Interest on a personal loan is calculated on the balance you still owe, not on the amount you originally borrowed. To find how interest is calculated on a personal loan, the lender takes your annual rate, divides it by 12 to get a monthly rate, and multiplies that monthly rate by your current balance. That figure is the interest portion of your next payment. Whatever is left over from your fixed monthly payment goes toward principal, which shrinks the balance and shrinks the next month’s interest charge.
The Monthly Calculation, Step by Step
Most personal loans are fully amortized. Each monthly payment is the same dollar amount from the first month to the last, and the balance reaches zero with the final payment. What changes from month to month is the split between interest and principal.
Start with the rate conversion. A 6% annual rate becomes 0.5% per month (6% ÷ 12). On a $10,000 loan in the first month, the interest charge is $10,000 × 0.005 = $50. If the fixed monthly payment is $304, then $50 covers interest and the remaining $254 reduces the principal. You now owe $9,746.
Month two runs the same formula on the new balance. $9,746 × 0.005 = about $48.73 in interest, so a slightly larger slice of the $304 payment attacks principal. The pattern accelerates. By the loan’s final months, nearly the whole payment is principal and only a few dollars go to interest.
The fixed monthly payment itself is set by a formula that balances the rate, loan amount, and number of payments so the debt clears exactly on schedule. Online amortization calculators will generate a full table showing every payment’s interest-principal split, and reviewing that table before signing is worth the few minutes it takes. It shows the total interest you’ll pay over the life of the loan and how the balance falls each month.
Why Early Payments Are Mostly Interest
Interest lands heaviest at the start of the loan for a simple reason: a bigger balance generates a bigger interest charge. It isn’t a penalty or a trick. But it has a practical consequence. If you pay off a three-year loan after one year, you’ll have paid more than a third of the total interest, because the balance was highest during those early months.
Loans That Accrue Interest Daily
Some lenders calculate interest daily rather than monthly. The daily rate is the annual rate divided by 365. On a $10,000 balance at 5%, that works out to roughly 0.0137% per day, or about $1.37 in interest daily.1Consumer Financial Protection Bureau. What Is a Daily Periodic Rate on a Credit Card The lender totals daily interest charges between payments, and that sum becomes the interest portion of your next installment.
Under daily accrual, the date your payment lands actually matters. Paying a few days early reduces the number of days interest accrues that cycle and trims the charge slightly. Paying late does the reverse. Some lenders use a 360-day year for this calculation instead of 365, which produces a slightly higher daily rate. Your loan agreement specifies which convention applies.
Interest Rate vs. APR
Your loan agreement shows two percentage figures. The interest rate is the annual cost of borrowing the principal alone. The APR folds in additional costs like origination fees and other finance charges, so it captures more of what the loan actually costs per year. On a loan with no fees, the two numbers match. On a loan with a 5% origination fee, the APR can run a couple of points above the stated rate.
Federal law requires lenders to show both. Under the Truth in Lending Act, the terms “annual percentage rate” and “finance charge” must appear more prominently than other loan terms in your disclosure documents.2Office of the Law Revision Counsel. 15 USC 1632 – Form of Disclosure; Additional Information For a closed-end personal loan, the lender must also disclose the amount financed, the total of all payments, and the payment schedule.3eCFR. 12 CFR 1026.18 – Content of Disclosures
When you’re comparing offers, the APR is the fairer comparison. A lender advertising a low interest rate but attaching hefty fees will show a higher APR, and that’s where the true cost becomes visible. The finance charge, which the lender must state as a total dollar amount, is the other number worth reading closely. It includes interest plus loan fees, service charges, credit report fees, and premiums for any insurance the lender requires to protect against default.4Office of the Law Revision Counsel. 15 USC 1605 – Determination of Finance Charge
How Origination Fees Change What You Pay Interest On
Many personal loan lenders charge an origination fee, typically 1% to 8% of the loan amount. Here is the part that catches people off guard: the fee is usually deducted from your loan proceeds before you receive the money, but you still pay interest on the full amount.
Borrow $10,000 with a 5% origination fee and $9,500 lands in your bank account. Your monthly payment, though, is calculated on the full $10,000. If you need a specific amount in hand, you have to borrow more than that amount to cover the fee. The APR captures this gap by incorporating the origination fee into its calculation, which is why the APR on a fee-heavy loan runs well above the stated interest rate.
Fixed vs. Variable Rates
A fixed-rate personal loan locks your interest rate for the entire term. The rate you sign at is the rate feeding the monthly calculation in month one and in month 36. Most personal loans are fixed-rate.
A variable-rate loan ties your interest rate to a benchmark index, often the prime rate. Your rate equals the index value plus a margin set by the lender. If the prime rate is 8% and your margin is 4%, your rate is 12%. When the index moves, your rate moves with it, and so does your monthly interest charge.5Consumer Financial Protection Bureau. For an Adjustable-Rate Mortgage ARM What Are the Index and Margin and How Do They Work The margin is negotiable when you apply but stays fixed once the loan closes.
Variable rates often start lower than comparable fixed rates. The tradeoff is uncertainty. Most variable-rate loans include caps that limit how much the rate can increase per adjustment period and over the life of the loan. A loan with a 2% periodic cap and a 6% lifetime cap on a starting rate of 8% can never exceed 14%. Read the cap structure before signing.
Watch for the Rule of 78s
The Rule of 78s is an older interest allocation method that front-loads interest more aggressively than standard amortization. On a 12-month loan, the lender assigns weights to each month based on months remaining: month one gets 12, month two gets 11, on down to 1 in the final month. The weights sum to 78. Each month’s interest is that month’s weight divided by 78, multiplied by the total interest for the loan.
Under this method, roughly 58% of total interest is collected in the first six months of a one-year loan, compared to about 50% under standard amortization. The consequence hits hardest if you pay off the loan early: because so much interest has already been assigned to the early months, your refund on unearned interest is smaller than you would expect.
Federal law prohibits the Rule of 78s for consumer loans with terms longer than 61 months.6Office of the Law Revision Counsel. 15 USC 1615 – Prohibition on Use of Rule of 78s For shorter-term loans, some lenders still use it. If you may pay off your loan ahead of schedule, check whether the agreement specifies the Rule of 78s or the actuarial method for calculating any interest refund. The difference can be hundreds of dollars.
How Extra Payments Reduce Total Interest
Because interest is recalculated on the remaining balance each period, any extra money you put toward principal immediately reduces every future interest charge. Even modest additional payments can shorten your loan and cut real dollars off the total interest.
The key is directing extra funds specifically to principal. If you don’t specify, some lenders will treat the extra as an advance on your next scheduled payment, which doesn’t reduce interest the same way. Most lenders let you designate the extra as a principal payment, but you may need to call or note it on the payment.
Before making extra payments, check whether your loan carries a prepayment penalty. If it does, the penalty can wipe out some or all of the interest savings, depending on how it’s structured and how far ahead you’re paying.