Loan interest: how much the borrowing really costs
Borrowing money always costs more than the sticker price, and the difference is the interest. A loan interest calculator reveals exactly how much of each payment goes to interest versus principal, how the total cost accumulates, and how a slightly lower rate or shorter term changes the final bill. Whether you are financing a car, a home, or a small business, knowing the interest math puts you in control of the negotiation.
TL;DR
- In an amortized loan, each monthly payment covers the interest accrued on the outstanding balance plus a portion of the principal
- Formula: PMT = P × r / (1 - (1 + r)^-n) | Total Interest = PMT × n - P
- Example 1: A €20,000 car loan at 6% for 5 years
How it works
In an amortized loan, each monthly payment covers the interest accrued on the outstanding balance plus a portion of the principal. Early payments are interest-heavy because the balance is highest; later payments shift toward principal. The calculator models this schedule for the full loan term and totals the interest paid over the life of the loan.
The math
The monthly payment is derived from the loan amount, the monthly rate (annual rate ÷ 12), and the number of months using the annuity formula. The interest portion of each payment is the outstanding balance times the monthly rate; subtracting it from the payment gives the principal reduction, which lowers the balance for the next period.
This is the standard amortization formula used by banks worldwide; the same calculation powers mortgages, auto loans, and personal loans.
Examples
Example 1: A €20,000 car loan at 6% for 5 years
- Monthly rate: 0.06 ÷ 12 = 0.005, months: 60
- PMT = €20,000 × 0.005 / (1 - 1.005^-60) ≈ €386.66
- Total paid: €386.66 × 60 = €23,199
- Total interest: €3,199
Example 2: The same loan with a 4% rate
- Monthly rate: 0.04 ÷ 12 ≈ 0.00333
- PMT ≈ €368.33
- Total paid: €22,100
- Saving: €1,099 over 5 years for a 2% rate drop
Real-World Uses of a Loan Interest Calculator
✦ Comparing Loan Offers
Borrowers translate APRs and terms into total cost, comparing offers from banks, dealers, and online lenders on equal footing.
✦ Budgeting Monthly Payments
Families check whether a payment fits their budget before signing, and simulate extra payments to shorten the loan and cut interest.
⚠ Common mistakes
- Focusing Only on the Monthly Payment: A longer term lowers the payment but can double the total interest paid.
- Ignoring Fees: Origination and processing fees are excluded from the interest formula but part of the real cost.
- Missing the Effective Rate: Advertised rates are often nominal; the effective APR includes fees and compounding differences.
FAQ (6)
What is the difference between APR and interest rate?
The APR includes fees and other costs, so it is always higher or equal to the nominal interest rate and better for comparisons.
Can I pay less interest by paying early?
Yes, extra principal payments reduce the balance and the interest accrued on it, shortening the term and the total cost.
If I type my salary or prices here, do you store them?
No — and that's deliberate. Everything runs locally in your browser, nothing leaves your device. We don't see your net salary, your margins, or the VAT numbers you test. For a finance tool that matters: you can run a quick check on a payslip or an invoice while on public Wi-Fi and close the tab knowing no trace is kept server-side.
Can I use it on the train without signal to check an invoice?
Yes. Load the page once, then it works offline. VAT, discount and mortgage math are all pure JavaScript — no API call. I often test it in airplane mode: open the mortgage page, type 250,000 at 4.5% for 25 years, you get the schedule instantly, even in a tunnel.
Can I file taxes with these results?
Use it as a fast sanity check, not as the final filing. Formulas are standard (e.g. Net = Gross / 1+VAT) and verified against official sources listed at the bottom, but rates and thresholds change — Italy's 22% vs Germany's 19%, or IRPEF brackets. For the actual declaration, double-check with your accountant or Agenzia delle Entrate / HMRC / IRS.
How do I send a calculation to my accountant?
Hit 'Share Link' under the results. It encodes the inputs in the URL — e.g. ?price=1200&vat=22 — so your accountant opens the exact same view, no screenshot needed. You can also copy the result with one tap and paste it into an email.
Sources & References
⚠ Financial Disclaimer
This tool provides estimates for informational purposes only and does not constitute financial, tax, or legal advice. Results may vary based on local regulations, rates, and individual circumstances. Always consult a qualified professional before making financial decisions.