The payment is the advertised number. The interest is the real one.
You are not buying a payment. You are renting money, and the rent is charged on the balance — which is why the early years are almost all interest. The term is the most expensive number on this page: stretching it shrinks the payment and quietly multiplies what you hand over.
Almost every calculator assumes one rate for twenty-five years. That is not the deal you are signing. You are signing a short fix and then a revert, and when the fix ends the lender recalculates what is left over what remains of the term. Budget for the payment after the cliff, not the one before it.
Interest is charged on what you still owe, so at the start you owe nearly everything and the payment is nearly all rent. The point where you start paying off more than you are paying for is years away — and on a long term it can be past the halfway mark.
An overpayment comes straight off the balance, so every future month's interest is charged on a smaller number. The same money paid in year two is worth far more than in year twenty — not because it is bigger, but because it has longer to stop working against you.
Overpayments here shorten the term rather than reduce the monthly payment, which is the usual default and the one that saves more.
A lower rate carrying a fee only beats a fee-free higher rate above a certain loan size, because the fee is flat and the interest saving scales with what you borrow. There is an exact crossover, and it is usually not where people guess.
Read across a row to see what half a point of rate is worth, and down a column to see what five more years costs. A longer term is not a cheaper mortgage; it is a smaller payment and a much larger bill.
Total interest over the full term, holding one rate throughout and ignoring overpayments, so the rate and the term are the only things moving. The outlined cell is where you are.