Mortgage repayment calculator
Calculate your repayments and total interest over the loan term.
Calculator mortgageLogic updated January 2026
This calculator estimates your regular mortgage repayment amount based on the loan principal, interest rate, and loan term. It uses standard amortisation to show how much you'll pay each period and the total cost over the life of the loan.
How this is calculated
Formula
M = P × [r(1+r)^n] / [(1+r)^n – 1]Step-by-step
- Convert the annual interest rate to a periodic rate by dividing by the number of payments per year
- Calculate the total number of payments (term in years × payment frequency)
- Apply the standard amortisation formula to determine the fixed periodic payment
- Multiply the periodic payment by the total number of periods to get the total repaid
- Subtract the original principal from the total repaid to get the total interest
- Rounding mode
- ROUND_HALF_UP
- Precision
- 20-digit internal precision (Decimal.js), rounded to 2 decimal places for display
- Logic last reviewed
Assumptions & limitations
What this calculator assumes
- Fixed interest rate for the entire loan term
- Repayments made at the end of each period (ordinary annuity)
- No offset account or redraw facility
- No fees, charges, or lenders mortgage insurance included
- Compounding frequency matches the repayment frequency
What this calculator doesn’t account for
- Does not model variable or split interest rates
- Does not include establishment fees, valuation fees, or ongoing account fees
- Does not account for rate changes during the loan term
- Does not model offset or redraw balances reducing interest
- Does not include lenders mortgage insurance for high-LVR loans
Worked example
A borrower takes out a 30-year mortgage for $500,000 at 6.5% per annum with monthly repayments.
| Input | Value |
|---|---|
| Loan amount | $500,000 |
| Annual interest rate | 6.5% |
| Loan term | 30 years |
| Repayment frequency | Monthly |
Monthly repayment: $3,160.34
Over 30 years, the borrower repays a total of $1,137,722.40 — meaning $637,722.40 is interest. The monthly rate is 6.5% ÷ 12 = 0.5417%, applied to a declining balance over 360 payments.
Frequently asked questions
What is an amortisation schedule?
An amortisation schedule is a table showing each repayment broken down into principal and interest components. Early repayments are mostly interest; later repayments are mostly principal. This calculator generates the schedule automatically.
How does repayment frequency affect total interest?
More frequent repayments (fortnightly or weekly instead of monthly) reduce total interest because the principal balance decreases faster. Fortnightly repayments on a 30-year loan can save several years of interest.
What happens if interest rates change?
This calculator models a fixed rate for the entire term. In practice, variable-rate loans change periodically. Use the extra repayment calculator to model the impact of rate changes on your repayment amount.
Why doesn't this calculator include fees?
Mortgage fees (establishment, valuation, ongoing) vary significantly between lenders and products. This calculator focuses on the core principal-and-interest repayment. Use the comparison rate calculator to see the effective rate including fees.
How accurate is this calculator?
This calculator uses the standard amortisation formula with 20-digit internal precision and banker's rounding. The results match those produced by major lending institutions for fixed-rate loans. However, actual repayments may differ due to fees, rate changes, or offset account balances.
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