How your monthly payment is worked out
A repayment mortgage is designed to hit zero on the exact month your term ends. To do that, the lender spreads the loan plus all the interest into equal monthly instalments - the same amount every month, from the first payment to the last. That flat figure is what the calculator calls your monthly payment.
The catch is what each payment is made of. Interest is charged on whatever you still owe, so at the start - when the balance is at its biggest - most of your payment is interest and only a sliver chips away at the actual loan. As the balance shrinks, the interest slice shrinks with it, and more of each payment goes on the house. By the final years almost every pound is buying you bricks. That is why the balance line on the Monthly payment chart starts out nearly flat and then falls faster and faster toward the end.
Why a bigger deposit does more than it looks
Your deposit does two jobs at once. The obvious one: every pound of deposit is a pound less you borrow, so a pound less you pay interest on for decades. The quieter one is your loan-to-value, or LTV - the size of your loan as a percentage of the property price. Lenders price mortgages in LTV bands, and the rate usually steps down as you cross the round numbers: 90%, 85%, 80%, 75%, and best of all 60%.
This is why nudging a deposit from, say, just under 10% to a clean 10% can be worth far more than the deposit itself - it can drop you into a cheaper band and lower the rate on the whole loan. If you are close to one of those lines, the calculator makes it easy to see what tipping over it would save.
Fixed, tracker or variable - what am I actually choosing?
The rate you pay comes in a few shapes. A fixed ratelocks your interest - and so your monthly payment - for a set period, usually two or five years. You trade the chance of a lower rate later for the certainty of knowing your payment won’t move. A trackerfollows the Bank of England base rate plus a set margin, so it falls when the base rate falls and rises when it climbs. A lender’s standard variable rate (SVR) is the rate you roll onto automatically when a fix or tracker ends; it is set by the lender, tends to be high, and is almost always worth remortgaging away from.
Whatever shape you pick, the calculator works in a single rate so you can compare like for like. When your deal is due to end, come back, put in the new rate you have been offered, and see what the payment becomes before you commit.
Overpaying: the closest thing to free money
Because interest is charged on the balance, paying a little extra off early removes not just that pound of debt but every future pound of interest it would have racked up. On a typical mortgage, £1 overpaid in the early years can come back as well over £1 in interest you never hand over - the overpayment tab spells out the exact multiplier for your numbers.
You can overpay a fixed amount every month or drop in a one-off lump sum; both shorten the term and shrink the total interest. One thing to check first: most fixed deals let you overpay up to 10% of the balance a year without penalty, but going beyond that can trigger an early repayment charge. Your mortgage offer spells out your limit.
How much can I borrow?
Lenders decide what they will lend on affordability, not just a flat multiple of your pay. As a rough guide, many cap lending at around four and a half times your income, but they also look at your outgoings, other debts, and how the payment would cope if rates rose - a stress test. Two people on the same salary can be offered very different amounts depending on their commitments. Treat the number the calculator produces as a way to test scenarios, not a promise; only a lender’s own decision in principle tells you what you can actually borrow.
The costs beyond the monthly payment
The payment is the headline, but it is not the whole bill. Budgeting for a home means leaving room for the one-off costs of buying - stamp duty (its name and thresholds differ across England, Wales and Scotland), a valuation or survey, the lender’s arrangement fee, and a solicitor to do the legal work. Then there are the running costs of owning: buildings insurance, which your lender will insist on, plus the repairs a landlord used to handle. None of these change your monthly mortgage figure, but they decide how much deposit is left once the keys are yours.