Principal Paid User Guide

Most people who take out a home loan or vehicle finance are surprised at how slowly the outstanding balance moves in the early years. Regular repayments are made, but the majority of each payment goes to the bank as interest — not to reducing the debt itself. This guide explains both calculators on Principal Paid: the bond and mortgage calculator and the car loan calculator, and the amortisation principles that underpin them.

¶Why your early repayments are mostly interest #

When you take out a fixed-rate loan your monthly repayment stays constant throughout the term, but the split between interest and capital changes dramatically over time. In the early years the vast majority of each payment goes to the bank as interest, with only a small portion reducing your outstanding balance. This is because interest is calculated on the remaining balance — which is at its highest at the start. As you pay down the principal the interest portion shrinks and the capital portion grows, but this shift happens slowly and only accelerates meaningfully in the final third of the term.

This matters for several practical reasons. If you sell a property or settle a vehicle loan early you may find you owe far more than you expected, because so little capital has actually been repaid. It also affects your equity position — homeowners who assume they have built up substantial equity after several years of repayments are often disappointed to discover how little the outstanding balance has moved. Understanding this curve is essential for decisions about refinancing, early settlement, or whether to make additional payments.

Making even small additional payments against the principal early in a loan has a disproportionately large effect on total interest paid. Every unit of principal you reduce today eliminates the interest that would have accrued on it for every remaining month — early capital reduction compounds in your favour in the same way that interest compounds against you.

¶Bond & mortgage calculator #

Start by entering your loan amount — the full purchase price of the property you are financing. If you are putting down a deposit, enter that separately and the calculator works out the net amount financed. Select your currency and country: the annual interest rate will pre-fill from the local prime lending rate, which is a reasonable starting point for most mortgage products. Enter your loan term in years or months, then hit Calculate.

The Principal Capital Paid After field is what sets this tool apart from a standard repayment calculator. Enter any point in your loan term — say year 5 on a 20-year loan — and the results panel shows exactly how much of the original loan you have paid off by that point versus how much has gone to interest. You can also click directly on any point along the chart to update this field instantly. Once calculated, use the Share button to copy a link that restores your exact inputs — useful for comparing scenarios or sending a calculation to someone else.

¶Paying more than the minimum #

The Custom Repayment Amount toggle lets you model the impact of paying more than the minimum each month. When enabled, it pre-fills a suggested round-number amount above the minimum — adjust this to any figure you can comfortably afford. The chart draws a second green line alongside the blue baseline, showing how much faster your principal reduces under the higher payment. On a typical 20-year bond, even a modest additional monthly payment can shorten the loan by several years and save a substantial amount in total interest.

The results panel updates throughout to reflect the custom schedule — the Total Interest Paid figure switches to the lower custom amount, and an Interest Saved line shows the difference against the minimum scenario. The amortisation table also reflects the custom repayment, with the capital column highlighted in green to distinguish it from the standard schedule. This makes it straightforward to compare both scenarios and understand exactly what a higher monthly commitment is worth over the life of the loan.

¶Lump sum payment #

Within the custom repayment panel is a Lump Sum Payment option. Enable custom repayment first, then check the lump sum box — two fields appear: an amount and a Paid at point in years or months. Enter a once-off amount you expect to pay into the loan — a year-end bonus, an inheritance, proceeds from selling an asset — and specify when in the term that payment would be made.

The lump sum applies to the green custom curve only, not the blue minimum baseline. At the month you specify, the full amount is credited directly against the outstanding principal. Because all subsequent repayments are then calculated on a lower balance, less interest accrues across every remaining month — the effect compounds forward through the entire remaining term. The earlier the lump sum is applied, the greater the saving: a payment in year two eliminates interest on that principal for the following eighteen-odd years, whereas the same payment in year fifteen saves interest only over the final five. Adjust the Paid at field to see how timing changes the outcome — the chart and results update immediately.

¶Car loan calculator #

The same amortisation principles apply to vehicle finance. Enter the purchase price, deposit, annual interest rate, and loan term to see your monthly repayment and a full amortisation schedule. The currency picker works the same way as the bond calculator — select your market and the local prime rate is pre-filled as a starting point. The Share button copies a link encoding all your inputs, including balloon amount and fee settings, so you can save or share any scenario directly.

¶Balloon payment #

Vehicle finance frequently includes a balloon payment (also called a residual value) — a lump sum deferred to the final month of the term. Enable the balloon toggle, enter the amount, and the calculator models the full impact: the monthly repayment falls because a portion of the principal is never progressively amortised, but the amortisation table shows the balance remaining elevated until the final settlement. You can see exactly how much interest accrues on the deferred portion over the term — often more than the lower monthly payment implies. A balloon makes sense if you need reduced monthly instalments and are confident you can settle the residual at term end, whether by refinancing, trading in, or paying cash.

¶Bank fees and effective rate #

Enable the Include bank fees toggle to add a once-off initiation fee and a monthly administration fee to the model. Defaults are pre-filled based on your selected currency — South African loans, for example, use the NCA-capped maximums. You can override these with the actual figures from your loan agreement.

When at least one fee is non-zero, an Effective Rate (APR) result appears. This is the single annual rate at which the present value of all your payments — monthly repayments, admin fees, and the initiation fee — equals the amount you borrowed. It is always higher than the nominal rate and is the most accurate single figure for comparing loan offers, because it captures the full cost of credit rather than just the stated interest rate. The Total Cost of Credit block shows the complete cash outflow over the term, including all fees.

¶Vehicle depreciation and negative equity #

The depreciation chart plots your estimated vehicle market value against the outstanding loan balance month by month. Select a vehicle category — new standard, luxury, electric, or used — and the calculator applies a three-tier depreciation curve reflecting how that type of vehicle loses value over time. You can customise all three rates (year 1, years 2–3, year 4 onwards) if you have more specific data for your vehicle.

The shaded red area on the chart highlights any period where the loan balance exceeds the estimated vehicle value — known as negative equity, or being underwater. This is common early in a loan, particularly with a small deposit, a high balloon payment, or a vehicle category that depreciates steeply in the first year. During this period, settling or trading in would leave a shortfall: you would owe more than the car is worth. The chart shows the crossover point — the month at which the vehicle value overtakes the remaining balance and you return to positive equity.

¶Local interest rates #

Selecting a currency in either calculator pre-fills the local prime lending rate automatically. For a broader view — including typical home loan ranges and rate context for your market — we maintain a searchable table of rates for 135 countries.