How the rate changes the cost
Simple interest uses a principal base
For one uninterrupted year, simple interest is principal times rate times time. Many installment loans calculate daily simple interest on the outstanding balance, so payment dates still matter.
Compounding creates a new base
When unpaid interest is added to the balance, later interest is calculated on principal plus prior interest. The effective annual rate rises with more frequent compounding at the same nominal rate.
$10,000 at 10% for one year with no payments
- Rate input
- Simple interest: $1,000
- Rate input
- Monthly compounding: about $1,047
- Rate input
- Daily compounding: about $1,052
- Rate input
- Difference comes from interest earning interest
- Rate input
- Installment payments change the balance throughout the year
Illustrative calculation. Rounded values can differ from a lender or card issuer schedule.
This decision belongs in The Real Cost of Borrowing. Use Add-On Interest Loans: Why the Quoted Rate Can Understate Cost to check the next assumption, then test the numbers in the Loan Total Cost Calculator.
For a revolving example of timing and compounding, see daily-balance credit card interest in practice.
Interpret the result in context
Amortization is a separate question
A loan can use simple daily accrual and still have amortizing monthly payments. Each payment first covers accrued interest and then reduces principal.
Late timing can matter without a late fee
On daily simple-interest loans, paying later means more days of accrued interest and less of the payment reaches principal. Confirm the servicer’s cutoff and posting rules.
Inputs that change the answer
The label matters less than the balance used for each accrual period. Payment dates, capitalization and day-count rules determine whether prior interest becomes part of the next calculation base.
Contract terms needed for the calculation
- Accrual frequency and day-count convention.
- When unpaid interest is added to principal.
- How early, late and extra payments are posted.
- Identify the accrual basis: daily, monthly or another period.
- Identify whether unpaid interest capitalizes and when.
- Use actual payment dates for a daily-accrual estimate.
- Compare effective annual yield when compounding frequencies differ.
- Confirm how extra payments are allocated.
Rebuild the calculation with your terms
Loan Total Cost Calculator
Separate the advertised rate from the dollars that leave your account.
Where rate comparisons go wrong
- Calling every installment loan compound interest.
- Dividing annual interest by twelve when the contract uses actual days.
- Assuming early payment always advances principal.
- Comparing nominal rates with different compounding frequencies.
Questions this decision usually raises
Is daily simple interest always cheaper?
No. It can be transparent, but the rate, term, fees and payment timing still control total cost.
Does APR include compounding?
APR conventions vary by product and law. Use the disclosure for compliance comparison and cash flows for your own economic comparison.
Can interest be charged on fees?
Yes, if fees are financed into the balance. A deducted fee instead reduces net proceeds.
Check the rules and definitions
This page explains a calculation, not a personal recommendation. Contract rules and consumer protections vary by product and jurisdiction. Check the current agreement and official guidance before acting.