fFinMath

Loan Payment Calculator

Monthly payment, total interest, amortization, extra-payment savings, and refinance comparison. Personal, auto, student, and business loans.

Loan Inputs

$
%
$
Additional principal payment each month, applied on top of the regular payment.
Monthly Payment
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USD
Total Interest
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Total Paid
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Payoff Time
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Interest Saved (extra)
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How Loan Payments Work

A standard amortizing loan (personal, auto, student, most business loans) is structured so each monthly payment is identical, but the split between principal and interest changes over time. Early payments are mostly interest; later payments are mostly principal. This calculator shows that split month by month in the amortization schedule.

The math:

M = P × r × (1 + r)n / ((1 + r)n − 1)

Where P is the loan principal, r is the monthly interest rate (annual ÷ 12), and n is the total number of monthly payments.

The Power of Extra Payments

An extra $50/month on a $30,000 auto loan at 7.5% for 5 years shaves about 5 months off the term and saves a few hundred dollars in interest. On a 5-year $100K business loan at 8.5%, an extra $200/month saves over $2,000 and closes the loan ~6 months early.

The savings compound on longer loans. Extra payments always apply to principal first, which shrinks the balance that interest is calculated on.

When Refinancing Pays Off

Refinancing replaces your existing loan with a new one — ideally at a lower rate. The decision depends on three numbers:

  1. Rate difference: a 1% rate drop is the common rule of thumb.
  2. Closing costs: origination fees, legal fees, prepayment penalties on the old loan.
  3. Break-even time: months of lower payments needed to offset closing costs.

Enable "Compare with refinance scenario" above to see net savings after closing costs.

APR vs Interest Rate

The interest rate is the cost of borrowing the money. APR (Annual Percentage Rate) adds origination fees, mortgage insurance (for home loans), and other required fees into a single annualized number. APR is always ≥ interest rate. Always compare loans by APR.

This calculator uses the interest rate directly. To model APR, simply enter the APR as the rate. The monthly payment shown will then include the amortized cost of fees.

Loan Types & Typical Terms

  • Personal loans — unsecured, 2-7 year terms, rates 7-36% depending on credit. Used for consolidation, emergencies, major purchases.
  • Auto loans — secured by the vehicle, 3-7 years, rates 4-12% for new cars. Dealer-financed rates typically higher than credit union.
  • Student loans — federal fixed-rate (4-8%) or private variable. Longer terms, some with income-driven repayment.
  • Business / SME loans — 1-10 years, secured or SBA-backed. Rates vary widely (6-15%+) with risk profile.

Limitations

This calculator assumes a fixed interest rate and equal monthly payments. Variable-rate loans (HELOCs, some private student loans, some business credit lines) shift payments when the underlying index moves. For mortgages, use the dedicated Mortgage Calculator with PMI, tax, and insurance. For bond yields, see the Bond YTM Calculator.

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