SAC Amortization Schedule: First N Payments

Line-by-line SAC amortization schedule

This tool lays out the schedule of a SAC (Sistema de Amortização Constante) loan one row at a time. Every month k shows four columns: the opening balance, the interest = balance · i, the amortization = PV / n, and the installment = amortization + interest. Amortization never changes. Because the balance drops by that fixed amount each month, the interest and the total installment both decline in a straight line.

Say you borrow R$ 100,000 at 1% per month over 100 months. Amortization holds at R$ 1,000. In month 1 the interest is 100,000 · 0.01 = 1,000, so the installment is R$ 2,000. By month 100 the balance is down to 1,000, interest is just 10, and the installment is R$ 1,010. Across the whole loan you pay about R$ 50,500 in interest, against roughly R$ 58,700 under Tabela Price for the same contract.

Real-world applications

For SFH/SFI mortgage planning the line-by-line view is hard to do without. Caixa has run SAC as its default for housing finance since 2018. You can use the schedule to forecast the first installment that feeds your debt-to-income ratio, to test extra payments where an unscheduled amortization immediately lowers the next installment and shortens the run, and to stack it up against Price on the same PV, i and n.

FAQ

Why does the installment fall linearly? Amortization is fixed and the balance drops by that same amount every month, which means the interest (balance · i) shrinks by a constant step as well.

What is the first installment worth in SAC? It comes to PV/n + PV · i. That's the biggest payment in the whole schedule, and it's the one banks lean on to test borrower capacity.

If I make an extra payment, what changes in the next row? The opening balance falls by the extra amount, so the next interest line and the next installment come down too. Amortization stays where it was.