Money & Budget
Loan / Installment Calculator
This calculator uses the standard amortisation formula to show what a loan costs each month and in total, including how much of it is interest.
Calculator
The calculation runs in your browser.
What is a Loan / Installment Calculator?
A loan installment calculator estimates the regular payment and total borrowing cost from the principal, interest rate and repayment period. It is useful for comparing scenarios, but lender fees, variable rates, insurance and payment schedules can change the actual amount you pay.
Formula and calculation method
For a standard amortizing loan, the payment is calculated from principal, periodic interest rate and number of payments. Total interest is the modeled total repayment minus the original principal.
What this calculator does
Uses the standard amortisation formula to turn a loan amount, annual interest rate and term into a fixed monthly payment, then shows the total repayment, total interest and how much of the first payment is interest.
How to interpret the result
The monthly payment is the number to test against your budget. Total interest shows the real cost of borrowing beyond the principal: comparing a short and a long term side by side usually reveals that the longer term costs far more overall despite the smaller payment.
Assumptions
- The interest rate is fixed for the full term.
- Payments are monthly and level for the whole loan.
- No arrangement fees, insurance or early-repayment charges are included.
Limitations
- Variable-rate loans can change payment size mid-term; this tool cannot model that.
- Fees and mandatory insurance raise the effective rate above the advertised one.
- Overpayments are not modelled directly — approximate by shortening the term.
How it works
The monthly payment uses P × r ÷ (1 − (1 + r)⁻ⁿ), where P is the amount borrowed, r is the annual rate divided by twelve, and n is the number of monthly payments. Total repayment is the payment multiplied by n, and interest is that total minus the amount borrowed.
Worked example
Borrowing 18,000 at 8.5% over five years gives a payment of about 369 a month. Across 60 payments that is roughly 22,150 repaid, of which about 4,150 is interest.
How to use it
- Enter the amount you will actually borrow, after any deposit.
- Use the annual rate, not a monthly one — the calculator divides it for you.
- Compare three and five year terms to see the trade-off between payment size and total interest.
- Check the payment against the margin from the Monthly Budget Calculator before committing.
Factors to consider
- Early payments are mostly interest, which is why paying extra early saves the most.
- Fees and mandatory insurance raise the effective rate above the advertised one.
- Variable-rate loans can change payment size mid-term; this calculator assumes a fixed rate.
Useful for
- Checking a quoted monthly payment against your budget before signing.
- Comparing a 3-year and a 5-year term to see the trade-off between payment size and total interest.
- Seeing how much a larger deposit (smaller loan amount) saves in interest over the term.
Understand what the monthly payment hides
A monthly installment is easy to compare but can hide the total cost of borrowing. Two loans can have similar payments while one lasts longer and accumulates much more interest. This calculator shows the payment, total repayment and modeled interest so you can see both sides of the decision. Start with the amount you actually need to borrow, not the maximum a lender is willing to offer. A smaller principal can reduce both the monthly payment and the total interest without changing the advertised rate.
Compare term length deliberately
Shorter repayment periods usually produce higher monthly payments but less total interest. Longer terms reduce the monthly burden while keeping the balance outstanding for more months. There is no universal best term because affordability matters, but the trade-off should be visible before you sign. Run at least two or three terms with the same principal and rate. Then compare the payment with the margin in your monthly budget. A payment that looks affordable in isolation may be uncomfortable once normal living costs and an emergency buffer are included.
Check the lender's full cost
The calculator models a standard amortising loan and cannot know every lender-specific charge. Arrangement fees, insurance, early repayment rules, variable rates and different compounding conventions can change the actual cost. When you receive an offer, compare its total repayment and effective rate with your calculator result. If they differ, ask why. The difference may be legitimate, but understanding it protects you from comparing a simple headline rate with a more expensive real contract.
Use the result before borrowing, not after
The most useful time to calculate a loan is before you commit. Test a smaller amount, a shorter term and a higher rate to see how much room you have. If the decision becomes unaffordable under a modest change in assumptions, consider that a warning. For existing loans, the calculator can also help you understand why an early overpayment may reduce interest, although your lender's terms determine what you are actually allowed to do. The calculation is a planning tool, not a substitute for reading the loan agreement.
Frequently asked questions
Why does a longer term cost more overall?
Interest accrues on the outstanding balance for longer, so a lower payment across more months usually means more total interest.
Does a 0% rate work?
Yes. The calculator divides the amount evenly across the months and reports no interest.
Can I model overpayments?
Not directly. As an approximation, shorten the duration and see how the total interest falls.