What the comparison has to get right
Housing is the one financial decision where the intuitive arithmetic is reliably wrong, which is what the rent versus mortgage comparison exists to fix. Here is what makes it hard to do in your head.
The question people ask is “is it cheaper to rent or to buy?” and the intuitive answer is buying, because rent is money you never see again while mortgage payments build equity. That framing is wrong often enough to be worth taking apart.
The costs the intuition skips
Interest is also money you never see again. In the early years of a mortgage, most of the payment is interest, not principal, and how lopsided it is depends on the rate. On a 30-year loan at 5 %, roughly three quarters of the first year’s payments are interest; at 3 % it is closer to three fifths. Either way that portion is exactly as gone as rent is.
The deposit could have been doing something else. This is the largest term people leave out. Money locked in a property is money not invested elsewhere, and the return you gave up is a real cost of buying even though it never appears on a statement. A comparison that ignores opportunity cost will favour buying every time, automatically, regardless of the numbers.
Transaction costs are front-loaded and large. Legal fees, transfer tax, valuation, agent commission and moving costs are paid once, on the way in and again on the way out. Spread over thirty years they are noise; spread over four, they can exceed everything you gained. This is why the honest answer to the question depends so heavily on how long you stay, and why short horizons favour renting even in expensive markets.
Maintenance does not stop. A roof, a boiler and a set of windows all have finite lives, and the owner pays for them. A common rule of thumb is around 1 % of the property value per year, averaged over the long run — lumpy in practice, zero for years and then not.
Rates and rents both move. Fixing a rate fixes it for a term, not for the life of the loan, and rents rise over the same period. A comparison run at today’s numbers for thirty years is a scenario, not a forecast.
How to use the result
Treat the output as a break-even horizon rather than a verdict: it tells you roughly how long you would need to stay for buying to come out ahead of renting under the assumptions you entered. If the answer is seven years and you might move in three, the calculation has told you something useful even though it did not tell you what to do.
Change one input at a time and watch what moves. If a plausible change to the interest rate or the growth assumption flips the answer, the two options are close enough that the financial case is not what should decide it.
Nothing you enter is transmitted anywhere — the calculation runs in the page, and your salary, deposit and loan terms stay in the tab.