Loan Interest Calculator
principal · rate · term · repayment method
Compare three repayment methods
Use the same loan amount, fixed annual rate and term to compare first and last payments, total interest and total repayment.
Enter the loan amount in USD, then the annual rate and term in years. Choose a repayment method. The optional initial interest-only period changes when principal repayments begin.
Enter the amount, annual rate and term.How to use the loan interest calculator
Enter the amount, annual interest rate and term to compare three repayment methods. Explore an initial interest-only period, four charts and the monthly schedule in US dollars.
This calculator compares three regular fixed-rate repayment models using the amount, nominal annual interest rate and term you enter. It shows the first and last monthly payments, total interest, total repayment and the full monthly schedule. The same assumptions are used for a side-by-side comparison. A mini donut summarises the interest share, while three larger chart views show payment composition, balance paths and principal versus interest. Enter US dollars directly, including cents. These are illustrative loan scenarios rather than current lender offers, an approval decision or a calculation of fees and APR.
Fixed payment spreads principal and interest into equal payments after any initial interest-only period. Equal principal repays the same principal amount each repayment month, with interest falling as the balance decreases. Principal at maturity pays interest each month and repays the entire principal in the final payment. The model holds the nominal rate constant. All calculations run in your current browser and the entered loan details are not saved or sent to a server.
When this tool is needed
- Estimate the repayment burden before discussing a mortgage or another fixed-rate loan.
- Compare the payment pattern and total interest of fixed payment and equal principal.
- See how an initial interest-only period changes later payments and total interest.
Check before entering
- Use USD for the amount, including cents if needed. Paste a grouped amount or enter plain digits; formatting is applied when you leave the field.
- Use the nominal annual interest rate for the model. This is not a calculation of APR, fees, tax or a variable-rate path.
- The optional initial interest-only period must leave at least one repayment month after rounding. Enter 0 if it does not apply. It is disabled for principal at maturity.
How to use
- Enter the loan amount in US dollars or use its slider. Include cents if needed.
- Enter the nominal annual interest rate and the term in years.
- Open the optional setting and enter any initial interest-only period in years.
- Choose fixed payment, equal principal or principal at maturity.
- Review the first and last payments, totals, charts and monthly schedule. Copy the results if you need to compare them elsewhere.
How the repayment methods differ
With fixed payment, the amount is constant after the initial interest-only period, so the later regular payment is easy to compare. A larger share goes to interest early in the amortising period. Equal principal starts with a larger payment that falls as the balance declines; with the same principal, rate, term and grace assumption, it has the lowest total interest of these three models. Principal at maturity has small interest-only payments until the final principal repayment, so the final payment is much larger. Compare both first and last payments alongside total interest rather than looking only at the first month.
Initial interest-only periods and limitations
Loan and initial interest-only periods are converted to months and rounded; the loan must have at least 1 month. Each schedule amount and the total interest are rounded separately to cents. Adding the rounded rows can therefore differ slightly from the separately rounded total. The charts and copied schedule use the same calculated amounts. Actual lender settlement, day-count and final-payment adjustments may differ.
An initial interest-only period reduces the early payments but leaves principal unchanged. Repaying principal over fewer remaining months can increase the later payment and total interest. The model assumes a fixed nominal rate, with payments at monthly period ends. It excludes variable-rate changes, discounts, prepayment fees, taxes, guarantees and other charges. The 30% maximum rate accepted by the input is a technical bound for this calculator, not a US legal interest cap.
These are standard fixed-rate model estimates, not an offer or an exact lender statement. Confirm the rate definition, payment dates, fees, rounding and contract terms with the lender before making a decision. The model does not calculate APR or decide eligibility.
Calculation basis and sources
- Calculation scope
- Calculate first and last payments, total interest and repayment, comparisons, four charts and a monthly schedule for fixed payment, equal principal and principal at maturity from your own inputs. Optional initial interest-only periods are supported; variable-rate changes and fees are excluded.
- Calculation method
- Monthly rate r = nominal annual rate ÷ 12 ÷ 100; n is the rounded number of months. Fixed payment = P · r · (1+r)ⁿ / ((1+r)ⁿ − 1) over repayment months after any initial interest-only period. Equal principal divides P by the remaining repayment months and adds balance × r. Principal at maturity pays P × r each month and the principal in the last payment.
- Limits and disclaimer
- A standard fixed-rate model without a guarantee of actual lender payments. Variable rates, discounts, prepayment charges, taxes, guarantees, fees, date conventions and lender-specific rounding can change the result.
- Personal information processing
- Input values are calculated only in the current browser and are not sent to a server or stored.
🗓️ Source and copy review 2026-11-01 · 🔁 Next review 2027-03-17
FAQ
How do fixed payment and equal principal compare?
With the same $300,000 principal, 3.5% fixed nominal annual rate, 30-year term and no grace period, this model gives about $184,968.26 total interest for fixed payment and $157,937.50 for equal principal. Equal principal starts with a higher monthly payment and then decreases. Compare first and last payments as well as interest. These are model examples, not current lender offers, and rounded schedule rows may not sum exactly to the separately rounded total.
What changes with an initial interest-only period?
You pay interest while principal stays unchanged during that initial period. The remaining months are then used for principal repayment, which can raise later payments and total interest compared with no grace period. The schedule shows both phases. Term and grace years are converted to rounded months; the grace period must leave at least one repayment month. The separate grace input is ignored and disabled for principal at maturity.
How does principal at maturity work?
You pay interest each month and repay the entire principal together with interest in the final payment. For the same fixed rate, principal and term, this model keeps principal outstanding for longest, so it has the highest total interest among the three compared methods. Its small early payment does not remove the large final-payment obligation. Check the final payment separately and confirm whether a lender offers such terms.
Will the result match an actual lender schedule?
Not necessarily. The model assumes a fixed nominal annual rate divided by 12, monthly payments at period ends and no additional charges. Real lenders can use different payment dates, day-count rules, variable rates, fees, tax and rounding. This tool rounds rows and total interest independently, which can also create small cent differences when adding displayed rows. It is not an APR calculation or a lending decision.
Can a beginner compare the methods?
Yes. Start with the amount in USD, the nominal annual rate and the term in years. The three repayment methods are compared automatically. Use the sliders or enter values directly; open the optional setting if an initial interest-only period applies. Review the first and last payments, interest totals, charts and full monthly schedule, then copy the results for your own comparison.