Loan & Mortgage Amortization Calculator

Turn a loan amount, an interest rate, and a term into a full amortization schedule. The actor returns one row per payment showing how much goes to principal, how much to interest, and what balance is left. It runs offline with no API and no key, so a run never breaks and never rate limits.

Good for mortgages, car loans, student loans, personal loans, and any fixed-rate installment loan. Feed it several loans at once and export the schedule as CSV, Excel, JSON, or an API feed.

What it does

For each loan you give it, the actor computes the periodic payment with the standard amortization formula, then walks the balance down payment by payment. You get the interest and principal split for every period, the running balance, and cumulative totals.

Add an extra payment and it shows how much interest you save and how many payments you skip. Switch the frequency to biweekly, weekly, quarterly, or annual and it recomputes the whole schedule.

Features

• Full amortization schedule, one row per payment
• Multiple loans in a single run
• Extra recurring payment, with interest saved and payments saved
• Monthly, biweekly, weekly, quarterly, or annual frequency
• Term set in years or in months
• Summary mode: one row per loan with the headline numbers
• Zero interest loans and irregular terms handled correctly
• Pure offline math, no third-party API, no key, no rate limits

Input

Field	Type	Description
loans	array	One or more loans. Each takes amount, annualRate (percent), and termYears or termMonths. Optional: label, extraPayment, startDate.
paymentFrequency	string	monthly, biweekly, weekly, quarterly, or annually. Default monthly.
outputMode	string	schedule for one row per payment, summary for one row per loan. Default schedule.
currency	string	A label added to each row. It tags the output, it does not convert money.
roundCents	boolean	Round money to two decimals. Default true.

Example input

{
  "loans": [
    { "label": "30-year mortgage", "amount": 300000, "annualRate": 6.5, "termYears": 30 },
    { "label": "5-year car loan", "amount": 25000, "annualRate": 7.9, "termYears": 5, "extraPayment": 100 }
  ],
  "paymentFrequency": "monthly",
  "outputMode": "schedule"
}

Output

Each schedule row has these fields:

Field	Description
loanLabel	The label of the loan this row belongs to
paymentNumber	Payment index, starting at 1
paymentDate	ISO date of the payment
beginningBalance	Balance before the payment
scheduledPayment	The regular payment amount
extraPayment	Extra amount applied this period
totalPayment	Scheduled plus extra
principalPaid	Part that reduces the balance
interestPaid	Part that covers interest
endingBalance	Balance after the payment
cumulativeInterest	Interest paid so far
cumulativePrincipal	Principal paid so far
currency	Currency label

Summary mode returns one row per loan with scheduledPayment, numberOfPayments, totalPaid, totalInterest, payoffDate, and, when an extra payment is set, interestSaved and paymentsSaved.

Example output row

{
  "loanLabel": "30-year mortgage",
  "paymentNumber": 1,
  "paymentDate": "2026-02-01",
  "beginningBalance": 300000.0,
  "scheduledPayment": 1896.2,
  "extraPayment": 0.0,
  "totalPayment": 1896.2,
  "principalPaid": 271.2,
  "interestPaid": 1625.0,
  "endingBalance": 299728.8,
  "cumulativeInterest": 1625.0,
  "cumulativePrincipal": 271.2,
  "currency": "USD"
}

How the math works

The periodic payment uses the standard formula P * r / (1 - (1 + r)^-n), where P is the amount, r is the periodic rate (annual rate divided by payments per year), and n is the number of payments. Interest each period is the current balance times r. The rest of the payment reduces the balance. A zero-rate loan splits the amount evenly across the term.

Notes

The currency field is a label only. The actor does not fetch exchange rates or convert between currencies. Results are estimates for planning and do not account for fees, insurance, taxes, or rate changes on variable-rate loans.