Loan Amortization Calculator (Price & SAC)

Enter principal, rate, and term to estimate fixed or decreasing installments, total interest, and a month-by-month table before you sign.

Loan details

Amortization system

Fixed paymentSame payment every monthDecreasing paymentConstant principal each month

Loan principal

Nominal annual interest rate (%)

Term (months)

Your data is not stored. Everything runs in your browser.

Your result

0/mo

Total interest: 0
Total paid: 0
Term: 0 months

Assumes a fixed rate and the amortization system you pick, with no extra fees, grace periods, or early-payment rules your contract may have. Real contracts may include clauses not modeled here.

What is amortization?

Amortizing means paying the loan in installments until the balance hits zero. With a fixed payment and fixed rate, early payments are mostly interest; over time more of the same payment reduces what you owe (principal). That is why total interest and the principal-vs-interest curve matter.

Amortization schedule with payment, interest, and balance over time

Want to use this amortization schedule with clearer criteria before you sign?Read the guide →

Why simulate before you sign?

The monthly payment is only part of the story. Total interest, term, and how principal versus interest evolves help you compare offers, see the effect of changing the rate or term, and check affordability. This page does not replace your lender or contract; it shows the usual textbook math behind a quote.

When to use this calculator

Use it to rough out mortgages, personal loans, payroll-linked loans, or any setup close to “fixed monthly payment + annual rate in %.” It also helps you see total interest if you keep the same term and rate. If your lender adds fees, insurance, or a different interest convention, treat this as an approximation.

How to read the table

Each row is one month after the payment. The columns mean:

Month: payment number (1, 2, 3…).
Payment: your fixed installment in this model.
Principal: how much of that payment reduces what you still owe.
Interest: how much of that payment is the cost of borrowing (charged on the balance before the payment).
Balance after: remaining debt after that payment.

Practical tips

Try different terms: longer terms often lower the payment but can raise total interest a lot.
Always verify the rate and APR / total cost in official disclosures; here we only use the nominal annual rate you type, converted to a monthly rate.
If the bank payment differs, look for origination fees, insurance, or rounding in the contract.
Use the summary (payment, total interest, total paid) to compare two scenarios on the same basis.

Limitations

Educational model: fixed rate, equal payments, simple monthly compounding from nominal annual rate. No grace periods, variable payments, indexation, taxes, insurance, or prepayment rules. For contractual decisions, use the lender’s simulator and documents.

How the payment is calculated

The goal is a fixed payment that each month covers interest on the remaining balance and uses the rest to pay down principal, reaching zero balance on the last month.

Symbols: P = principal, r = monthly rate = (nominal APR %) ÷ 100 ÷ 12, n = months, M = payment. If APR is 0%, payment is simply P ÷ n.

r = (APR/100) / 12
M = P × r(1+r)^n / ((1+r)^n − 1)
Each month: interest = balance × r; principal part = M − interest; new balance = old balance − principal partr=APR1200M=P r(1+r)n(1+r)n−1I=B r,Ppr=M−I,Bnew=B−Ppr\begin{aligned}r&=\dfrac{\mathrm{APR}}{1200}\\M&=P\,\dfrac{r(1+r)^n}{(1+r)^n-1}\\I&=B\,r,\quad P_{\mathrm{pr}}=M-I,\quad B_{\mathrm{new}}=B-P_{\mathrm{pr}}\end{aligned}rMI​=1200APR​=P(1+r)n−1r(1+r)n​=Br,Ppr​=M−I,Bnew​=B−Ppr​​

Standard amortizing-loan payment formula; see e.g. Wikipedia “Amortizing loan” or finance textbooks.

FAQ

What is the difference between principal and interest in the table?

Principal is what you borrowed and are paying down. Interest is the cost on what you still owe. With a fixed payment, interest is larger early on because the balance is larger.

Why doesn’t my bank match exactly?

Contracts may add fees, insurance, different day-count rules, or rounding. This tool uses the textbook monthly model only.

Is APR the same as my contract rate?

You should enter the nominal annual rate your disclosure uses for monthly payment quotes. When unsure, check your loan documents.

What about extra payments or paying off early?

Many contracts allow prepayment or payoff with their own rules (fees, discounts, recalculation). This calculator does not model that. Use the output as the “full schedule” baseline and confirm rules with your lender.

Which system should I pick?

Fixed payment is common on personal loans. Decreasing payment (constant principal) appears on many mortgages, especially in Brazil and parts of Latin America. Compare total interest, not just the first payment.