How to Use This Loan Calculator
The Standard tab handles the most common task: enter your loan amount, the annual interest rate your lender has quoted, and the loan term in months. The calculator returns your fixed monthly payment, total interest, total repayment cost, and a full month-by-month amortisation schedule — everything you need to understand exactly what the loan will cost before you sign.
If you are considering making extra payments, the Extra Payment tab shows you the exact interest saved and how many months sooner you will pay off the loan. The Compare tab lets you evaluate two competing offers side by side, including origination fees — because a lower rate with high fees can easily cost more than a higher rate with no fees. The Affordability tab works in reverse: enter a maximum comfortable payment and the calculator tells you the most you can borrow.
What Is Loan Amortisation?
Amortisation is the process of repaying a loan through equal periodic payments, where each payment covers the interest owed for that period and reduces the outstanding principal by the remainder. The word comes from the Old French amortir — to deaden or extinguish — and that is precisely what the schedule does: it extinguishes the debt gradually, payment by payment.
The counterintuitive reality of amortisation is that your early payments are mostly interest. On a $25,000 loan at 7.5% over 60 months, the first payment of $500.76 sends $156.25 to interest and only $344.51 to principal. By the final payment, the ratio has reversed completely — the last payment is almost entirely principal. This front-loading of interest is not a lender trick; it is a mathematical consequence of applying a percentage rate to a balance that starts at its maximum.
The Loan Payment Formula
Every fixed-rate monthly payment is derived from the standard amortisation formula:
Where M is the monthly payment, P is the principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments (years × 12).
The term (1 + r)ⁿ is the compound growth factor over all payment periods. The ratio of this factor applied to r over the denominator produces a payment that perfectly decreases the balance to exactly zero on the final payment — no more, no less. This elegant property is why the formula has been used by financial institutions for over a century.
How Interest Accrues Each Month
Each month, interest is calculated on the remaining balance: Interest = Balance × (Annual Rate ÷ 12). This amount is subtracted from your fixed payment, and the remainder reduces the principal. Because principal decreases each month, the interest charged also decreases — allowing a growing portion of each subsequent payment to reduce the principal. This is the mechanics behind the amortisation schedule's shifting split.
Step-by-Step Example
Daniel borrows $18,000 for a home renovation at 8.25% APR over 48 months.
Monthly rate r = 8.25% ÷ 12 = 0.6875%. Total payments n = 48.
Monthly payment = $18,000 × [0.006875 × (1.006875)⁴⁸] ÷ [(1.006875)⁴⁸ − 1] = $440.97
Over 48 months: total repaid = $21,166.56. Total interest = $3,166.56 — 17.6% of the loan amount paid purely for the privilege of borrowing. By making an extra $75 per month, Daniel reduces this to roughly $2,540 and finishes 4 months early, saving $626 in interest.
Understanding the Donut Chart
The donut chart visualises the single most important fact about any loan: the ratio of principal to total interest in your complete repayment. On a short-term loan at a moderate rate, interest might be 10–15% of the total. On a 30-year mortgage, it commonly exceeds 100% of the original loan amount — meaning you pay for the house twice over.
The percentage shown in the centre is the interest cost as a proportion of total repayment. A number below 10% is efficient borrowing. Above 30%, you should seriously consider whether a shorter term or a larger down payment changes the picture meaningfully. Above 50%, the interest cost has become larger than the principal and you should explore every option to reduce it.
How to Read the Amortisation Schedule
The schedule shows five columns: payment number (or month/date), total payment amount, the portion applied to principal, the portion applied to interest, and the remaining balance. Toggle between Monthly and Yearly views — the yearly summary is easier to scan for long-term loans like mortgages, while monthly detail is useful for planning extra payments.
Two things to look for in the schedule: first, identify the crossover month — the payment where principal exceeds interest for the first time. On a 7.5% loan, this happens around month 38 of a 60-month term. Second, note how quickly the balance drops in the final third of the schedule compared to the first third. This asymmetry is where extra payments do the most damage to the loan.
The Real Cost: APR vs Interest Rate
The interest rate is the base cost. The APR is the true annual cost after incorporating all mandatory fees — origination fees, broker fees, mortgage insurance, and required points. Under US federal law, lenders must disclose the APR under the Truth in Lending Act (TILA). In the UK, lenders must quote representative APR under the Consumer Credit Act.
This calculator computes a fee-adjusted APR: it adds any origination fee to the total interest cost and then uses a reverse calculation to find the effective annual rate you are actually paying. Two loans with identical stated rates but different fees will show different true APRs here — making it the definitive comparison figure.
Factors That Affect Your Loan Rate
Credit Score
This is the single largest determinant of your rate in consumer lending. A 760+ FICO score typically qualifies for the best available rates. Dropping to 680 can increase your rate by 2–3 percentage points on a personal loan. On a $25,000 loan over 5 years, a 3% rate difference costs approximately $2,000 in additional interest.
Loan Term
Longer terms carry higher rates in most lending categories — the lender faces more uncertainty over a longer period and prices that risk accordingly. A 36-month auto loan typically carries a lower rate than a 72-month loan for the same vehicle, even from the same lender.
Loan Type and Purpose
Secured loans (where the lender holds collateral — a car, a house) carry lower rates than unsecured personal loans because the lender has recourse if you default. Mortgages and auto loans are typically lower-rate products than personal loans for this reason. Student loans occupy a unique category with federally set rates that do not vary by credit score.
Debt-to-Income Ratio
Lenders look at your total monthly debt payments as a percentage of gross monthly income. A DTI below 36% is generally considered healthy by mortgage lenders. Above 43%, most conventional mortgage lenders will not approve the loan. The Affordability tab calculates your DTI automatically if you enter your monthly income.
How Extra Payments Work
Every dollar of extra payment you make reduces the principal directly. That reduced principal then accrues less interest in every subsequent month — the savings compound over the remaining life of the loan. This is why paying an extra $100 per month on a 5-year loan saves far less in absolute terms than the same extra $100 on a 30-year mortgage: the compounding period is shorter on the personal loan, leaving less time for interest to accumulate on the saved principal.
Common Mistakes When Taking Out a Loan
Focusing on Monthly Payment Instead of Total Cost
Lenders and dealers know that most people shop by monthly payment, not total cost. Extending a car loan from 48 to 72 months drops the monthly payment noticeably — but adds thousands in interest and leaves you "underwater" on the vehicle (owing more than it's worth) for a longer period. Always compare total cost alongside monthly payment.
Not Shopping Multiple Lenders
A 1% rate difference on a $30,000 loan over 5 years saves roughly $800. Rates vary significantly across banks, credit unions, and online lenders even for borrowers with identical credit profiles. Multiple hard inquiries for the same loan type within a 14–45 day window are typically treated as a single inquiry by FICO scoring models — so shopping around does not damage your score as much as many people fear.
Ignoring the Origination Fee
A loan advertised at a 6% rate with a $500 origination fee can be more expensive than one at 6.5% with no fee, depending on the loan amount and term. Enter both scenarios in the Compare tab to find the break-even point.
Variable Rate Risk
This calculator models fixed-rate loans. If you are taking a variable-rate product (ARM mortgage, variable personal loan), your actual payments will change when rates adjust. The initial rate is a starting point, not a guarantee. Model your maximum possible payment using the rate cap disclosed in your loan agreement.
When to Speak with a Financial Professional
This calculator gives you precise numbers for any fixed-rate loan. Speak with a licensed professional in these situations:
- You are taking a mortgage and considering points, ARM products, or interest-only periods — these require scenario analysis beyond a basic calculator.
- Your DTI is above 43% and you are still considering a significant loan — a certified financial planner (CFP) can help identify whether and how to proceed safely.
- You are consolidating multiple debts into a single loan — the maths of consolidation often look favourable on a per-payment basis while hiding a higher total cost over a longer term.
- You are a business owner financing equipment or operations — business loan structures, SBA guarantees, and tax deductibility of interest require professional input.
In the US, NFCC member agencies offer free or low-cost financial counselling. For mortgage-specific questions, HUD-approved housing counsellors are available nationwide. For investment-linked borrowing (margin loans, HELOC for investing), a fee-only financial adviser familiar with leverage risk is essential.