Borrowing and Repayment
A practical guide to loans and mortgages that explains amortisation, payment structure, interest allocation, and the trade-off between payment size, total interest, and repayment speed.
Key formulas
Where i is the periodic rate and n is the number of payments.
Each period starts by charging interest on the remaining balance.
The rest of the payment reduces the balance.
Useful for comparing term choices.
Amortisation is the key idea
A loan repayment is not just a monthly bill. It is a structured split between interest on the outstanding balance and repayment of principal. Early in the term, more of the payment usually goes to interest because the balance is still large. Later, more of the same payment goes to reducing principal.
That changing split is what amortisation describes. Once you understand that, monthly-payment calculators become easier to interpret and compare.
The payment depends on four things
At a basic level, the repayment is driven by the amount borrowed, the interest rate, the payment frequency, and the number of payments. Change any one of those and the payment changes. Stretching the term usually lowers the payment but increases total interest. Shortening the term usually does the reverse.
That trade-off is often the central borrowing decision. A lower payment can improve monthly affordability while quietly raising the lifetime cost of the debt.
Worked example: standard repayment loan
Take a 20000 loan at 7 percent annual interest over 5 years with monthly payments. The calculator gives a payment that is high enough to cover the interest charged that month and still chip away at principal. Over time the interest share shrinks and the principal share grows.
What matters is not only whether the payment is affordable today, but also what total interest bill you accept in exchange for that affordability.
Worked example: mortgage term comparison
A 25-year mortgage and a 30-year mortgage on the same balance can differ meaningfully in monthly payment, but the total interest bill is often where the real contrast shows up. The longer term spreads the burden but gives interest more time to accumulate.
That is why borrowers should compare more than the payment headline. The term changes the whole cost profile of the loan.
Why extra payments matter so much
Extra payments usually work best when they directly reduce principal, because every principal reduction lowers the future interest base. Even modest recurring overpayments can shorten the term and reduce total interest far more than many people expect.
The exact effect depends on the lender rules and loan structure, but the general principle is stable: principal reduced early has the most time to save later interest.
Common interpretation mistakes
- Treating the monthly payment as the full cost of borrowing instead of checking total interest over the whole term.
- Comparing loans without matching payment frequency and compounding assumptions.
- Ignoring fees, insurance, taxes, or other borrowing costs that sit outside the pure formula.
- Assuming an introductory or variable rate will behave like a fixed rate for the whole term.
- Forgetting that affordability stress-testing matters if rates or income conditions change.
Apply the topic straight away.
Loan Payment Calculator
Calculate a fixed-payment loan from the amount borrowed, rate, and term, using only the values you enter.
Mortgage Payment Calculator
Estimate a standard repayment mortgage from property price, deposit, rate, and term with clear monthly costs and supporting figures.
Present Value Calculator
Discount a future amount back to today's terms using the annual rate and time period you enter.
Compound Interest Calculator
Project compounded growth from a starting balance, contribution schedule, rate, and compounding frequency using only the values you enter.