How to calculate the total interest on a loan

Once you have the payment, the total cost of the loan is two more keystroke pairs, because the calculator keeps every value you used.

Problem

On a $350,000 mortgage over 30 years at 6.5%, how much do you pay in total, and how much of that is interest?

Keystrokes

fFINx≷y
0.00
Start clean.
30g12×n
360.00
360 monthly payments.
6.5g12÷i
0.54
Monthly rate.
350000PV
350,000.00
The amount borrowed.
PMT
−2,212.24
The monthly payment.
RCLn×
−796,405.71
Payment times the number of payments: everything you pay over the life of the loan.
RCLPV+
−446,405.71
Adds back the amount you borrowed. What’s left is interest.

Answer

You pay $796,405.71 over 30 years, and $446,405.71 of it is interest.

Why it works

RCLn brings back the 360 you stored and multiplies it by the payment still in the display. Because the payment is negative, the total is negative too: money out.

Adding the positive amount you borrowed cancels the part of those payments that repaid principal. The remainder is the interest, still negative because you paid it.

A note on rounding

The calculator keeps the payment to full precision, so this total can differ by a few cents from 360 times the rounded payment on your statement. To match the statement, key the rounded payment in yourself, for example 2212.24CHSPMT, before the last two steps.

Questions

Does paying extra each month reduce the total interest?

Yes, often by a lot, because each extra dollar stops accruing interest for the rest of the loan. See the guide on extra payments.