How Personal Loan Interest Is Calculated: Formula & Example
See how personal loan interest is calculated: simple interest, APR vs. interest rate, and the monthly payment formula, with a worked $12,000 example.
Short answer: most personal loans charge simple interest on the unpaid principal balance. Each month the lender multiplies your remaining balance by the monthly rate (the annual rate divided by 12, or a daily rate for the days in the period). The payment stays the same, but the interest portion shrinks and the principal portion grows as the balance falls.
We may earn a referral fee from lenders in our network if you apply through links on this site. See how we work. Examples are illustrations, not quotes.
Interest Rate vs. APR
A loan offer shows an interest rate and an APR, and they are not the same number. The CFPB explains that the interest rate is the cost you pay the lender for borrowing money, while the APR is "a measure of the interest rate plus the additional fees charged with the loan," including origination charges. The CFPB advises comparing APRs to APRs and not APRs to interest rates.
Lenders must disclose the finance charge and APR on closed-end loans before you commit, per Regulation Z, section 1026.18. Use the disclosed APR when comparing, and read origination fees vs. APR to see how fees change the picture.
Simple Interest: How Each Month's Interest Is Computed
The CFPB describes simple interest as interest calculated on a daily or monthly basis, based on your actual balance, so paying more reduces the balance that interest is charged on. (The CFPB page is about auto loans; the same logic applies to other simple interest installment loans. Your loan agreement states how yours works.)
Monthly interest = remaining balance x (annual rate / 12)
If your lender accrues daily, the formula becomes balance x annual rate / 365 x days in the period. Daily accrual explains why interest can differ slightly between 28-day and 31-day months: on an $11,733.97 balance at 14.99%, interest for 28, 30 and 31 days would be $134.93, $144.57 and $149.39.
Worked Example: $12,000 at 14.99% Over 36 Months
All numbers below were computed in Python with the standard payment formula, so you can check any lender's math.
M = P x r / (1 - (1 + r)^-n), where P is the loan amount, r is the monthly rate (annual rate / 12) and n is the number of payments.
For P = $12,000, r = 0.1499 / 12 = 0.012492 and n = 36, the monthly payment is $415.93. Total paid is $14,973.31, so total interest is $2,973.31.
| Payment | Interest portion | Principal portion | Remaining balance |
|---|---|---|---|
| 1 | $149.90 | $266.03 | $11,733.97 |
| 2 | $146.58 | $269.35 | $11,464.63 |
| 3 | $143.21 | $272.71 | $11,191.91 |
Month 1 interest is $12,000 x 0.012492 = $149.90. Subtracting that from the $415.93 payment leaves $266.03 for principal. Month 2 interest is computed on the new, lower balance of $11,733.97, so it is $146.58.
Does the Monthly Payment Change Each Month?
For a fixed-rate, fully amortizing loan, the total monthly payment does not change. What changes is the split: the interest share falls and the principal share rises every month, because interest is charged on a shrinking balance. For a full table, see our amortization schedule explainer.
A payment can change if the rate is variable (see fixed vs. variable rate loans), if you pay extra and your lender recalculates the payment, or if fees or late charges are added.
What a Higher APR Costs in Dollars
Moving from 9% to 36% on the same $10,000 and 24 months adds $3,207 in interest ($4,171 less $964). The monthly payments rise from $456.85 to $590.47. At 14% the payment is $480.13 and total interest is $1,523.09.
For context, the Federal Reserve's G.19 consumer credit release (published October 7, 2026) reported a preliminary average interest rate of 11.90% for August 2026 on 24-month personal loans at commercial banks. That is an average across banks, not what you will be offered, and it can differ from rates at other lender types.
Same Rate, Different Terms: Why the Payment Is Not the Whole Story
Interest accrues for as long as a balance exists, so a longer term costs more even at the same rate. Using the $12,000 at 14.99% example:
| Term | Monthly payment | Total interest |
|---|---|---|
| 24 months | $581.78 | $1,962.79 |
| 36 months | $415.93 | $2,973.31 |
| 48 months | $333.91 | $4,027.59 |
| 60 months | $285.42 | $5,124.97 |
Stretching from 36 to 60 months cuts the payment by $130.51 but adds $2,151.66 in interest. The CFPB cautions that a longer loan can reduce the monthly payment but you pay more interest over the life of the loan. Our term length comparison goes deeper.
Try It Yourself: A Second Example
Take $8,000 at 18% over 24 months (illustrative, no fees). Month 1 interest is $8,000 x 0.18 / 12 = $120.00. The payment is $399.39, so $279.39 reduces principal in month 1. Total interest over the term is $1,585.43. If you add $50 to every payment, the loan is repaid in 21 payments instead of 24 and interest falls to $1,376.83, a saving of $208.60. Interest savings depend on your rate, balance and when you pay extra.
Common Mistakes When Estimating Interest
- Multiplying the rate by the loan amount for the whole term. That ignores the falling balance and overstates simple interest on an amortizing loan.
- Comparing a rate to an APR. Use APR for APR.
- Ignoring fees deducted from proceeds. The amount you receive may be less than the amount you repay.
- Assuming every loan uses the same method. Precomputed interest, which the CFPB distinguishes from simple interest, works differently, and extra payments may not reduce the interest owed. Check your agreement.
What Influences the Rate You Are Offered
Lenders price each application individually. Common factors include:
| Factor | How it can matter |
|---|---|
| Credit history | Lenders weigh it heavily; see credit score requirements |
| Debt-to-income ratio | Higher obligations relative to income signal more risk; see DTI and personal loans |
| Loan term | Different terms are priced differently; see term length comparison |
| Loan amount | Fixed lender costs can weigh more on small loans |
| Lender type | Banks, credit unions and online lenders price differently |
| Collateral | Secured loans may be priced differently; see secured vs. unsecured |
How Origination Fees Change the Effective Rate
What happens to your loan amount when fees are applied? If a lender deducts a fee from the proceeds, you receive less than the amount you repay. On a $10,000 loan with a 12% rate and a 5% fee over 24 months, you receive $9,500 but your payment is still based on $10,000: $470.73 a month. Treating the $9,500 as the amount you actually borrowed, the effective annual rate is about 17.2%, not 12%. A higher fee or a shorter term raises that effective rate further.
Frequently Asked Questions
How is interest calculated on a personal loan? Usually as simple interest: remaining balance x annual rate / 12 for each month (or a daily rate for each day), recalculated as the balance declines.
Does the total monthly payment change each month on a personal loan? Not on a fixed-rate loan. The payment stays constant while the amount going to interest falls and the amount going to principal rises. If the rate is variable, the payment can change.
What changes over time if the payment does not? The balance, the interest charged each month and the principal portion of each payment.
What is the difference between interest rate and APR? The interest rate is the cost of borrowing the principal; the APR adds certain fees, such as origination charges, into a single yearly rate. Compare APR to APR.
Is personal loan interest compounded? Most personal loans use simple interest on the unpaid principal, so interest does not accrue on prior interest. Confirm in your agreement.
How can I check my lender's math? Use the formula above, or the loan calculator and APR calculator.
What to Do Next
Once you can read an offer, compare several. Use the calculator to test rates and terms, learn how paying off early reduces interest, and get started here to prequalify.
General education only, not financial advice.