How Loan Amortization Works

Amortization is the process of paying a loan down through scheduled installments. Each payment covers the interest for that period and reduces principal. A schedule shows this movement from the opening balance to zero.

What each schedule column means

• Payment number: where you are in the agreed repayment sequence.
• Payment date: the expected due date, subject to the lender’s calendar rules.
• Installment: the total scheduled payment for that period.
• Principal: the portion that reduces the amount borrowed.
• Interest: the cost charged for that period.
• Closing balance: principal remaining after the payment.

Why early payments contain more interest

With an amortised reducing-balance loan, the outstanding balance is largest at the start. The first interest charge is therefore larger. Because the total installment is level, less of that early payment goes to principal. As the balance falls, interest falls and more of the same installment reduces principal.

The standard equal-payment formula

The periodic payment uses principal, the interest rate per repayment period and the number of payments. An annual nominal rate must first be converted to the repayment period—for example, divided by 12 for monthly repayments. If the period rate is zero, payment is simply principal divided by the number of installments.

In words, the formula selects one level payment whose present value equals the amount borrowed. Each period, interest is first calculated from the opening principal balance. The remainder of the payment reduces principal. Repeating that process produces a closing balance close to zero after the final scheduled payment.

Worked first two payments

Consider KES 300,000 repaid monthly over 24 months at a nominal annual reducing-balance rate of 12%, with no fees. The monthly rate is 1%, and the estimated level payment is KES 14,122.04.

The payment stays the same, but the interest portion falls by about KES 111 between these two rows because the second calculation starts with a smaller balance. The principal portion rises by the same amount.

Over the full example schedule, estimated payments total about KES 338,929 and estimated interest is about KES 38,929. The final line may differ by a few cents because a correctly implemented schedule adjusts for accumulated rounding rather than leaving a tiny balance.

Amortised payment is not the only reducing-balance structure

Some loans use equal principal instead of equal total payments. With KES 300,000 over 24 months, equal principal would be KES 12,500 each month. Interest would be added to that amount based on the current balance, so the first total payment would be higher and later payments would fall.

Both structures calculate interest from a reducing balance, but their cash-flow patterns differ. Ask whether the quoted installment is level or declining.

What an extra payment can change

An extra principal payment reduces the balance used for future interest. Depending on the contract and system, the lender may keep the same installment and shorten the term, or recalculate the installment while keeping the original maturity date. Tell the lender how the extra amount should be applied and obtain an updated schedule.

Paying an installment early is not always the same as making an extra principal payment. It may simply satisfy the next due amount. Confirm the allocation on the statement.

How to audit a schedule quickly

You can perform four useful checks without rebuilding the entire loan:

1. The opening balance on a row should equal the preceding closing balance.
2. Principal plus interest should equal the installment, excluding separately shown charges.
3. Closing balance should equal opening balance minus principal.
4. The final balance should reach zero, subject to a disclosed final rounding adjustment.

If the schedule contains fees, penalties, insurance or rate changes, those items should be identifiable rather than silently described as principal or ordinary interest.

Why your official schedule may differ

• Fees or insurance may be financed or deducted.
• A lender may use equal principal rather than equal total installments.
• Dates may use exact day counts, business-day adjustments or payroll cycles.
• Rates may change or be stated per month rather than per year.
• Each payment may be rounded, with the final installment adjusted.
• Late, early or extra payments change the actual balance.

The official statement can also differ from the original schedule after a missed payment, partial payment, restructuring or rate change. A schedule is a plan; the account statement records what actually happened.