How to measure the power consumption of my device

Hello all! im noob here but im sure this is the right place to post my problem.
I was working with a EMG device and arduino , a important place of the design is to be “low energy” well…
how can i measure the duration of my battery in my device?? (knowing the voltage every time and so on)
One way could be to take measures every time a whole day till my batteries death but, is there a way to do it with maths?

please i didnt study electronics so i dont know how to do it…i appreciate your patience here.


i just saw how to do it right here x.x sorry can you remove?