Question

how do I calculate NPV for a mortgage with a down payment, a discount rate, and an interest rate?

Answer 1

It is very simple. First of all calculate installment to be paid for the laon taken. This can be found using forumla of present value of annuity.
Installment = Loan/PVIFA(r%,n)
r= Interest rate and n = number of years.
Downpayment is consider as cash flow for year 0
Let us understand this with an simple example
Loan is of 10,000$ and Downpayment is of 1000$, hence loan amount = 9000$, Interest rate is 10% and n = 5 years
Installment = Loan/PVIFA(10%,5)
=9000/3.7908
2374.1777
Now cash flow will be as under

Year	Cash outflow
0	1000
1	2374.177
2	2374.177
3	2374.177
4	2374.177
5	2374.177

Now to fing NPV just bring this cash flow in present terms using discount rate.