From three numbers: the amount borrowed, the interest rate, and how many months you have to repay. The payment is the level amount that clears the balance in exactly that many months, with interest charged each month on whatever is still owed. Every instalment is the same size, but the split between interest and principal shifts toward principal as the balance falls.
The interest rate covers the interest alone. APR folds in the fees a lender charges to set the loan up, so it is the fairer number to compare two offers with. Put the rate in the rate field and any set-up fee in the fees field, and the cost of borrowing figure shows the total either way.
Usually, though not always. Extra payments reduce the balance, and interest is charged on the balance, so each one lowers every charge that follows. Some agreements carry an early repayment charge that eats part of the saving, so check that clause before overpaying a large amount.
Yes, for any loan that repays in level monthly instalments at a fixed rate. Car and personal loans are usually quoted in months, so switch the term unit. Student loans on an income-based plan work differently, because the payment moves with earnings rather than staying level.
Enter zero in the rate field and the payment becomes the amount split evenly across the term. The cost is then whatever fee came with it, which is why the fees field is separate. Watch the end of the offer period too: a rate that starts later applies to whatever balance is left at that point.
A loan is one line among many. Aventurine holds the whole ledger, categorised and charted, in an encrypted file on your own machine.