The Same 3 Percent, Two Very Different Interest Bills

The two methods, in one sentence each

Amortization for small lenders comes down to two methods. Flat add-on charges the rate against the original principal for the whole term, no matter how much has been repaid; declining balance charges the rate only against what is still owed. Same quoted rate, very different totals.

Most small financing companies in the Philippines quote flat add-on because it is easy to compute and easy to explain. Banks and most regulated products use declining balance. Neither is wrong. What is wrong is not knowing exactly what your own schedule implies.

How much does 3 percent flat add-on really cost?

The flat method is three lines of arithmetic:

• Monthly interest: ₱120,000 times 3 percent equals ₱3,600
• Total interest: ₱3,600 times 12 months equals ₱43,200
• Monthly payment: (₱120,000 plus ₱43,200) divided by 12 equals ₱13,600

Every installment is ₱13,600. Every installment carries the same ₱3,600 of interest and ₱10,000 of principal. Month 1 and month 12 look identical, even though by month 12 the borrower only owes ₱10,000 of principal. That is the defining trait of flat add-on: the borrower pays interest on money already returned.

What does the same loan cost on declining balance?

At 3 percent monthly on the declining balance, the standard amortization formula gives a monthly payment of about ₱12,055. The total paid over 12 months is about ₱144,666, so total interest is about ₱24,666.

Inside each payment, the mix shifts. Month 1: interest is ₱3,600 (3 percent of ₱120,000) and principal is about ₱8,455. By the final month, interest is only about ₱351 and nearly the whole payment is principal. The borrower pays interest only on what is still outstanding.

Put the two side by side and the gap is plain: ₱43,200 of interest on flat add-on versus about ₱24,666 on declining balance. The flat method collects roughly 75 percent more interest at the same quoted rate on this loan. As a rule of thumb, a flat add-on rate is equivalent to a declining balance rate close to double.

This is not an argument to abandon flat pricing. Flat add-on is legitimate, common in Philippine lending, and borrowers understand a fixed installment. The argument is for honesty with yourself: know your effective yield, disclose what the contract requires, and price deliberately instead of by habit.

Why this matters beyond pricing

The method you choose drives everything downstream. Early settlement is the sharpest case. On declining balance, a borrower who settles at month 6 simply pays the outstanding principal. On flat add-on, you need a rebate policy for unearned interest, and if that policy lives in someone's head, every early settlement becomes a negotiation.

Restructures, partial payments, and penalty bases all depend on the schedule too. If a payment is short by ₱2,000, is the penalty computed on the shortfall or the whole installment? Your schedule and your rules have to answer that the same way every time.

What your system should generate

Whichever method you use, the schedule should never be typed by hand in a spreadsheet. A proper lending system takes principal, rate, method, and term, and generates the full amortization table the moment the loan is booked: every due date, every installment split into principal and interest, and the running balance after each row. Payments post against specific installments, so the outstanding balance is always computed, never estimated.

The same generated schedule becomes the backbone of everything else your operation needs: the daily list of installments falling due, the aging of anything unpaid, the penalty base when a due date passes, and the borrower's printed statement. One computed table, produced once at booking, referenced everywhere. That is the quiet reason lending systems beat spreadsheets: the schedule stops being a document someone maintains and becomes a fact the rest of the business reads.

When a borrower asks why their balance is what it is, the answer should be a schedule the software produced, not a formula someone remembers writing.