Loan Calculator (Fixed Payment and Constant Principal)

You enter the loan amount, the monthly interest rate, the number of payments and the amortization system. The tool calculates each payment, the total paid at the end and how much of that is interest, and builds the month-by-month table.

The difference between the two systems is how the debt is paid down:

• Fixed payment (the annuity method, also known as the Price table) keeps the monthly payment the same from start to finish. Early on, most of the payment is interest and only a small part reduces the debt; over time, that proportion flips.
• Constant principal (the SAC method) keeps the amount paid toward the debt the same each month, so the payment starts higher and decreases as the balance — and the interest on it — shrinks.

In the table, you follow, for each payment, how much was interest, how much reduced the balance and how much is still owed. That makes it clear where your money goes over the life of the loan.

The simulation is useful before signing any loan — a car, a home or a personal loan. It helps you understand the real weight of the debt: how much you'll pay in interest overall, how the payment behaves over time, and which system suits your budget.

Comparing the two systems side by side is especially valuable for long loans, like mortgages. The constant-principal method usually has a higher first payment but pays less interest overall; the fixed payment fits the budget better at the start, with a predictable amount. Seeing the concrete numbers helps you decide clearly, rather than in the dark.

The most common mistake is confusing the monthly rate with the annual rate. A rate of 1.5% per month is very different from 1.5% per year — and because of compounding, it isn't simply twelve times either. Entering the rate for the wrong period completely distorts the simulation. This tool works with the monthly rate.

Another error is looking only at the payment amount and ignoring the total interest. A smaller payment can hide a long term that, in the end, costs much more. The total paid is the number that reveals the real cost of the loan.

It's also worth remembering that the simulation doesn't include mandatory insurance, administrative fees or taxes, which appear in real contracts and vary by lender. The result is a good basis for comparison, but the true annual percentage rate quoted by the lender will be somewhat higher.