Monthly Debt Interest Cost CalculatorFree online financial calculator — instant results, no sign-up
Estimate one month's interest on two unchanged balances at their stated annual rates.
Estimates only, based on the inputs and assumptions shown. This is not financial, investment, tax, or legal advice. Prices, rates and local rules may change; crypto scenarios do not predict returns. Confirm important decisions with a qualified professional.
Sign in to save results and favorites.
About the Monthly Debt Interest Cost Calculator
The Monthly Debt Interest Cost Calculator estimates one month's interest on two unchanged debt balances using their stated annual rates. It shows the estimated interest for each balance separately and the combined monthly interest, using each annual rate divided by 12.
- First balance ($)
- The first debt's balance in dollars, assumed unchanged for the month, with a default of $5,000.
- First annual rate (%)
- The annual interest rate applied to the first balance, entered as a percentage, with a default of 24%.
- Second balance ($)
- The second debt's balance in dollars, assumed unchanged for the month, with a default of $9,000.
- Second annual rate (%)
- The annual interest rate applied to the second balance, entered as a percentage, with a default of 12%.
Formula and method
I₁ = B₁ × (r₁ / 100) / 12; I₂ = B₂ × (r₂ / 100) / 12; I_total = I₁ + I₂.
- B₁First unchanged debt balance in dollars.
- r₁First annual interest rate as a percentage, entered as 24 for 24%.
- B₂Second unchanged debt balance in dollars.
- r₂Second annual interest rate as a percentage, entered as 12 for 12%.
- I₁Estimated monthly interest on the first balance in dollars.
- I₂Estimated monthly interest on the second balance in dollars.
- I_totalCombined estimated monthly interest on both balances in dollars.
Convert each annual percentage rate to a decimal by dividing by 100, then divide by 12 to obtain its monthly rate. Multiply each unchanged balance by its monthly rate and add the two interest amounts. With the default inputs, the first monthly interest is $5,000 × (24 / 100) / 12 = $100.00; the second monthly interest is $9,000 × (12 / 100) / 12 = $90.00. Combined monthly interest is $100.00 + $90.00 = $190.00.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- First balance5000 $
- First annual rate24 %
- Second balance9000 $
- Second annual rate12 %
Results
- First monthly interest$100.00
- Second monthly interest$90.00
- Combined monthly interest$190.00
How to use the Monthly Debt Interest Cost Calculator
- 1Enter the first balance in dollars; the default is 5000.
- 2Enter the first annual rate as a percentage; the default is 24.
- 3Enter the second balance in dollars; the default is 9000.
- 4Enter the second annual rate as a percentage; the default is 12.
- 5Review the first, second, and combined monthly interest estimates, keeping in mind that both balances are assumed unchanged.
Reading your result
The First monthly interest row shows the estimated interest attributable to the first balance: $100.00 with the default inputs. The Second monthly interest row shows the estimate for the second balance: $90.00. The Combined monthly interest row adds these amounts to give $190.00. These are interest costs only, not required payments or amounts that reduce principal.
Limitations
This is a simple annual-rate-divided-by-12 estimate, not a lender statement calculation. It assumes both balances and annual rates remain unchanged throughout the month and does not model payments, new borrowing, fees, compounding, or promotional and grace-period rules. Actual charges may depend on daily balances, billing-cycle length, day-count conventions, and account terms. The calculator does not determine minimum payments, payoff time, or total interest over the life of either debt.
Common questions
Why does the smaller first balance produce more monthly interest in the default example?+
The first balance is $5,000 at 24%, producing $100.00 in estimated monthly interest. The second is $9,000 at 12%, producing $90.00. The first rate is twice the second rate, which more than offsets its smaller balance.
Does the $190.00 combined monthly interest equal my total monthly payment?+
No. The $190.00 represents estimated interest only. Required payments may also include principal and fees and are determined by each lender's terms.
Why might my statements show interest different from $100.00 and $90.00?+
The calculator uses unchanged balances and one-twelfth of each annual rate. Your lenders may calculate interest using daily balances and the actual billing-cycle length, and payments, purchases, or grace periods may change the charges.
Does this calculator apply one average rate to both debts?+
No. It applies 24% to the $5,000 first balance and 12% to the $9,000 second balance separately, then adds the resulting $100.00 and $90.00 monthly interest estimates.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
You might also like
Credit Utilization Calculator
Measure revolving utilization and the payment needed to reach a target percentage.
Minimum Payment Trap Calculator
Compare a percentage-of-balance minimum payment with a fixed monthly payment.
Balance Transfer Promotion Calculator
Calculate the payment needed to clear a zero-interest transfer before its promotion ends.
Daily Simple Interest Cost Calculator
Calculate simple interest using a selected annual day-count denominator.
Pawn Loan Redemption Cost Calculator
Estimate pawn redemption cost with simple monthly interest and a fixed fee.
Title Loan Simple Cost Calculator
Estimate a single-payment title loan's simple interest and fees without renewals.
Results are estimates for informational purposes and are not financial, medical or legal advice.