What does the Loan Interest Calculator do?
It converts the entered loan assumptions into payment, interest, total cost, and schedule-style outputs using deterministic formulas.
Finance Calculator
Loan Interest Calculator
Compare simple interest, flat-rate interest, and reducing-balance amortized interest for the same principal, rate, and term.
Last reviewed: June 21, 2026Loan interest method set v1.0.0Finance method set v1.0.0: fixed-payment, amortization, payoff, comparison, flat-rate, and simple-interest formulas
What the Loan Interest Calculator does
The Loan Interest Calculator compares how interest changes under simple, flat-rate, and reducing-balance methods.
How to use the Loan Interest Calculator
Enter the values that describe the loan or interest scenario, then review the result, schedule, warnings, and assumptions before using the number.
- Enter principal, rate, and term.
- Compare interest methods.
- Review why flat rate is not APR.
Formula
Simple I = P x r x t. Flat interest uses the same total-interest idea but divides total principal plus interest over payments. Reducing balance uses amortization.
Variables
The calculator uses the following variables in its formula layer.
- P = principal
- r = annual decimal rate
- t = years
- n = payments
Assumptions
These assumptions keep the calculation deterministic and transparent.
- Simple and flat interest do not use reducing balance.
- Reducing balance uses periodic amortization.
- No official APR is calculated.
Calculation steps
NexaCalc applies the formula in a fixed sequence so the output can be tested and repeated.
- Calculate simple interest.
- Calculate flat installment.
- Generate reducing-balance schedule.
- Compare interest totals.
Worked examples
100,000 at 8% for 18 months produces 12,000 simple interest.
The flat-rate installment divides 112,000 across 18 payments.
Reducing-balance interest usually differs because balance declines each payment.
Result interpretation
The same rate label can produce different costs depending on the interest method.
Limitations
The result is a model, not a lender quote or official disclosure.
- No precomputed rebate rules.
- No official APR.
- No lender-specific compounding conventions.
Frequently asked questions
Is the interest rate the same as APR?
No. The entered rate is used for the modeled interest calculation. APR may include other costs and lender disclosure rules.
Why can my lender's numbers differ?
Lenders can use different accrual conventions, rounding, fee timing, payment posting rules, taxes, insurance, and legal disclosures.
Does changing currency convert the amount?
No. Currency changes formatting only. NexaCalc does not fetch exchange rates or convert values.
Can I use this for approval decisions?
No. The calculator does not estimate eligibility, creditworthiness, approval probability, or suitability.
Are extra payments guaranteed to save interest?
The model applies extra payments to principal, but actual savings depend on lender prepayment terms and posting rules.
Does the schedule use daily accrual?
No. The amortization schedule uses periodic interest based on the selected frequency unless the page explicitly uses simple-interest day counts.
Is this financial advice?
No. It is a general education calculator and should be checked against lender disclosures and qualified advice when decisions matter.
References
Financial disclaimer
This calculator is for general educational use only. It is not financial, legal, tax, lending, or investment advice. Lender disclosures, compounding conventions, fees, taxes, insurance, prepayment rules, and local regulations can change actual loan costs.