SAC Amortization Schedule

How the SAC amortization system works

In the SAC system (Sistema de Amortização Constante), the principal slice of each installment never changes. You get it from Amort = PV / n, where PV is the financed amount and n the number of installments. The interest for a given month is whatever the outstanding balance is, multiplied by the rate i, so the full installment comes out to PMT_k = Amort + saldo_(k−1) · i. Since the balance keeps dropping, so does the interest, and the installments shrink along with it. That makes the first payment the heaviest and the last one the lightest.

Say you finance R$ 100,000 at 1% per month over 100 months. The constant amortization works out to R$ 1,000. Your first installment is 1,000 + 100,000 · 0.01 = R$ 2,000, and the last one is 1,000 + 1,000 · 0.01 = R$ 1,010. Add it all up and you pay roughly R$ 50,500 in interest. That's a good deal less than Price would charge on the same contract, since the balance comes down faster here.

Real-world applications

When it comes to long-term housing finance in Brazil under SFH and SFI, SAC is the go-to. Caixa Econômica Federal has made it the standard for new mortgages since 2018, largely because installments that decline over time hold up well against inflation and rising salaries. You'll also find SAC in BNDES long-term project finance, infrastructure debt, agribusiness credit and equipment leasing, situations where the borrower's cash flow usually grows faster than the original installment falls.

FAQ

Why are SAC installments higher at first? The principal portion stays the same every month, but at the start your balance is at its highest, which means the interest charge is too. Once you start chipping away at the balance, the interest part shrinks and the installments come down.

Does SAC really pay less total interest than Price? For the same rate and term, yes. SAC pays down principal faster, so your average outstanding balance stays lower, and interest is only ever charged against that smaller balance.

Can I switch between SAC and Price mid-contract? Usually not, since the system gets locked in when you sign. A few banks will let you renegotiate in a way that rewrites the schedule, but you should count on new fees and a fresh CET calculation if they do.