All forms of borrowing, from 100% mortgages through to loans are subject to APY. APY stands for annual percentage yield, or effective annual rate (AER), which is the compounded interest rate.
For 100 mortgage interest rates, the compounded interest will be added up and formulated in order to ascertain the overall total repayment due on the repayment of the mortgage for the given term of mortgage and the rate. Of course it does not simply get calculated on the capital sum which you borrow for the 100 mortgage, as this is only part of the overall debt needing repayment.
Equally the interest for the total 100 mortgage and the total term will be repayable, or at least shown as outstanding, from day 1 of the mortgage, which is why, when someone makes monthly repayments, they are simply repaying interest for upto half the term of the mortgage, as if 100k is borrowed, and charged interest at 10%, then it may well be calcuated approx as follows;
Year One – 100% Mortgage
100k *10% = 110k
110k * 10% = 121k
121k * 10% = 131.1k
131.1k * 10% = 146.41k
146.41k * 10% = 161.051K
161.051K * 10% = 177.1561k
177.1561k * 10% = 194.8717k
194.8717k * 10% = 214.35888k
214.35888k * 10% = 235.79476k
235.79476k * 10% = 259.37423k
So as you can see, 100k * 10% 100 mortgage over 10 years is not repayable as 200k, (100k * 10K), but as 259.4k with no repayments deducted. Obviously, there will be repayments and so figure will lower, ie
100k * 10% = 110k – 2.4k (200 per month) = 107.6k and so forth, but as you can see, at this level of interest, borrowing, 100k, and the interest of 10k being added, with 2.4k in repayments, the outstanding balance still consists of 100k capital and interest of 7,6k, as the remaining 2.4k of the 10k is repaid.
Until the interest is eaten iup and rebalanced, the capital of the 100% mortgage debt does not decrease, and this is one way of estimating how much will be repaid for 100 mortgage apy.