How loans actually work

Every amortising loan behaves the same way regardless of what it is buying: a constant payment, interest charged on a falling balance, and a principal component that starts small and grows. Understanding that one mechanism explains mortgages, car loans, student loans, and the reason paying extra early is worth so much more than paying extra late.

The amortisation mechanism

A fixed-rate loan is solved for the single constant payment that exactly retires the balance over the term. Each month, interest is charged on whatever principal remains; the rest of the payment reduces the balance. Because the balance falls, the interest portion falls, and the principal portion grows to compensate.

The consequence is that early payments barely touch the debt. On $350,000 at 6.5 percent over 30 years, the payment is about $2,212 and the first one splits roughly $1,896 to interest and $316 to principal. It takes around 19 years before more than half of each payment goes to principal, and after a decade of on-time payments the balance is still about $287,000.

This is not a fee structure imposed by lenders; it falls directly out of charging interest on a declining balance with a level payment. But it has a practical implication worth acting on: a dollar of extra principal in year one eliminates every interest charge that dollar would have generated across 29 remaining years, while the same dollar in year 28 saves almost nothing. Extra payments are enormously front-loaded in value.

It also explains why refinancing late in a term can cost more in total interest even at a lower rate. Resetting to a new 30-year schedule puts you back at the front of the curve, where the interest share is highest. Comparing total remaining interest rather than the monthly payment is the only way to evaluate this properly.

Term costs more than rate

Both matter, but the term is the variable people trade away most casually because it is the one that lowers the payment most visibly.

On $30,000 at 6 percent: 48 months costs about $3,800 in interest; 60 months about $4,800; 72 months about $5,800; 84 months about $6,800. Each extension buys roughly $60 to $80 off the monthly payment for about $1,000 more in total cost.

On a mortgage the effect is larger still. Moving $350,000 at 6.5 percent from a 30-year to a 15-year term raises the payment about 38 percent and cuts total interest from about $446,000 to about $199,000 — a $247,000 difference on the same loan at the same rate.

Rate is not trivial either. One percentage point on that 30-year mortgage is worth about $224 a month and roughly $80,000 in total interest, which is more than most buyers save by negotiating the purchase price. But rate is largely set by the market and your credit; term is a choice made at the table, which is why it deserves the more deliberate decision.

The subtler cost of a long term is equity. A depreciating asset financed over 84 months leaves you owing more than it is worth for most of the loan, which means a total loss or an unplanned sale requires cash to settle. On appreciating assets this matters less; on cars it matters a great deal.

APR, APY, and simple interest are different numbers

Three rate conventions are in common use and they are not comparable without conversion.

A nominal rate is stated annually but applied per period. Six percent nominal compounded monthly charges 0.5 percent a month, which compounds to an effective 6.17 percent over a year. The effective rate is the one that reflects what you actually pay.

APR on a loan is intended to include origination fees and points alongside the interest rate, so it is comparable across offers with different fee structures. APY on a deposit reflects compounding. Both exist precisely because nominal rates can be made to look better than they are.

Simple interest charges only on the original principal and does not compound. It is rare in consumer lending but appears in short-term instruments and some auto loans, where daily simple interest means your payment timing genuinely affects total cost — paying a few days early reduces the interest accrued.

The practical rule when comparing offers is to compare the same convention, and where fees differ, to compare total cost over the expected holding period rather than the rate. A lower rate with two points of origination is often worse than a higher rate with none, particularly if you expect to refinance or sell within a few years.

Where the money actually goes

Loan payments frequently include components the amortisation formula does not produce, and comparing a calculated payment to a lender quote without accounting for them causes a lot of confusion.

On a mortgage, escrowed property taxes, homeowners insurance, and private mortgage insurance commonly add 30 to 40 percent to the principal-and-interest figure. Taxes alone at 1.5 percent of a $400,000 assessed value are $500 a month. PMI applies below 20 percent equity and can usually be removed once you reach it, which is worth tracking rather than waiting for the servicer to notice.

On a car loan, sales tax, registration, and documentation fees are typically financed, so the amount borrowed exceeds the negotiated price. Add-on products presented at signing — extended warranties, gap coverage, protection packages — are also financed, and a $2,000 package at 6 percent over 60 months costs about $39 a month and $320 in interest.

On revolving debt the mechanism is different again. Credit card minimums are a percentage of the balance, so they shrink as the balance does and the payoff curve flattens indefinitely. On $5,000 at 22 percent, a 2 percent minimum starts at about $100 of which $92 is interest. A flat extra payment behaves entirely differently, because it does not shrink — which is why any fixed additional amount is so much more effective than it appears.

Paying debt down in the right order

With multiple debts, the ordering of extra payments has a real cost attached to it. Highest interest rate first — the avalanche method — minimises total interest by definition, since a dollar applied to a 24 percent balance saves twice what it saves on a 12 percent balance.

The competing approach, smallest balance first, costs more in interest and is sometimes still the right choice, because a completed payoff is a visible milestone and the common failure mode of a debt plan is abandonment rather than inefficiency. On an $8,000 balance at 22 percent and a $12,000 balance at 9 percent with $600 a month available, the ordering difference is about $1,300 and four months.

A sensible resolution: where rates differ sharply, take the avalanche and accept the slower first win. Where rates are similar, ordering barely matters and you should pick whatever you will sustain. Either way, confirm with your servicer that extra payments are applied to principal rather than held against the next scheduled payment or split proportionally across balances, which quietly defeats the strategy.

And check whether refinancing beats ordering entirely. A genuine zero-percent balance transfer stops accrual for the promotional period, and consolidating revolving debt into a fixed-rate instalment loan gives a defined end date. Both change the arithmetic more than any payment sequence can.

Frequently asked questions

Calculators referenced in this guide