Down Payment Optimiser
Compare two deposit amounts to see how much interest the larger down payment saves over the loan term.
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What this tool does
This calculator compares the total interest paid on a car loan at two different deposit levels. It uses standard amortisation algebra to compute the monthly payment for each scenario, multiplies by the term, subtracts the principal, and reports the difference. Inputs are vehicle price, APR, loan term in months, and two deposit amounts; the output is the interest saved (in pounds) by choosing the larger deposit.
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Formula
How the Down Payment Optimiser works
The tool calculates the monthly payment for a car loan at two different deposit amounts, then multiplies each payment by the number of months to find total repayment. Subtracting the amount borrowed (car price minus deposit) from total repayment yields total interest paid. The difference between the two interest figures is the saving generated by the larger deposit.
The formula
For each deposit scenario, the monthly payment M is computed using the standard loan amortisation formula:
M = P × [r(1 + r)n] / [(1 + r)n − 1]
where P is principal (price − deposit), r is the monthly interest rate (APR ÷ 12 ÷ 100), and n is the term in months. Total interest for that deposit is then (M × n) − P. The tool subtracts the larger-deposit interest from the smaller-deposit interest to show the saving.
Where this method is most accurate
The calculation assumes a fixed-rate loan with equal monthly instalments and no early-repayment penalties, fees, or balloon payment. It does not account for arrangement fees, payment-protection insurance, or changes in APR over the term. Results are most reliable when the APR entered reflects the actual annual percentage rate offered by the lender, inclusive of any compulsory fees rolled into the rate.
What this tool does not do
This calculator does not include taxes, registration fees, insurance, or any jurisdiction-specific charges. It does not evaluate whether a given deposit level is financially optimal in the context of alternative investments, emergency-fund requirements, or opportunity cost. The output is purely a comparison of interest charges under the two deposit scenarios entered; it does not recommend a specific deposit amount or endorse any lending product.
Disclaimer
This tool is provided for educational purposes and produces results based solely on the values entered and the published formula. It does not constitute financial advice, tax advice, or a recommendation to enter into any loan agreement. Users should verify loan terms with lenders and consult a qualified financial adviser before making borrowing decisions.
Questions
- Why does a larger deposit reduce total interest?
- A larger deposit reduces the principal borrowed. Because interest accrues on the outstanding balance, a smaller loan generates less interest over the same term and APR.
- Does this calculator include arrangement fees or early-repayment charges?
- No. The tool uses only the car price, APR, term, and two deposit amounts. Arrangement fees, payment-protection insurance, and early-settlement penalties are not included in the calculation.
- Can I compare more than two deposit amounts?
- The current implementation accepts exactly two deposit values. Running the tool multiple times with different pairs will produce a series of comparisons.
- What if my lender quotes a flat rate instead of APR?
- Flat rates and APR are calculated differently; entering a flat rate in the APR field will produce incorrect results. Convert the flat rate to an equivalent APR before using this tool, or ask the lender for the APR figure.
- Does the saving account for the opportunity cost of tying up cash in a deposit?
- No. The calculator shows only the difference in interest charges on the loan itself. It does not model alternative uses of the deposit funds, such as investment returns or liquidity needs.
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Sources & Methodology
The tool applies the standard fixed-rate amortisation formula M = P × [r(1+r)^n] / [(1+r)^n − 1] to compute the monthly payment for each deposit, multiplies by the term to find total repayment, subtracts the principal to isolate interest, and reports the difference. Formula sourced from established loan-amortisation algebra used in consumer finance.
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