How Loan Payments Are Calculated
Whether it's a personal loan, auto loan, or student loan, most consumer loans use the same amortization formula to calculate fixed monthly payments. Each payment includes both principal (paying down what you borrowed) and interest (the lender's charge for the loan). Early payments are interest-heavy; later payments shift more toward principal.
The interest rate, loan term, and loan amount are the three levers that determine your payment. Extending the term lowers your monthly payment but increases total interest paid — a tradeoff worth weighing carefully depending on your priority between cash flow and total cost.
The Loan Payment Formula
M = P × [r(1+r)^n] / [(1+r)^n − 1]
M = monthly payment
P = loan principal
r = monthly interest rate (annual ÷ 12)
n = number of payments (months)
Worked Example
Example
A $15,000 personal loan at 7.5% APR over 36 months results in a monthly payment of about $466.67. Over the life of the loan, total interest paid comes to roughly $1,800 — meaning the loan ultimately costs $16,800 in total repayment.
Tips Before Taking a Loan
- Compare APR, not just interest rate — APR includes fees and gives a more accurate picture of total borrowing cost.
- Shorter terms save money — a 36-month loan costs significantly less in total interest than a 60-month loan for the same amount, even though monthly payments are higher.
- Check for prepayment penalties — some loans charge a fee for paying off early; this matters if you plan to pay extra or pay off the loan ahead of schedule.
- Your credit score drives your rate — improving your credit score before applying can meaningfully lower your interest rate and total cost.
Frequently Asked Questions
What's the difference between APR and interest rate? ▼
The interest rate is the base cost of borrowing the principal. APR (Annual Percentage Rate) includes the interest rate plus any additional fees (origination fees, etc.), giving a more complete picture of the loan's true annual cost. Always compare APR when shopping between lenders.
Does paying extra reduce my loan term or payment? ▼
For most amortized loans, extra payments go directly toward principal, which shortens your loan term while keeping your required monthly payment the same — unless you specifically request "re-amortization" from your lender, which recalculates a lower required payment based on the reduced balance.
How does my credit score affect my loan rate? ▼
Lenders use your credit score to assess risk — higher scores typically unlock lower interest rates since you're seen as more likely to repay reliably. Moving from a "fair" to "good" credit tier can often reduce your rate by several percentage points, which adds up to significant savings over the loan term.
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