Your rate dropped from 2.8% to 1.6%. Why does the monthly instalment barely move?

If you have looked at refinancing recently, you may have run into a puzzling moment. Rates have come down meaningfully. You pull up a calculator, plug in a rate of 1.6% instead of the 2.8% you are paying today, and the monthly instalment falls by a few hundred dollars. After bracing yourself for something dramatic, the number feels almost disappointing. Is it really worth the paperwork?

It is. The instalment is simply the wrong number to be looking at, and it understates what is actually happening by a wide margin.

The worked example

Take a homeowner with $1,000,000 outstanding and 25 years remaining on the loan.

	At 2.8%	At 1.6%	Difference
Monthly instalment:	$4,639	$4,047	$592 lower, or 13%
Interest over the full term:	$391,623	$213,956	$177,667 lower, or 45%

The rate fell by 43%. The interest bill fell by 45%. The monthly instalment fell by 13%.

Three numbers describing the same change, and only one of them is small. That one happens to be the number most people use to make the decision.

Why the instalment hides the saving

Your monthly instalment is not a bill for the cost of borrowing. It is two very different things bundled into a single figure:

The interest portion: this is the genuine cost. It is money that leaves your household and does not come back. This is the only part of the instalment that responds to the interest rate.

The principal portion: this is not a cost at all. It is your own money moving from your bank account into the equity of your property. It is forced savings. Crucially, it does not change when rates change, because the amount you owe and the years you have left to repay it are what determine how much principal you must clear each month.

At 2.8%, roughly half of that $4,639 payment is interest and roughly half is principal. When the rate is cut by 43%, it can only act on the interest half. The principal half sits there unmoved, diluting the percentage change in the total. A large cut to half of something looks like a modest cut to the whole thing.

Then the maths flattens it further. The full $177,667 saving is spread evenly across 300 months, which is precisely how you arrive at $592. A very large number divided by a very long time always produces a small number. That is arithmetic, not a verdict on whether the saving is worth having.

The part almost nobody sees

Here is where the instalment comparison becomes genuinely misleading rather than merely undramatic.

Most Singapore packages come with a lock-in period of two years, so two years is the sensible horizon for judging a refinance. Over the first 24 months of the same $1,000,000 loan:

	At 2.8%	At 1.6%	Difference
Cash paid out:	$111,330	$97,116	$14,214 less
Interest paid:	$54,490	$30,992	$23,498 less
Principal cleared:	$56,840	$66,125	$9,285 more
Balance still owing:	$943,160	$933,875	$9,285 lower

Look at the middle two rows together. The homeowner avoids $23,498 of interest over two years, but only $14,214 of it shows up as lower cash outflow. The remaining $9,285 never appears in the monthly figure at all, because it arrives as additional equity in the property.

At the lower rate you pay out $14,214 less and still knock $9,285 more off the loan. The instalment comparison captures roughly 60% of the benefit and stays completely silent about the other 40%.

That is the whole illusion in one line. The instalment tells you what leaves your account. It says nothing about how much of what leaves your account is actually being kept.

What the saving is worth in practice

$177,667 over 25 years is a real number, but it is also a long way off, and rates will change several times before then. So let us stay with the two-year window, which is the horizon you are actually committing to.

$23,498 of interest avoided over 24 months. Set against the typical costs of refinancing, which are legal fees in the region of $2,500 to $3,000 plus a valuation fee, and often partly or fully subsidised by the incoming lender, the arithmetic is not close. This is not a marginal decision that hinges on whether you can be bothered with the forms.

There is also a second way to take the benefit, which many homeowners prefer once they see it. Rather than pocketing the lower instalment, keep paying $4,639 a month at the new rate of 1.6%. The loan then clears in roughly 21 years and 2 months instead of 25 years, and total interest falls to about $179,779. Same cash outflow as today, close to four years of repayments removed from your life, and around $212,000 of interest avoided.

Nothing about your monthly budget changes. Only the finish line moves.

The number to look at instead

When you compare mortgage options, the instalment is useful for one thing only: checking that the payment fits your cash flow. It is a budgeting figure. It is not a measure of cost, and it is not a basis for comparison.

For that, look at the interest instead:

• The interest component of your next payment: multiply your outstanding balance by the annual rate and divide by twelve. Do it for both rates. That comparison is honest.
• Total interest over the lock-in period: this is the number you are actually deciding on when you commit to a package.
• The outstanding balance at the end of the lock-in: this captures the equity effect that the instalment ignores.

A drop from 2.8% to 1.6% is a large event in the life of a mortgage. It reduces the cost of your loan by nearly half. The fact that it presents itself as a $592 line item is a quirk of how instalments are constructed, and it is a quirk that quietly costs homeowners a great deal of money every year.