Interest is charged on the balance you still owe. Every other fact about paying off debt follows from that one sentence, including why an extra payment in year one is worth several times the same payment in year fifteen.
Why early payments are worth more
On a twenty-five year loan at five and a half percent, the first payment is mostly interest and barely touches the balance. By year twenty the same payment is almost entirely principal. An extra amount paid in month one removes that principal from every future interest calculation — three hundred of them. The same amount paid in month two hundred removes it from a handful.
This is why the standard advice to overpay early is not a moral preference for discipline. It is arithmetic, and you can watch it happen in the amortisation table: enter your loan, add an extra monthly amount, and compare the total interest figure with and without it.
Avalanche or snowball
When you have several debts, you pay the minimum on all of them and send every spare amount to one. Which one is the whole question.
Avalanche: highest rate first
Target the debt with the highest interest rate, regardless of size. This is mathematically optimal: it always produces the lowest total interest and the shortest payoff time. If you will stick to it, nothing beats it.
Snowball: smallest balance first
Target the smallest balance, regardless of rate. You clear individual debts faster, which frees up their minimum payments and produces visible wins early. It costs more in interest, sometimes meaningfully.
Worked example
Three debts: 1,200 at 8%, 6,000 at 24%, 3,500 at 14%, with 600 a month spare. Avalanche clears the 24% card first and saves roughly a few hundred in interest over the run. Snowball clears the 1,200 balance in two months, which for many people is the difference between continuing and quitting in month four.
Pick avalanche if the gap between your highest and lowest rate is large, which is usually the case when a credit card is involved. Pick snowball if you have abandoned a payoff plan before. The best method is the one still running in month nine.
Refinancing: when it is worth it
Refinancing replaces an expensive debt with a cheaper one. It genuinely works, and it is also where most of the traps live.
- Compare total cost, not the monthly payment. A lower payment over a longer term usually costs more overall. Run both in the loan calculator and read the total interest line.
- Count the fees. Arrangement fees, valuation fees and early-repayment charges on the old loan can erase the saving. Add them to the new loan's cost before deciding.
- Watch the term reset. Refinancing eighteen years into a twenty-five year mortgage back to a fresh twenty-five years is often a loss dressed as a saving.
- Balance-transfer cards. The zero percent window is real, and so is the transfer fee and the rate that applies afterwards. It only works if you clear the balance inside the window, so divide the balance by the number of months and check that the payment is one you can make.
The order to attack, once and for all
- Pay every minimum, every month. Missed payments cost more than any strategy saves.
- Hold a small cash buffer, one month of essential spend, so a surprise does not go back on the card you are trying to clear.
- Clear anything above roughly ten percent aggressively. No investment reliably beats a guaranteed return of that size.
- Capture any employer retirement match, which is an immediate return no debt rate matches.
- Then choose: keep attacking mid-rate debt, or start investing, depending on whether the rate is above or below what you would realistically earn.
Low-rate debt is not an emergency. A subsidised loan at two percent while inflation runs higher is a debt you can pay on schedule while your money works elsewhere. Not all debt deserves the same urgency.
Making the extra payment count
Tell the lender the extra amount is for the principal. Some lenders otherwise treat it as an early payment of next month's instalment, which does far less for you. Check the balance after the first overpayment to confirm it was applied the way you intended, and keep checking for the first few months.