Accounting → Debt & financing → Loan calculator.
Every field is typed in. It reads nothing from your books, because the loan does not exist yet — these are the numbers the lender quoted you.
What it shows
The payment, the interest over the life of the loan, and the date it finishes. For a mortgage, enter the property tax, insurance and PMI per year and it shows what each adds to every payment — the quoted "payment" from a lender is often only the loan part, and the real one is larger.
PMI stops when you owe less than 80% of the original amount, so the calculator shows the year that happens instead of pretending the payment never changes.
Paying extra
Enter an amount and how often. It compares against the plain schedule and tells you the interest saved and how much earlier the loan ends.
When matters as much as how much. The same money paid once a year saves less than a twelfth of it paid every month, because interest is charged on the balance every period — the calculator shows the difference rather than describing it.
After you take it
The button at the bottom carries the amount, rate, term and frequency into Add a debt, so the loan is recorded without retyping. From then on each payment coming out of your bank can be split into principal and interest in one press — see Record a credit you owe, and file its payments.