An interest loan calculator estimates how much interest a fixed-rate loan will cost and how much a lower rate or shorter term could save. Enter the loan amount, annual interest rate, and term to see the monthly payment and total interest; use the estimates to compare scenarios, not as a lender quote.
Formula
The calculator uses the standard fixed-rate, fully amortizing loan formula. It first finds the monthly payment, rounded to cents, then subtracts the amount borrowed from all scheduled payments to estimate total interest:
Monthly payment = round(P × r ÷ [1 − (1 + r)−n], 2)
Total interest = monthly payment × n − P
- P is the principal, or loan amount, in dollars.
- r is the monthly interest rate, calculated as APR ÷ 1,200 when APR is entered as a percentage per year.
- n is the number of monthly payments, calculated as years × 12.
For example, an APR of 8% becomes a monthly rate of 0.08 ÷ 12, or about 0.006667. The formula assumes the same payment is made every month and that interest accrues monthly. The total-interest output uses the rounded monthly payment, so it reflects the displayed payment rather than an unrounded theoretical amount.
Worked Example
With the default inputs—$20,000 borrowed, 8% APR, and five years—the term is 5 × 12 = 60 monthly payments. The monthly rate is 8 ÷ 1,200 = 0.0066667.
Substitute those values into the payment formula:
$20,000 × 0.0066667 ÷ [1 − (1 + 0.0066667)−60] = $405.53 per month, rounded to the nearest cent.
Total scheduled payments are $405.53 × 60 = $24,331.80. Subtract the $20,000 principal: estimated total interest is $4,331.80.
Assumptions
- The APR stays fixed for the full term.
- Payments are made monthly, at the end of each payment period.
- Interest compounds monthly at APR ÷ 12.
- The calculation excludes fees, taxes, insurance, PMI, and escrow; none are included as inputs.
How to Estimate Interest Savings
“Savings” is the difference between the interest amounts in two comparable scenarios. First, calculate the original loan. Then change one input—such as the APR or term—and calculate the alternative. Subtract the alternative’s total interest from the original’s total interest. A positive difference means the alternative produces a lower modeled interest total, not that it will necessarily reduce the full cost of borrowing.
For instance, if one scenario shows $4,331.80 in interest and another shows $3,600, the difference is $731.80 in estimated interest. Compare like with like: keep the principal and term unchanged when isolating the effect of a rate change. To examine a term change, hold the principal and APR constant. Changing several inputs at once makes it harder to tell which change accounts for the difference.
A shorter term generally means fewer payments, but the monthly payment calculated for that term will be higher. A lower APR can reduce both the payment and total interest when the principal and term stay the same. These are mathematical effects under the model’s assumptions, not a recommendation to select a particular loan.
How to Use and Read the Results
Enter the principal you expect to borrow, the annual APR as a percentage, and the term in years. The calculator returns a monthly payment and the sum of modeled interest over the full schedule. Use APR as a yearly percentage: enter 8 for 8%, not 0.08. The input limits allow positive loan amounts, APRs from 0.01% to 100%, and terms from 0.0833 to 50 years.
For an actual offer, use the lender’s disclosed loan amount, APR, and repayment term. If the APR or payment can change, or if the offer includes costs not represented here, this fixed-rate estimate will not reproduce the complete repayment schedule. Check whether a quoted payment includes optional products or other charges before comparing it with this calculator’s payment.
The monthly payment is not the same as the total cost. Multiply it by the number of payments to get the modeled amount repaid; subtract the principal to isolate interest. The result does not show how much principal is outstanding at a particular date, and it does not account for extra payments. A separate balance or payoff calculation would be needed to model those details.
Common Limits and Mistakes
- Confusing APR and a monthly rate: APR is entered per year, and the formula divides by 12 to get a monthly rate.
- Comparing different loan amounts: A smaller principal can reduce total interest independently of the rate or term.
- Ignoring payment rounding: Real schedules may adjust the final payment by a small amount. This calculator applies the rounded monthly payment to all periods.
- Treating interest as every borrowing cost: Fees and other charges are not included, so the result is not a complete cost comparison.
- Assuming extra payments are modeled: They are not. Additional principal payments can change a real payoff schedule, but they must be calculated separately.
At a 0% rate, the payment formula has a zero-rate edge case; this calculator sets a minimum APR of 0.01% rather than modeling that case. Very small rates will be close to, but not exactly, principal divided by the number of payments.
Frequently Asked Questions
How do I calculate interest savings on a loan?
Calculate total interest for the original and alternative scenarios using the same principal and comparable assumptions. Subtract the alternative’s interest total from the original’s. The difference is an estimate based on the inputs, not a guaranteed reduction in total borrowing costs.
Does a lower interest rate always lower the monthly payment?
With the same principal and term in this fixed-rate model, a lower APR lowers the calculated payment and total interest. If the term, fees, or other loan features differ, the comparison may not hold for the complete cost.
Why can a shorter loan term cost less interest?
A shorter term has fewer payment periods, so interest accrues over less time in this model. It also requires a larger monthly payment for the same principal and APR.
Is APR the same as the interest rate?
APR is an annual percentage figure used here as the rate input. A lender’s APR may reflect details this calculator does not model, so use the disclosed figures and review what is included when comparing offers.
Does this calculator include fees or extra payments?
No. It assumes regular level payments and excludes fees, taxes, insurance, PMI, escrow, and extra principal payments. Those can change the amount paid or the payoff schedule.
This result is an estimate; the lender’s disclosure gives the exact figures.
Related
Formula used
in p | Loan amount | $ | 20000 | 1 | 100000000 in apr | Interest rate (APR) | % | 8 | 0.01 | 100 in yrs | Loan term | years | 5 | 0.0833 | 50 out pmt | Monthly payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2 out int | Total interest | $ | pmt * yrs * 12 - p | 2
Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work
Method: the standard loan formula for this calculation, fixed rate and monthly compounding unless stated, rounded as each result states. Not financial advice; no lender, rate or product is recommended. See formulas and assumptions, the disclaimer or report an error.
More in this section
- Total Monthly Car Cost Calculator: Full Ownership Costs$920.00
- Total Cost to Own a Car Calculator and Guide$405.53
- Total Cost of Vehicle Calculator: What It Includes
- Total Cost to Lease a Car: What You’ll Pay$16,100.00
- Total Cost of Car Ownership Calculator$36,500.00
- Car Registration Renewal Cost: What You’ll Pay$75.00