How to Use This Calculator
- 1
Enter Loan Amount
Input the total principal sum you are borrowing.
- 2
Enter Annual Percentage Rate (APR)
Input the yearly interest rate charged by the lender.
- 3
Select Term Unit
Choose whether your loan term is measured in Years or Months.
- 4
Set the Loan Term
Enter the duration of the loan in the unit you selected.
- 5
Review Your Results
The calculator displays your Monthly Payment, Total Interest Paid, Total Amount Paid, and Interest-to-Principal ratio. The insights panel shows the true cost of borrowing, daily cost, and a principal vs. interest breakdown. Scroll down to see the balance chart and full amortization schedule.
Example Calculation
An individual is planning a $25,000 personal loan at 7.5% APR for 5 years.
Loan Amount
$25,000
Annual Percentage Rate (APR)
7.5%
Term Unit
Years
Loan Term
5
Results
Monthly Payment
$500.95
Total Interest Paid
$5,056.92
Total Amount Paid
$30,056.92
Interest-to-Principal
20.2%
Insights card shows true borrowing cost, daily cost, and principal vs.
Tips
Compare APRs Carefully
Even a 2.5% rate difference matters. A $25,000 loan over 5 years costs $5,057 in interest at 7.5% but only $3,307 at 5% — saving $1,750.
Shorten the Term to Save
The same $25,000 at 7.5% costs $5,057 over 5 years but $2,996 over 3 years. The monthly payment rises from $501 to $778, but you save $2,061 in interest.
Check the Amortization Table
Scroll below the chart to see a month-by-month breakdown of how each payment splits between principal and interest. Early payments are mostly interest.
Use the Chart to Visualize Progress
The balance chart shows your remaining balance declining over time alongside cumulative interest, making it easy to see when you've paid more principal than interest.
Understanding Loan Interest and Payments
Knowing how much a loan truly costs — beyond just the monthly payment — is essential for financial planning.
This Loan Interest Calculator breaks down your borrowing costs by showing the monthly payment, total interest, and total amount repaid.
For example, a $25,000 loan at 7.5% APR over 5 years requires monthly payments of $500.95, with $5,056.92 going to interest — meaning you pay back $1.20 for every $1 borrowed.
The Amortization Formula
The calculator uses the standard amortization formula for fixed monthly payments:
M = P x r / (1 - (1 + r)^-n)
Where:
P= Loan amount (principal)r= Monthly interest rate (APR / 1200)n= Total number of payments (term in months)
From the monthly payment, total costs are derived:
Total Amount Paid = Monthly Payment x n
Total Interest = Total Amount Paid - Loan Amount
Interest-to-Principal Ratio = (Total Interest / Loan Amount) x 100%
Worked Example: $25,000 Personal Loan
An individual takes out a $25,000 personal loan at 7.5% APR over 5 years.
- Convert to months: 5 years x 12 = 60 months
- Monthly rate: 7.5% / 1200 = 0.00625
- Monthly payment: $25,000 x 0.00625 / (1 - (1.00625)^-60) = $156.25 / 0.31191 = $500.95
- Total paid: $500.95 x 60 = $30,056.92
- Total interest: $30,056.92 - $25,000 = $5,056.92
- Interest-to-principal: ($5,056.92 / $25,000) x 100 = 20.2%
The 20.2% interest-to-principal ratio means for every $1 borrowed, $0.20 goes to interest.
How Amortization Works
Each monthly payment covers two components: interest on the remaining balance and principal reduction.
In the early months, most of the payment goes to interest because the balance is large.
Over time, as the balance decreases, more of each payment goes to principal.
For the $25,000 loan at 7.5%:
- Month 1: $156.25 interest, $344.70 principal (31.2% to interest)
- Month 30: $87.99 interest, $412.96 principal (17.6% to interest)
- Month 60: $3.11 interest, $497.84 principal (0.6% to interest)
This shift is visible in the amortization chart above the table.
The Impact of Rate and Term on Total Cost
Small changes in rate or term have outsized effects on total interest:
| Scenario | Monthly Payment | Total Interest |
|---|---|---|
| $25,000 at 5% for 5 years | $471.78 | $3,306.85 |
| $25,000 at 7.5% for 5 years | $500.95 | $5,056.92 |
| $25,000 at 7.5% for 3 years | $777.66 | $2,995.60 |
| $25,000 at 10% for 5 years | $531.18 | $6,870.57 |
A 2.5% rate reduction (7.5% to 5%) saves $1,750, while shortening the term from 5 to 3 years saves $2,061 — though at a higher monthly payment.
Frequently Asked Questions
What is a loan interest calculator used for?
A loan interest calculator estimates your monthly payment, total interest paid, and overall cost based on the loan amount, interest rate, and term. It helps you compare different loan offers and plan your budget before committing.
How is the monthly payment calculated?
The monthly payment uses the standard amortization formula: M = P x r / (1 - (1 + r)^-n), where P is the principal, r is the monthly rate (APR/1200), and n is the total number of payments. For example, $25,000 at 7.5% for 60 months = $500.95/month.
What is APR and why does it matter?
APR (Annual Percentage Rate) is the yearly cost of borrowing expressed as a percentage. It provides a standardized way to compare loan offers. A loan at 7.5% APR costs more than one at 6% — on $25,000 over 5 years, that 1.5% difference means $1,058 more in interest.
Can I reduce total interest by paying extra?
Yes. Making extra payments reduces your principal faster, which means less interest accrues each month. Even an extra $50/month on a $25,000 loan at 7.5% can save hundreds in interest and shorten the loan term. Our Loan Calculator includes an Extra Monthly Payment feature for this analysis.