eDPI (otherwise known as "true sensitivity") is a convenient way of comparing sensitivity in video games because it takes into consideration both the game sensitivity and mouse DPI.
This is useful because a low DPI and high sense could actually be the exact same as high DPI and low sense, but if you ignore DPI the sensitivities look very different.
This calculator provides a simple way to show you your eDPI for different sensitivities and DPIs.
In order to calculate you eDPI simply select whether the sensitivity for the game you play is in percentage or decimal (most games use decimal). Then enter your in game sensitivity as well as the DPI for your mouse and your eDPI will be instantly calculated.
The formula to calculate eDPI is simply game sensitivity * mouse DPI. So if you have a sensitivity of 2 with a DPI of 400 your eDPI is 800, similarily if your sensitivty is 0.5 and your DPI is 1600 your eDPI is also 800. As you can see the true sensitivities are equal, even though the in game sensitivities are very different.