. Are you confused with the formula?
There is a trick for speed:
\(\dfrac{1}{T-1}\sum\limits_{t=1}^{T}(r_t-\overline{r}) ^2= \dfrac{T}{T-1}\left(\left(\frac{1}{T}\sum\limits_{t=1}^{T} r_t^2\right) -\overline{r}^2\right)\)
There is a trick for speed:
\(\dfrac{1}{T-1}\sum\limits_{t=1}^{T}(r_t-\overline{r}) ^2= \dfrac{T}{T-1}\left(\left(\frac{1}{T}\sum\limits_{t=1}^{T} r_t^2\right) -\overline{r}^2\right)\)
In other words, the variance is the difference between the average of the squares and the square of the average multiplied by T/(T-1)
Whether you use the usual formula or the trick, the memory of the calculator (key M+) will be very useful.