Reducing balance interest

Reducing balance interest is calculated on the principal still outstanding, recalculated each period as the borrower repays. As the balance falls, so does the interest charged. It is also called declining balance, diminishing balance or amortised interest.

The calculation for each period is:

Interest for the period = outstanding principal × periodic rate

The principle behind it is simple: the borrower pays for the money they actually have. Once principal has been repaid, no further interest is charged on it.

This is the standard method in most formal lending and is increasingly mandated by regulators, because the alternative — flat rate — charges interest on money the borrower has already returned.

How it works: a worked schedule

$6,000 at 24% per annum over 6 monthly instalments.

Monthly rate = 24% ÷ 12 = 2.0%
Instalment   = $1,071.15

Month   Opening      Instalment   Interest   Principal    Closing
1       $6,000.00    $1,071.15    $120.00    $951.15      $5,048.85
2       $5,048.85    $1,071.15    $100.98    $970.17      $4,078.68
3       $4,078.68    $1,071.15    $81.57     $989.58      $3,089.10
4       $3,089.10    $1,071.15    $61.78     $1,009.37    $2,079.73
5       $2,079.73    $1,071.15    $41.59     $1,029.56    $1,050.17
6       $1,050.17    $1,071.15    $21.00     $1,050.15        $0.00

Total repaid   = $6,426.90
Total interest = $426.90

The instalment never changes. The interest inside it falls from $120 to $21, and the principal portion rises correspondingly.

The interest/principal split — and a common misunderstanding

Borrowers often notice that early payments are interest-heavy and conclude that the lender has "front-loaded" the interest. On a reducing balance loan, it has not.

The split shifts for one reason only: interest is charged on the balance, and the balance is largest at the start. Nothing has been loaded anywhere. If the borrower settles at any point, they owe the principal outstanding at that date and no more.

This is genuinely different from flat-rate or pre-computed lending, where total interest is fixed at inception and a rebate method determines what — if anything — comes back on early settlement. On reducing balance, no rebate is needed, because unearned interest was never charged.

Two variants that are both reducing balance

"Reducing balance" describes how interest is calculated, not how the loan is repaid. Two repayment structures both use it, and they are frequently conflated.

Equal instalment (annuity). The payment is constant; the interest/principal split shifts. This is the table above and by far the more common structure.

Equal principal (straight-line). The principal portion is constant; the payment declines as interest falls.

Same $6,000 at 24% over 6 months, repaid as equal principal of $1,000 per month:

Month   Opening     Principal   Interest   Instalment
1       $6,000      $1,000      $120       $1,120
2       $5,000      $1,000      $100       $1,100
3       $4,000      $1,000       $80       $1,080
4       $3,000      $1,000       $60       $1,060
5       $2,000      $1,000       $40       $1,040
6       $1,000      $1,000       $20       $1,020

Total interest = $420.00  (versus $426.90 on equal instalment)

Equal principal costs slightly less, because principal is repaid faster early on. It demands more cash in the first months, which is why equal instalment dominates in consumer lending — predictability outweighs the small saving.

Daily accrual versus periodic calculation

Two implementations produce slightly different numbers.

Periodic (monthly) rate. The annual rate is divided by twelve and applied to the balance at each instalment date. Simple, and the basis of most published schedules.

Daily accrual. Interest accrues each day on the balance outstanding, typically on an Actual/365 basis, and is billed monthly:

Daily interest = outstanding balance × annual rate ÷ 365

The difference matters when payments arrive off-schedule. Under daily accrual, paying three days early genuinely reduces interest, because the balance drops three days sooner. Under a monthly periodic calculation, it usually makes no difference at all.

Daily accrual also makes the loan sensitive to the day count convention — a 31-day month accrues more than a 28-day month under Actual/365, and exactly the same under 30/360.

Reducing balance versus flat rate

This is the comparison that matters most to borrowers, because the same headline rate produces very different costs.

Flat rate charges interest on the original principal for the full term, regardless of repayments made. Using the same loan:

Flat:      6,000 × 24% × 0.5 years  = $720.00 interest
Reducing:                             $426.90 interest

The same quoted 24% costs 69% more under flat.

• Interest base: Reducing balance recalculates on the outstanding balance; flat rate is fixed on the original principal.
• Interest per period: Reducing balance falls over the term; flat rate remains constant.
• Cost at same quoted rate: Reducing balance is lower; flat rate is substantially higher.
• Early settlement: Reducing balance requires no rebate; flat rate relies on a rebate method to determine any benefit.
• Overpayment: Reducing balance genuinely reduces future interest; flat rate may save nothing without a rebate.
• Transparency: Reducing balance is high (reconciles directly to the balance); flat rate is low (rate understates cost).

Converting between the two

As a working approximation for a fully amortising loan:

Reducing-balance equivalent ≈ flat rate × 1.8 to 2.0

The multiplier depends on the term. In the six-month example above, $426.90 of reducing-balance interest corresponds to a flat rate of about 14.2% — so 24% reducing ≈ 14.2% flat over six months. Over twelve months and longer, the ratio moves closer to 2×.

The direction is always the same: a flat rate understates the true cost of credit by roughly half. A borrower comparing a 14% flat quote against a 24% reducing quote is looking at two near-identical loans.

Why reducing balance matters downstream

The interest method determines whether several other borrower protections have any effect.

• Overpayment reduces future interest on a reducing balance loan. On a flat loan without a rebate, it may achieve nothing.
• Early settlement requires no rebate calculation — the settlement figure is principal plus accrued interest to date.
• Restructuring recalculates cleanly from the outstanding balance, with no unearned-interest complication.
• Statements reconcile. The balance a borrower sees is the amount they would need to pay, which removes an entire category of dispute.

Operational notes for lenders

• Choose and document the accrual basis — daily versus periodic, and the day count convention. Publish it in the agreement.
• Separate accrual from billing. Interest accrues daily; instalments fall monthly. Systems that conflate the two misstate settlement figures and arrears.
• Handle rounding at the final instalment. Two-decimal rounding across a schedule leaves a residual, normally absorbed in the last payment.
• Recalculate on every balance-changing event — overpayment, restructure, rate change, capitalisation of arrears — and reissue the schedule.
• Apply arrears interest deliberately. Interest continuing to accrue on an overdue balance is not the same as a penalty charge, and the two should be separately disclosed and separately reported.
• Quote reducing-balance rates. Where competitors quote flat, publishing an equivalent reducing-balance rate or an APR is a real differentiator with informed borrowers, and increasingly a regulatory expectation.