Updated August 31, 2026
In this guide
SAC formula
The Constant Amortization System divides principal by the term. For financed amount PV and n payments:
Each month, Iₜ = OBₜ₋₁ × i and Pₜ = A + Iₜ + costs. With no indexation or extra event, declining interest makes the payment fall.
Sample calculation
For BRL 300,000 over 360 months, theoretical amortization before cent adjustments is BRL 833.33 per month. The interest reference is shown inside the generated module.
BCB reference rate for fixed-rate real-estate financing: 2026-07, median of 3 institutions.
Parameters used
- Financed amount
- R$300,000.00
- Term
- 360 months
- Interest rate
- 14.9797% effective per year
- System
- SAC
- Monthly adjustment
- —
- Extra amortization
- —
Calculated results
| Scenario | SAC |
|---|---|
| First payment | R$4,343.33 |
| Last payment | R$844.29 |
| Total interest | R$633,557.33 |
| Total paid | R$933,557.33 |
| Effective term | 360 months |
| Accumulated adjustment | R$0.00 |
Selected installments
| Payment no. | Payment | Interest | Amortization | Closing balance |
|---|---|---|---|---|
| 1 | R$4,343.33 | R$3,510.00 | R$833.33 | R$299,166.67 |
| 12 | R$4,236.08 | R$3,402.75 | R$833.33 | R$290,000.04 |
| 60 | R$3,768.08 | R$2,934.75 | R$833.33 | R$250,000.20 |
| 180 | R$2,598.09 | R$1,764.76 | R$833.33 | R$150,000.60 |
| 360 | R$844.29 | R$9.76 | R$834.53 | R$0.00 |
What the curve shows
Without indexation, the balance falls almost linearly. Payments fall faster early on because each balance reduction removes more interest from the next period. The engine adjusts cents in the final row to close the balance.
SAC does not guarantee the lowest cost under every contract. Rate, index, term, insurance and other charges must be considered together.
What may differ in a contract
- Amortization remains constant only without balance indexation, grace periods, capitalized charges or extraordinary events.
- The collected payment may include insurance and fees outside the SAC formula.
Limits of this analysis
- The scenario excludes arrears, construction phases, grace periods and renegotiation.
- A repeated fixed adjustment rate is a hypothesis, not a forecast.
Sources and review
- Central Bank of Brazil — Citizen Calculator · July 27, 2026