Interest is automatically accrued (added to the balance of your account) once a month - you do not need to manually add the interest charges. The day of your account's due date is used as the accrual date. For example, if you have an account that is due on 3/29, the interest will be accrued every month on the 29th day. Undebt.it supports promo interest rates, so if you have your account set to be in a promo, then the promo interest rate is what will be used to calculate the current month's interest. There's also an area on the debt details page where you can see a history of the monthly interest charges. Note that your bank or lender may calculate interest differently, so it may be necessary to occasionally update your accounts balance manually.