About Lumera and how we calculate
What Lumera is
Lumera is a personal project by Luis Yoshi Vera Perez, who builds free financial calculators. There is no account to create and nothing to install: every calculation runs inside your browser, and the figures you type are never sent to us. The site is funded by advertising, which is why you will see ads on the calculator pages. Lumera does not sell loans and does not recommend any financial product.
How the loan calculator works
The calculator uses the standard method for fixed-rate loans repaid in equal monthly payments — the same one lenders use for most mortgages, car loans and personal loans. The monthly payment is:
where P is the amount borrowed, r is the monthly interest rate (the annual interest rate divided by 12) and n is the number of monthly payments. Each month, interest is charged on the balance still owed (balance × r), and the rest of the payment reduces that balance. That is why the interest part of each payment is largest at the start and shrinks over time.
Extra payments
An extra payment goes straight to the balance in the month you make it, after that month's regular payment. The monthly payment stays the same, so the loan ends sooner: the calculator shows how many months earlier, and how much less interest you pay.
You can combine a recurring extra (monthly, quarterly or yearly, starting in the month you choose), a one-off lump sum, and the biweekly-equivalent option, which adds one twelfth of the monthly payment every month — the same as one extra payment a year. Some lenders let you keep the original end date and lower the monthly payment instead; the calculator currently models the shorter-term option only.
Fees and the true total cost
Fees are kept separate from the interest rate. One-off costs, such as an origination fee, are counted once. A monthly fee, such as mandatory insurance, is counted for every month the loan actually runs — so paying off early saves those fees too.
True total cost = amount borrowed + all interest + all fees. When you compare scenarios, the winner is the one with the lowest true total cost, not the lowest monthly payment or the lowest rate.
Which rate to enter
Enter the loan's interest rate, not its APR. The APR already includes fees, so entering the APR and the fees separately would count them twice. The Consumer Financial Protection Bureau explains the difference between an interest rate and an APR.
What the calculator does not model
- Variable or adjustable rates that change during the loan.
- Early-repayment (prepayment) penalties your lender may charge.
- Taxes, escrow, and changes in insurance premiums.
- Your lender's own rounding and day-count rules, which can make its schedule differ by a few cents per payment.
Results are an estimate to help you decide, not an offer or financial advice. Always check the figures with your lender before acting.
How we check the numbers
The formulas are checked against cases worked out by hand. For example, $100,000 at 6% over 30 years gives a payment of $599.55 a month, and €100,000 at 3.5% over 25 years gives €500.62. The calculator also runs an automatic self-check against reference cases every time the page loads.
Sources and further reading
- Consumer Financial Protection Bureau — What is the difference between a mortgage interest rate and an APR?
- Banco de España, Portal del Cliente Bancario — official loan simulators (in Spanish)
Contact
Found an error or have a question? Write to soporte@lumera-one.com.