Intro
A mortgage calculator is useful for comparing assumptions, but the monthly payment is only one part of the cost of borrowing. The loan amount, interest rate, term, taxes, insurance, and fees all matter.
For a standard fixed-rate repayment loan, each payment's interest is the current balance multiplied by the periodic rate. The principal part is the payment minus that interest, and the next balance is the old balance minus the principal part. This guide explains that calculation and the limits of a basic estimate.
Loan amount comes after the deposit
The amount borrowed is the property price minus the deposit. If a calculator asks for a deposit percentage, the equivalent calculation is loan = property price * (1 - deposit percentage / 100). A larger deposit reduces the balance on which interest is calculated and can also change the lending products available.
Why term changes total interest
A longer term spreads repayment across more months, often reducing the monthly payment. It also gives interest more time to accumulate, so total paid can be higher.
What calculators leave out
A simple estimate normally excludes taxes, insurance, maintenance, legal costs, arrangement fees, points, and variable-rate changes. Add those separately when building a real budget.
What the monthly estimate represents
For a repayment mortgage with a fixed periodic rate, the payment covers interest on the outstanding balance plus a principal amount. The familiar amortisation formula assumes a constant rate and regular payments; real products may use different compounding, payment dates, introductory rates, or overpayment rules. Treat the result as a model, not an offer.
A deposit changes the loan amount and often the loan-to-value band, which can affect the available rate. A longer term usually lowers the required monthly payment but gives interest more time to accumulate. Compare both the monthly affordability and the total amount paid, not one number in isolation.
The NAB Tools calculator treats its annual interest-rate input as a nominal annual rate and divides it by 12 to model monthly payments. That is an assumption, not a definition of every mortgage rate. APR can include fees and other costs, and lenders may calculate interest using different compounding or payment-date rules. Do not enter an APR as though it were the same as the calculator's rate without checking what the lender means.
Costs an online estimate may miss
Budget for arrangement or valuation fees, legal work, insurance, taxes, service charges, maintenance, and changes after a fixed-rate period. A lender may assess income, existing debt, credit history, property value, affordability stress, and repayment type. Variable rates can change the payment; interest-only lending has a separate principal repayment problem.
Use realistic scenarios: a higher rate, a lower income, a larger repair, and a shorter or longer term. Record whether the calculator uses a monthly or annual rate and whether fees are financed. Rounding each payment can produce a small difference from a lender's schedule.
This calculator is useful for exploring assumptions before speaking to a lender or adviser. It is not financial advice, a credit decision, or a guarantee that a particular property or payment is affordable.
The repayment formula and a worked example
For a standard fixed-rate repayment loan, the periodic payment can be represented as M = P * [r(1+r)^n] / [(1+r)^n - 1], where P is the principal, r is the periodic interest rate, and n is the number of payments. The formula assumes regular payments and a rate that remains constant for the model. A lender's schedule may differ because of compounding, payment dates, fees, rounding, or product rules. If the rate is zero, the payment is simply P / n.
For illustration only, a $200,000 loan over 25 years at a 5% nominal annual rate uses a monthly rate of approximately 0.05 / 12 and 300 payments. The estimated payment is $1,169.18. In the first month, interest is $833.33 ($200,000 * 0.05 / 12), so $335.85 of that payment reduces principal and the balance becomes about $199,664.15. The useful lesson is not the exact number; changing the principal, rate, or term changes both the payment and the total interest. Recalculate rather than applying a percentage shortcut.
Read an amortisation schedule
To calculate any row in an amortisation schedule, use interest = opening balance * periodic rate, then principal = payment - interest, then closing balance = opening balance - principal. Early payments normally contain a larger interest portion because interest is calculated on a larger outstanding balance. At a zero rate, the interest part is zero and the whole regular payment reduces principal. An overpayment can reduce total interest, shorten the term, or trigger an early-repayment charge depending on the mortgage contract. Compare the lender's illustration rather than assuming every overpayment has the same effect.
APR and the headline rate are not interchangeable. APR-style measures may include certain fees and assumptions, but coverage varies by jurisdiction and product. Compare like with like, ask which costs are included, and keep the currency, payment frequency, rate type, and term visible in any comparison.
Practical takeaway
Use a mortgage estimate to compare assumptions, not to make a final affordability decision. Test rates, terms, fees, and ownership costs, keep the assumptions visible, and confirm the result with a lender or qualified adviser. The calculator is most useful when it exposes trade-offs rather than producing one reassuring monthly number.
FAQ
How do I calculate the principal paid in one mortgage payment?
Multiply the opening balance by the periodic interest rate to get that payment's interest. Subtract the result from the scheduled payment; what remains is the principal paid. Then subtract that principal amount from the opening balance to get the next balance. Check the lender's schedule when compounding, payment dates, fees, or rounding differ from the simple model.
Does a mortgage calculator include taxes and insurance?
The basic estimate here includes principal and interest only. Taxes, insurance, and fees are excluded.
Why can a longer mortgage cost more overall?
The balance is outstanding for more monthly periods, so the accumulated interest can be higher even if each payment is smaller.
Is the result a guaranteed mortgage offer?
No. It is an estimate for comparison. A lender will use its own product terms, affordability checks, fees, and rate.