What a Loan Actually Costs (And Why the Monthly Payment Hides It)

Ask a lender what a loan costs and you’ll get a monthly figure. It’s a reasonable answer to the question “can I afford this?” and a terrible answer to the question “should I do this?”

The monthly payment is the smallest number in the deal. Two loans with nearly identical payments can differ by tens of thousands over their lifetimes. If you only ever look at the payment, you’ll pick the wrong one roughly half the time.

Where the monthly payment comes from

Every amortising loan uses the same formula. With P as the amount borrowed, r as the monthly rate (annual rate ÷ 12 ÷ 100) and n as the number of months:

Payment = P × r × (1 + r)ⁿ / ((1 + r)ⁿ − 1)

You don’t need to compute this by hand. What matters is that n sits in an exponent, which is why stretching the term reduces the payment quickly at first and then barely at all — while the total cost keeps climbing.

The three numbers to compare

Whenever you’re offered a loan, write down three things:

1. The monthly payment — can you afford it, every month, including the month the car breaks?
2. The total repaid — payment × number of payments.
3. The total interest — total repaid minus the amount borrowed. This is the price of the loan.

Number three is the one lenders don’t lead with, and it’s the one that should drive the decision.

A concrete example

Borrow $250,000 at 6.5%:

Term	Monthly payment	Total interest	Total repaid
15 years	$2,178	$142,000	$392,000
20 years	$1,864	$197,000	$447,000
25 years	$1,688	$256,000	$506,000
30 years	$1,580	$319,000	$569,000

Going from 25 years to 30 saves $108 a month. It costs $63,000.

That is the entire trade, stated plainly. Sometimes it’s still the right choice — if the lower payment is the difference between managing and not managing, take it. But make the decision knowing the price, not just the relief.

Why your early payments barely touch the balance

Interest is charged on what you still owe. At the start, you owe nearly everything.

In month one of a 30-year mortgage at 6.5%, about $1,354 of your $1,580 payment is interest. Roughly $226 comes off the balance. You have paid $1,580 and reduced your debt by the price of a weekly shop.

That ratio flips over time, but slowly. On a 30-year loan you’re typically past the halfway point — where more of each payment goes to principal than interest — somewhere around year 18.

This is not a scam. It’s arithmetic. But it explains why people who sell after five years are often surprised at how little equity they’ve built.

Overpaying is the highest guaranteed return you can get

An extra payment goes entirely against the principal. That removes not only the amount you paid, but every future month of interest that amount would have generated.

On the $250,000 loan at 6.5% over 30 years, adding $150 a month:

• Pays the loan off 4 years and 7 months early
• Saves about $62,000 in interest

You paid in roughly $45,000 of extra contributions and got $62,000 back. That’s an effective, guaranteed, tax-free return equal to your interest rate — which for most people beats any savings account available.

Two cautions:

• Check for early repayment charges. Some fixed-rate deals cap overpayments at 10% of the balance per year and charge a penalty beyond that.
• Clear higher-rate debt first. Overpaying a 6% mortgage while carrying a 22% credit card balance loses money every month. Highest rate first, always.

The rate is not the whole price

The interest rate is the headline; the APR is closer to the truth, because it folds in fees. But even APR misses things:

• Arrangement, valuation and legal fees — often $1,000–3,000, sometimes rolled into the loan so you pay interest on them for 30 years.
• Mandatory insurance — some loans require cover you’d otherwise not buy.
• Rate resets — a two-year fixed rate is a two-year promise, not a 30-year one. Model what happens if it resets three points higher.
• Early repayment charges — they cost you the flexibility to refinance if rates fall.

Compare loans on total cost over the period you’ll realistically hold them, not on the rate alone.

The questions to ask before you sign

• What is the total interest over the full term?
• What is the APR, and what is included in it?
• Can I overpay, and is there a limit or a penalty?
• What happens when the fixed period ends — what rate do I revert to?
• What is the early repayment charge, and when does it stop applying?
• Are there fees added to the loan rather than paid upfront?

A lender who won’t answer these plainly is telling you something.

One thing to try right now

Take whatever loan you’re considering — or already have — and model it twice: once at the term you were offered, and once five years shorter. Look at the difference in total interest, not the difference in monthly payment.

That single comparison changes more borrowing decisions than any amount of general advice.

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This is general information about how loan arithmetic works, not financial advice. Your circumstances, tax position and local regulations all matter. Talk to a qualified adviser before making a decision this size.