What an interest rate actually costs you over five years

What a rate is actually pricing

A quoted annual rate is a price per unit of money per unit of time. On most instalment loans it is applied monthly: the annual figure is divided by twelve, and that monthly slice is charged on the balance outstanding at that moment. Nothing is charged on the part you have already repaid.

That makes the balance the thing to watch. A 10 per cent annual rate becomes roughly 0.833 per cent a month. In the first month of a $20,000 loan, the interest is $20,000 multiplied by 0.833 per cent, which is $166.67. The payment on a five-year schedule at that rate is $424.94, so once the interest is covered, $258.27 comes off the balance. The following month the interest is charged on $19,741.73 instead of $20,000, so it is very slightly smaller, and slightly more of the same fixed payment reaches the balance.

Repeat that sixty times and the loan clears. This arrangement — a fixed payment that covers accrued interest first and reduces the balance with whatever is left — is called amortisation. It is why two loans with the same rate can cost very different amounts, and why the monthly payment on its own tells you almost nothing about the price.

The same loan at three rates

Holding the amount and the term fixed isolates what the rate alone does. The figures below run $20,000 over sixty monthly payments at three rates, with payments rounded to the nearest dollar.

Two things stand out. The first is how modest the monthly gaps look: four percentage points of rate moves the payment by about $40 a month here, the sort of number that disappears inside a household budget. The second is what those small gaps add up to. Moving from 6 per cent to 14 per cent more than doubles the interest, from about $3,200 to about $7,900 — money that leaves the account in $38 and $40 increments over five years.

The rates used here are chosen to be legible, not typical. What an actual application attracts depends on credit record, income, whether anything secures the debt, the term requested, the lender's own funding costs and prevailing market conditions. Advertised rates on unsecured instalment borrowing have illustratively spanned the mid single digits to well above twenty per cent, and that spread moves over time.

Where each payment actually goes

Because interest is charged on the outstanding balance, the split inside a fixed payment shifts over the life of the loan. Early payments are mostly interest. Late payments are almost entirely principal, meaning the part that reduces the debt itself. The table below takes the 10 per cent case above and shows the split at six points in the schedule.

The implication catches people out. Twelve payments of $424.94 total about $5,099, but the balance has fallen by only about $3,245. The missing $1,854 went on interest — roughly 36 per cent of everything paid in the first year. In the fifth year, interest takes about $266, or around 5 per cent of the payments made.

This is not a penalty or a trick in the paperwork. It falls straight out of charging a rate on a balance: the balance is at its largest at the start, so that is when the charge is largest. It does explain the sense of running hard and barely moving in a loan's first year, and why a settlement figure requested early can look uncomfortably close to the amount originally borrowed.

What one extra payment a year does

Every dollar paid ahead of schedule stops being charged interest for the rest of the term, so extra payments do disproportionate work when they arrive early. A common pattern is thirteen payments a year rather than twelve — one additional payment, perhaps from an annual bonus or a tax refund.

Applied to the 10 per cent loan, that means an extra $425 at the end of each of the first four years, alongside the ordinary schedule. The loan then clears at payment 55 instead of 60. Total interest falls from about $5,496 to about $5,039, so the whole exercise costs about $457 less than the standard schedule while finishing five months sooner. Both the interest saving and the five months come from the same source: the balance was smaller for longer.

The size of that benefit tracks the rate. Run the same routine at 6 per cent and the saving is about $237; run it at 14 per cent and it is about $736. Overpaying is worth most precisely where borrowing is most expensive, which is also where it is usually hardest to find spare money.

Two mechanical details govern whether any of this happens. The first is how a lender treats an unscheduled payment: it can reduce the balance and shorten the term, reduce the balance and recalculate a lower monthly payment, or sit in credit as an advance payment and do nothing to interest at all. The three outcomes are not equivalent and the agreement decides which applies. The second is whether early repayment carries a charge, which some agreements apply and others do not.

How the term changes the total

Lengthening a loan lowers the monthly payment while raising the price, because the same rate is charged over more months on a balance that shrinks more slowly. The rate is identical in all three rows below; only the term moves.

Stretching from five years to seven cuts the payment by about $93 a month and adds about $2,394 in interest. Notice that the seven-year loan at 10 per cent costs roughly what the five-year loan at 14 per cent cost in the earlier table. A longer term at a good rate can be more expensive than a shorter term at a poor one, which is why a payment quoted without its term is not information.

What the quoted rate leaves out

Interest is usually the largest cost of credit, but it is rarely the only one, and several common practices change the effective price without changing the headline rate.

• Fees folded into the balance. Suppose a $600 arrangement fee is added to the amount advanced. Borrowing $20,600 at 10 per cent over five years makes the payment about $438, and the total paid exceeds the $20,000 actually received by about $6,261. On the money that reached the account, that is equivalent to roughly 11.3 per cent rather than 10 per cent.
• Flat or add-on quoting. Some quotes charge a rate on the original amount for every year of the term, ignoring the fact that the balance falls. A flat 5.5 per cent a year on $20,000 for five years produces $5,500 of interest — almost exactly what the 10 per cent reducing-balance loan cost. A flat rate is worth roughly double its face value, though the exact multiple depends on the term and the rate — illustratively between about 1.7 and 1.9 times on instalment terms of three to seven years.
• Annual percentage rates. An APR is a standardised annual figure that folds compulsory charges into one number, so offers of the same term can be compared. It assumes the agreement runs as written, which early settlement or a missed payment breaks.
• Variable rates. If the rate can move, the schedule above is a snapshot rather than a plan. A change is typically absorbed by adjusting the payment or extending the term, and those two responses have very different total costs.
• Daily accrual and lapsing offers. Revolving credit such as a card commonly accrues interest daily on the balance, so timing matters within a month, and promotional periods end on a date after which the standard rate applies to whatever remains.
• Charges triggered by events. Late payment fees, arrears interest and early settlement charges are not in the rate, and they can dwarf a small difference in it.

What this means in practice

The arithmetic above is worth reproducing with figures from an actual agreement. A few things generalise.

The rate, the amount and the term are three separate levers, and the total cost responds to all three. A comparison that holds two of them fixed is informative; one that changes several at once, as a lower monthly payment over a longer term usually does, is not. Total interest expressed as a share of the amount borrowed makes the price legible in a way that a percentage on its own does not — 40 per cent of the sum borrowed is a different sort of statement than 14 per cent a year.

Where the money goes inside each payment matters for anyone thinking about settling early or refinancing. In the first year of the five-year loan at 10 per cent, a little over a third of what is paid covers interest — illustratively nearer a quarter at 6 per cent and nearer a half at 14 per cent — so the balance falls more slowly than the payments suggest. That is normal, and it is also why an overpayment early in the term buys more than the same amount late in it.

Finally, the headline rate is a starting point rather than a conclusion. Fees added to the advance, flat-rate quoting, variable rates and event-driven charges all shift the real cost, sometimes by more than the difference between two rates being weighed against each other. The documents supplied before an agreement is signed are where those details live, and reading the total amount payable is quicker than reconstructing it.

None of this settles whether a particular borrowing decision is sensible. That depends on what the money is for, how secure the income repaying it is, and what happens if circumstances change — questions arithmetic cannot answer. A qualified professional who knows your situation is better placed to weigh those than any general explainer, this one included.