Loan Overpayments: How Much Interest Do They Really Save?
“Pay a bit extra each month and you’ll save a fortune” is advice most people have heard and few have checked. It turns out to be true, sometimes by more than expected — but the size of the saving depends almost entirely on when the extra money goes in. Here is the maths, with real numbers.
Every figure below comes from the Loan Calculator, which rounds interest to the cent each month the way a lender’s statement does.
How an amortising loan spends your payment
A standard loan has a fixed monthly payment. Each month the lender charges interest on whatever you still owe, takes that out of the payment first, and uses the rest to reduce the balance.
Take a $300,000 mortgage at 6% over 30 years. The payment is $1,798.65. In the first month, interest on $300,000 is $1,500.00 — so only $298.65 of the payment reduces what you owe. Across the whole first year, 83% of everything you pay is interest.
By payment 300 the picture has flipped: the balance is lower, so interest is $471.82 and $1,326.83 goes to principal. Over the full 30 years you pay $347,515.44 in interest — more than the house cost.
Why an overpayment is worth more than it looks
A scheduled payment is mostly interest in the early years. An overpayment is not: every penny of it reduces the balance directly. And that money then stops being charged interest for every remaining month of the loan. A dollar overpaid in year one avoids 29 years of interest; a dollar overpaid in year 29 avoids one.
Example 1: $200 a month on a mortgage
Add $200 a month to that $300,000 mortgage, from the first payment:
| No overpayment | +$200 a month | |
|---|---|---|
| Payments | 360 | 279 |
| Total interest | $347,515.44 | $256,341.57 |
| Interest saved | — | $91,173.87 |
| Paid off | 30 years | 6 years 9 months sooner |
An extra $200 a month adds up to about $55,800 over the 279 payments, and it saves $91,000 in interest. That is the compounding working for you rather than the lender.
Example 2: $100 a month on a car loan
Now a $25,000 car loan at 6.5% over 5 years. The payment is $489.15 and the total interest is $4,349.20. Adding $100 a month clears it in 49 payments instead of 60 and saves $865.40.
That is still worth having, but it is a far smaller effect than on the mortgage. The difference is time. A five-year loan simply does not give an overpayment long to avoid interest. Overpaying matters most on large, long loans.
Example 3: timing a lump sum
Say you get a $5,000 bonus and put it against the mortgage in one go:
- At payment 12, it saves $22,367.98 and ends the loan 15 months early.
- At payment 240, the same $5,000 saves $4,006.24 and 5 months.
Same money, more than five times the effect. If you plan to overpay, sooner is worth much more than later.
Overpaying versus a shorter term
The same mortgage over 20 years costs $2,149.29 a month and $215,831.10 in interest — less than the 30-year loan even with $200 extra a month, because the whole balance is on a faster schedule from day one.
The trade-off is flexibility. A shorter term commits you to the higher payment every month. Overpaying a longer loan gets you part of the way there while letting you stop if money gets tight. Which is better depends on how stable your income is, not on the maths.
The rate detail that changes every answer
A loan’s rate can be quoted two ways, and mixing them up shifts every figure.
A nominal annual rate is divided by 12 to get the monthly rate. It is what most loan contracts and US lenders mean. An effective annual rate — which is what a UK APR is — already includes the effect of monthly compounding, so its monthly equivalent is a little lower.
On our mortgage, treating 6% as effective rather than nominal gives a payment of $1,768.11 instead of $1,798.65. That is $30.54 a month and about $11,000 over the term, from one setting. Check which kind of rate your documents quote before trusting any calculator, and set the calculator to match.
Before you overpay
Overpaying is a guaranteed return equal to the loan’s interest rate, which is hard to beat. Still, check four things first:
- Early repayment charges. Many fixed-rate loans cap penalty-free overpayments — in the UK often at 10% of the balance a year.
- Dearer debt. Clear credit cards and overdrafts before a 6% mortgage.
- An emergency fund. Money paid into a loan is hard to get back out.
- What your savings earn. If an account pays more after tax than the loan costs, saving may win.
Do the sums yourself
The best way to decide is to put your own loan in and try amounts and dates. The calculator shows the interest and time saved side by side, charts both balances and exports the full schedule. Your figures stay in your browser, which matters more than usual for money — the reasoning is in private online tools for everyday tasks. And when the cost is shared rather than borrowed, the Bill Splitter handles the pennies the same careful way.
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