How to Calculate Total Loan Interest
Learn how total interest can be estimated from payments and principal.
Key idea
Learn how total interest can be estimated from payments and principal.
Formula or method
Worked example
Start with the values in your question, substitute them into the formula, and keep the units consistent. For example, if a rate is given as 15%, use 0.15 when a formula requires a decimal.
Common mistakes
- Mixing percentage points with percentage change.
- Using different units for the same calculation.
- Rounding too early when several steps are involved.
Why the result can differ
Real-world calculations can include assumptions, rounding, fees, tax rules, calendar conventions, or other details. Treat simple calculator results as estimates when the underlying situation is more complex.
Interest depends on the balance
For an amortizing loan, interest is calculated from the outstanding balance according to the loan’s rate and schedule. As principal is repaid, the interest portion generally changes.
Why total interest matters
Two loans can have similar monthly payments but different total costs because their terms differ. Looking at total interest alongside the payment gives a fuller picture of the mathematical cost under the stated assumptions.
Check the contract
Actual borrowing costs may include fees or other charges that are not represented by a basic interest calculation. Use lender disclosures for a final cost comparison.