When to use this calculator
Calculate interest over an equal-principal repayment schedule. Assumes equal principal payments at month-end and a fixed simple monthly rate.
Explore the loan & repayment calculators for other questions in this topic. For a related calculation, use the Flat Rate Loan Interest Calculator: Calculate interest under a flat-rate-on-original-principal loan model.
PUTTING IT INTO PRACTICE
Equal Principal Total Interest: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Loan principal | 12,000 |
| Nominal annual rate (%) | 6 |
| Monthly payments (integer) | 12 |
| Result | 390 currency units |
- Check the input labels and enter the example values above.
- Apply the relationship: Total interest = principal × monthly rate × (months+1)/2.
- The result is 390 currency units, rounded for display.
How to interpret your result
Assumes equal principal payments at month-end and a fixed simple monthly rate.
Use a fixed-rate amortising model for loans repaid by equal monthly instalments. An interest-only payment leaves the principal outstanding. Fees and contractual APR calculations require more than the interest rate alone.
Frequently asked questions
Does a lower monthly payment mean a cheaper loan?
Not necessarily. Extending the term can lower payments while increasing total interest. Compare both monthly affordability and overall cost.
When is the equal principal total interest calculator useful?
Calculate interest over an equal-principal repayment schedule. Assumes equal principal payments at month-end and a fixed simple monthly rate.
How do I use the equal principal total interest calculator?
Enter loan principal, nominal annual rate (%), monthly payments (integer). The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Total interest = principal × monthly rate × (months+1)/2.
What should I check before using the result?
Assumes equal principal payments at month-end and a fixed simple monthly rate.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
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