🏦 Loan Calculator

🔧 Mode

📝 Loan details

📤 Summary

{{ primarySummary }}
Enter loan details — your payment and totals will appear here.

📊 Headline metrics

Monthly payment {{ formatMoney(monthlyPayment) }}
Total interest {{ formatMoney(totalInterest) }}
Total paid {{ formatMoney(totalPaid) }}
Payoff months {{ payoffMonths }}
Payoff date {{ payoffDateIso }}

🗂️ Amortization schedule

Showing the first 12 and last 12 months. Download the CSV for the full schedule ({{ scheduleRows.length }} rows).

# Date Principal Interest Extra Balance

📅 Year-by-year totals

Year Principal paid Interest paid Balance at year-end
{{ year.year }} {{ formatMoney(year.principalPaid) }} {{ formatMoney(year.interestPaid) }} {{ formatMoney(year.balance) }}
⚠️

ℹ️ About Loan Calculator

What is an amortizing loan?

An amortizing loan is a loan repaid in fixed, equal periodic payments. Each payment splits between interest (the lender's charge for the outstanding balance) and principal (the amount that reduces what you still owe). Early in the schedule most of the payment goes to interest; over time the share flips and most of it goes to principal.

This calculator answers the three questions borrowers ask most often: what is my monthly payment, how much interest will I pay over the life of the loan, and how does paying a little extra each month change the outcome.

Common use cases

  • Home mortgage: a 30-year fixed mortgage on $300,000 at 6.5% APR has a monthly payment near $1,896.20.
  • Auto loan: a 5-year $25,000 car loan at 7% has a monthly payment of about $495.
  • Student loan: compare a 10-year and 15-year payoff plan to see how the term affects total interest.
  • Personal loan / refinance: model how a lower rate or shorter term changes the lifetime cost.
  • Extra payment strategy: add an extra $200/month to a 30-year mortgage and watch the payoff date come forward by years.

The amortization formula

For a fixed-rate amortizing loan, the monthly payment P is derived from the present-value-of-annuity identity: 

P = L · r · (1 + r)^n / ((1 + r)^n − 1)

where
  L = loan amount (principal)
  r = monthly interest rate = annual rate / 12 (e.g. 6.5% APR → 0.005417)
  n = number of months = term in years × 12

For a zero-rate loan the formula collapses to P = L / n. Each month, interest charged is balance × r; the rest of the payment chips away at principal until the balance reaches zero.

What an extra payment does

An "extra payment" goes entirely to principal, in addition to the scheduled payment. Because the next month's interest is calculated on a smaller balance, each extra dollar of principal saves all of the interest that would have been charged on that dollar for the rest of the loan.

On a 30-year, $300,000, 6.5% mortgage, an extra $200/month cuts roughly 6 years and tens of thousands of dollars of interest off the total cost. The exact savings depend on rate and term — run the schedule mode below to see the impact for your loan.

Modes

  • Compute: monthly payment, total interest, total paid, payoff date, and a per-year summary.
  • Schedule: the full month-by-month amortization table — every interest and principal payment, plus the running balance. Download as CSV for spreadsheets.

References

Notes

  • Results are illustrative. Actual lender disclosures may include origination fees, mortgage insurance, escrow, and taxes that this tool does not model.
  • Calculations run entirely in your browser unless you call the API endpoint below.
  • The same TypeScript logic powers the API — you'll get identical results from either path.

API Usage Example:

POST /api/tools/math/loan-calculator
Content-Type: application/json

{
    "operation": "compute",
    "principal": 300000,
    "annualRatePercent": 6.5,
    "termYears": 30,
    "extraMonthlyPayment": 0
}
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