Take a $500,000 loan at 6 per cent. Over 30 years the repayment is $2,997.75 a month and the total interest comes to $579,190.95. Over 25 years the repayment is $3,221.51 and the total interest is $466,452.10. The extra five years costs $112,738.85, and buys a monthly saving of $223.76.
The rate is identical in both cases. Nothing about the loan changed except how long the balance stays outstanding.
Interest is rent on the balance
Interest is charged on what you still owe, every month, for as long as you owe it. A longer term does not reduce the rate; it extends the period over which the rate applies. That is the whole mechanism.
This is why the comparison people usually make, monthly payment against monthly payment, is the wrong one. The monthly payment is what the loan costs to service. The total repaid is what the loan costs.
Why the early years feel like nothing is happening
On that same $500,000 at 6 per cent, the first month's interest is $2,500. The payment is $2,997.75, so $497.75 goes to the balance. In the first month, 83 per cent of the payment is interest.
That ratio shifts every month as the balance falls, but it shifts slowly at first, which is why a mortgage can feel static for years. The loan payment calculator shows the full amortisation, and the shape of it is worth looking at once: the crossover, where more of the payment goes to principal than to interest, arrives far later than most people expect.
The lever that actually works
Extra repayments come off the balance directly, so they stop that balance earning interest for every remaining month of the loan. Adding $200 a month to the 30-year loan above pays it off in 25.5 years and cuts total interest to $476,046.06, a saving of $103,144.89.
Compare that with simply taking the 25-year loan: it costs $3,221.51 a month, while the 30-year loan plus $200 costs $3,197.75 and finishes at a similar time. Slightly less committed each month, similar outcome, and the flexibility to stop the extra payment in a bad month. The shorter term is a promise; the extra repayment is a choice you renew monthly.
When the longer term is still right
None of this makes a 30-year term a mistake. A payment you can meet in a difficult year is worth more than a theoretical interest saving you default on. The point is only that the longer term should be chosen for the cash-flow room it buys, with the total cost known, rather than chosen because the monthly figure looked more affordable and the total was never calculated.
Run both terms through the loan payment calculator before deciding, and read the total interest line, not just the monthly one. For a property loan with deposit and equity figures alongside it, the mortgage calculator covers the same arithmetic in that context.