Interest earnings can be calculated in different ways.
Depending on your account and the financial institution you have opened it with, you'll earn a different interest rate.
The interest rate you'll earn for savings accounts is based on compound interest, i.e., both the initial deposit and all the accumulated interest will be considered.
The interest is usually calculated using the APY (annual percentage yield), which is how much your account earns in one year, including compound interest.
If you have a fixed interest rate, there is a simple formula to find out the interest you'll earn:
multiply the account balance by the interest rate for the selected period of time you were planning on leaving the money in the account without withdrawals. You'll quickly find out how much interest the account should earn during that time.
On the other hand, if your account earns a variable interest rate, it will be trickier to calculate since the interest can change from time to time.
A lot will depend on your principal, the original sum you put in the account, how much time you want to let the funds in the account grow, and the APY your bank offers.
Also, keep in mind that some financial institutions compound interest monthly. In contrast, others do it daily, and so on, and this can also change how quickly your savings will grow.