A loan interest calculator estimates the interest a loan accrues under amortizing or flat (simple-interest) methods. Enter the principal, annual rate, and term with monthly, quarterly, or weekly periods; optional fees and extra payments can also be modeled. Results use fixed periodic rates, not daily accrual, and the displayed payment may be an average rather than a contractual installment.
The result is a model, not a lender quote. Loan contracts can use different payment dates, day-count conventions, fees, and payment-allocation rules.
How to Use the Loan Interest Calculator
- Enter the loan basics: Type the Loan Amount, pick the currency, and enter the Annual Interest Rate (% p.a.).
- Choose the interest method: Select Amortizing for interest charged on the declining balance, or Simple (Flat) for interest charged on the full amount for the whole term; the method you pick changes the total substantially.
- Set the term and frequency: Enter the Loan Term as Years and Months (1 to 600 months) and choose Monthly, Quarterly, or Weekly payment frequency.
- Add the optional items: Enter an Extra Payment per Period to model additional principal paydown, and Origination Fees to include the one-time cost in the total.
- Calculate and review: Click Calculate to see the Total Interest, Monthly Payment, Total Paid, and interest as a percentage of principal.
- Compare and inspect the schedule: The panel also shows the flat-versus-reducing comparison and, when an extra payment is entered, the interest saved (verify against the schedule — see the limits below); open the amortization schedule for the month-by-month breakdown.
The displayed payment may be an average rather than a contractual installment, since real contracts use different payment dates and day-count conventions.
What Loan Interest Represents
Principal is the amount borrowed. Interest is the financing charge calculated under the loan's rate and balance method. In a reducing-balance loan, each payment covers the interest for that period and then reduces principal; later interest calculations use the smaller remaining balance. That accrual math is label-agnostic — a car loan, a mortgage, or any other amortized loan accrues the same way at the same rate and term — so across the schedule the payment splits into interest versus principal, early payments are mostly interest, and the share flips as the balance falls.
Interest Versus APR
Interest is not the same as APR: APR may include certain borrowing fees as well as the interest rate. This tool reports entered fees separately and does not calculate an official APR disclosure. For a comparison of loan interest rates and APR, see the CFPB's loan APR guide.
Amortizing and Flat-Interest Methods
The Reducing-Balance (Amortizing) Calculation
With the amortizing method, the calculator applies a periodic rate to the outstanding balance and builds a payment schedule. Its periodic rate is the entered annual rate divided by the selected periods per year: 12 for monthly, 4 for quarterly, or 52 for weekly payments.
A standard level-payment estimate can be expressed as:
Payment = Principal x periodic rate / [1 - (1 + periodic rate)-number of periods]
When the rate is 0%, the principal is divided evenly across the modeled payment periods. The schedule rounds amounts to cents, so its final payment can differ slightly from earlier payments.
One period in practice: take the calculator's test case, $20,000 at 8.00% over 60 monthly periods. The modeled payment is $405.53, and month 1 accrues $20,000 × 0.08 ÷ 12 = $133.33 of interest, so $272.20 of that payment reduces principal and the balance falls to $19,727.80. Each later month charges interest on a smaller balance, which is why the interest portion of the payment declines over the schedule.
The Flat-Interest Calculation
The flat-interest option estimates interest on the original principal for the whole term:
Flat interest = Principal x annual rate x (term in months / 12)
It does not reduce the interest base as principal is repaid, so it is not the same calculation as reducing-balance amortization. Use it as a mathematical comparison only; confirm how an actual lender calculates and discloses its product.
Payment Frequency, Term, Fees, and Extra Payments
- Term: Enter a term from 1 to 600 months. The calculator converts the term to a whole number of payment periods.
- Payment frequency: Choose monthly, quarterly, or weekly. These choices change the number of modeled periods and the rate used per period.
- Fees: The optional fee amount is added to the reported total paid; it is not included in the interest calculation.
- Extra payment: The optional amount is added to each modeled amortizing payment period. Check the schedule and your contract to confirm how a lender would apply extra payments.
The displayed payment may be an average across the schedule rather than the regular installment amount. Review the schedule rows and verify the actual required payment with the lender before using a result for budgeting.
Comparing Frequency, Term, and Interest Method
Take $20,000 at 8.00% and change one input at a time. The engine returns the following set, which separates the effects that people usually conflate:
| Scenario | Payment per period | Modeled interest | Total paid |
|---|---|---|---|
| Monthly, 60 months | $405.53 a month | $4,331.62 | $24,331.62 |
| Weekly, 60 months | $93.39 a week | $4,281.08 | $24,281.08 |
| Quarterly, 60 months | $1,164.89 a quarter | $4,462.72 | $24,462.72 |
| Monthly, 36 months | $626.73 a month | $2,562.15 | $22,562.15 |
| Monthly, 84 months | $308.06 a month | $6,184.96 | $26,184.96 |
| Flat interest, 60 months | $466.67 a month | $8,000.00 | $28,000.00 |
The three frequency rows differ by only $181.64 across the full term, from $4,281.08 weekly to $4,462.72 quarterly. Paying more often helps because the balance falls sooner, but at this rate the effect is small enough that an account fee or a minimum balance rule can erase it. The term rows are the expensive variable: moving from 36 to 84 months saves $318.67 a month and adds $3,622.81 of interest. The method row is the one that matters most and is the one borrowers least often check. Flat interest on the original principal is $8,000.00 against $4,331.62 on a reducing balance, a gap of $3,668.38 on identical numbers. If a quote states a flat rate, translate it before comparing it with anything else.
Daily Interest and Model Limits
Per-Diem Interest and Other Limits
Some loans accrue simple interest by day. A common estimate for one day is outstanding balance x annual rate / 365, but contracts may use a different day-count convention. This calculator does not compute daily per diem interest or use payment calendar dates; it divides the annual rate by the selected payment frequency instead.
The amortization schedule can illustrate how interest and principal change over modeled periods, but its payment-savings display may not reliably show the effect of extra payments. Do not rely on that savings figure without checking the schedule or comparing it with the lender's figures. The calculator also does not model revolving credit-card balances, late payments, variable rates, tax treatment, or every possible lender fee.
For a regular payment estimate, use the Loan Payment Calculator. To view a payment-by-payment principal and interest breakdown, use the Amortization Calculator. For a fee-inclusive annualized borrowing-cost estimate, see the APR Calculator.
Last verified: October 2026. Calculator figures are outputs for the inputs entered; they are not live quotes, current rates, or an offer.