With YNAB, if you categorize all of your income as for the next month, or if you're keeping an emergency reserve, or if you have rainy day categories, you might have a large amount of money in your bank accounts. I do this but try to keep my checking account balance low, and put the rest of the reserves in a savings account to get a slight boost in interest. (monthly income + emergency fund + avg. rainy day balance) * (savings interest rate) = yearly interest by using a savings account