What this calculator does
Estimate payments and total interest for a mortgage with an initial interest-only period followed by amortizing payments.
How it works
The calculator evaluates interest-only payment = principal × annual rate ÷ 12; remaining principal is amortized over the remaining term. Inputs are checked before calculation, and invalid values such as zero denominators or out-of-range selections produce a clear error instead of a misleading result.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
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?loanAmount=Mortgage principalMoney · Required · Default: 400,000
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?annualRate=Annual interest ratePercentage · Required · Default: 6.5
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?totalTermYears=Total mortgage term in yearsNumber · Required · Default: 30
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?interestOnlyYears=Interest-only yearsNumber · Required · Default: 10
Outputs
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interestOnlyPaymentMonthly interest-only paymentCurrency · Primary output
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amortizingPaymentMonthly payment after interest-only periodCurrency
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paymentIncreasePayment increaseCurrency
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totalInterestTotal interest over full termCurrency
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totalPaidTotal paidCurrency
Inputs and results
The inputs are Mortgage principal, Annual interest rate, Total mortgage term in years, Interest-only years. Results include Monthly interest-only payment, Monthly payment after interest-only period, Payment increase, Total interest over full term, Total paid.
Important notes
Mortgage terms, escrow, insurance, taxes, fees, and adjustable-rate provisions can materially change actual payments.