Interest-Only Mortgage Calculator Formula

Understand the math behind the interest-only mortgage calculator. Each variable explained with a worked example.

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Formulas Used

Interest-Only Monthly Payment

io_monthly = io_payment

Amortizing Monthly Payment

amort_monthly = amort_payment

Payment Increase at Amortization

payment_shock = amort_payment - io_payment

Total Interest (IO period)

total_io_interest = io_payment * io_period_years * 12

Total Interest (amortizing period)

total_amort_interest = amort_payment * amort_months - loan_amount

Total Interest Over Loan Life

grand_total_interest = io_payment * io_period_years * 12 + amort_payment * amort_months - loan_amount

Variables

VariableDescriptionDefault
loan_amountLoan Amount(USD)500000
interest_rateInterest Rate(%)7
io_period_yearsInterest-Only Period(years)10
total_term_yearsTotal Loan Term(years)30
rDerived value= interest_rate / 100 / 12calculated
io_paymentDerived value= loan_amount * rcalculated
amort_monthsDerived value= (total_term_years - io_period_years) * 12calculated
amort_paymentDerived value= r > 0 ? loan_amount * r * pow(1 + r, amort_months) / (pow(1 + r, amort_months) - 1) : loan_amount / amort_monthscalculated

How It Works

Interest-Only Mortgages

An interest-only mortgage allows you to pay only the interest for an initial period (typically 5-10 years), after which the loan converts to fully amortizing payments.

How Payments Work

Interest-Only Period: Monthly Payment = Loan Balance x (Annual Rate / 12)

Amortizing Period: Standard P&I payment calculated on the full balance over remaining years

Advantages

  • Lower initial payments for improved cash flow
  • Useful for investors who plan to sell before amortization begins
  • Can invest the difference between IO and fully amortizing payments

    • Risks

  • No equity built during IO period (except through appreciation)
  • Payment shock when amortization begins
  • Higher total interest paid over the loan life

    • Worked Example

    A $500,000 loan at 7% interest, 10-year IO period, 30-year total term.

    loan_amount = 500000interest_rate = 7io_period_years = 10total_term_years = 30
    1. 01Monthly rate: 7% / 12 = 0.5833%
    2. 02IO payment: $500,000 x 0.005833 = $2,916.67/month
    3. 03IO period interest: $2,916.67 x 120 = $350,000
    4. 04Amortizing period: 20 years = 240 months
    5. 05Amortizing payment: $500,000 over 240 months at 7% = $3,876.83/month
    6. 06Payment shock: $3,876.83 - $2,916.67 = $960.16 increase
    7. 07Amortizing period interest: $3,876.83 x 240 - $500,000 = $430,439
    8. 08Total interest: $350,000 + $430,439 = $780,439

    Ready to run the numbers?

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