To calculate a monthly loan payment, you need three numbers: principal, interest rate, and loan term. The formula is: M = P × [r(1+r)^n] / [(1+r)^n − 1]. A $20,000 personal loan at 9% for 5 years has a monthly payment of $415 and costs $4,910 in total interest — 24.6% more than the amount borrowed. Total interest, not just monthly payment, is the number that matters.
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What Is a Loan Payment Calculator?
A loan payment calculator computes your monthly payment, total interest cost, and full repayment schedule for any amortizing loan — personal loans, auto loans, student loans, mortgages, or business loans — given the principal, annual interest rate, and loan term.
An amortizing loan has equal monthly payments throughout its life, but the split between interest and principal changes with every payment. Early payments are mostly interest; later payments are mostly principal. This is because interest is calculated on the remaining balance, which shrinks over time. A calculator that shows the full amortization schedule lets you see exactly how much principal you've paid at any point in the loan.
The core formula — M = P × [r(1+r)^n] / [(1+r)^n − 1] — looks complex but requires only three inputs: P (principal), r (monthly interest rate = annual rate ÷ 12), and n (total number of monthly payments = years × 12). Plug in any loan and it returns the exact monthly payment.
The most valuable output beyond the monthly payment is total interest paid. A $300,000 mortgage at 7% for 30 years has a $1,996 monthly payment — but the total interest paid over the life of the loan is $418,527, more than the original loan amount. Seeing this number motivates paying extra toward principal, which reduces both the total interest and the payoff timeline.
How to Use the Loan Calculator
- Enter the loan amount. The principal — the amount you are borrowing.
- Enter the annual interest rate. The rate quoted by the lender. For existing loans, check your statement or original loan documents.
- Enter the loan term. In months or years. Common terms: personal loans 1–7 years; auto loans 3–7 years; mortgages 15–30 years; student loans 10–25 years.
- Select amortization type. Standard amortizing loans (most personal, auto, and mortgage loans) have equal monthly payments. Interest-only loans have lower initial payments but require a balloon payment or full payoff at term end.
- Read the results. Monthly payment, total interest, total amount paid, and payoff date. The amortization schedule shows principal vs. interest breakdown for each payment.
The optional "extra payment" field shows the powerful impact of paying even $50–100 extra per month — calculated as months shaved off the loan and total interest saved.
The Loan Payment Formula
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
Where:
- M = monthly payment
- P = principal loan amount
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of monthly payments
Worked examples across loan types:
| Loan Type | Amount | Rate | Term | Monthly Payment | Total Interest | |-----------|--------|------|------|----------------|----------------| | Personal loan | $10,000 | 10% | 3 years | $323 | $1,616 | | Auto loan | $35,000 | 6% | 5 years | $676 | $5,574 | | Student loan | $50,000 | 5.5% | 10 years | $542 | $15,038 | | Mortgage | $300,000 | 7% | 30 years | $1,996 | $418,527 |
The mortgage example illustrates the dramatic effect of long loan terms: the total interest paid ($418,527) is larger than the original loan amount ($300,000). This is not unusual for 30-year mortgages and demonstrates why paying extra on mortgages — or choosing 15-year terms — saves enormous amounts.
How Loan Term Affects Total Interest
Shorter loan terms mean higher monthly payments but dramatically less total interest.
$20,000 auto loan at 6.5%:
| Loan Term | Monthly Payment | Total Interest | Interest % of Loan | |-----------|----------------|----------------|-------------------| | 36 months | $614 | $2,096 | 10.5% | | 48 months | $474 | $2,758 | 13.8% | | 60 months | $391 | $3,468 | 17.3% | | 72 months | $337 | $4,246 | 21.2% | | 84 months | $298 | $5,061 | 25.3% |
Extending from 3 to 7 years reduces the monthly payment by $316 (52%) but increases total interest by $2,965 (141%). The 7-year loan's interest cost is nearly equal to a quarter of the original vehicle price — before accounting for depreciation.
The general principle: The longer the loan term, the lower the monthly payment but the higher the total cost. Always calculate total interest, not just monthly payment, when evaluating a loan.
Frequently Asked Questions
What is the difference between APR and interest rate? The interest rate is the base cost of borrowing, expressed as a percentage of the principal. APR (Annual Percentage Rate) includes the interest rate plus fees — origination fees, points, and other loan costs — expressed as an annualized percentage. APR gives a more complete picture of the true cost of borrowing. When comparing loans, use APR for an apples-to-apples comparison. For a loan with no fees, APR and interest rate are identical.
What happens if I miss a loan payment? The consequences depend on the lender. Most have a grace period of 10–15 days before charging a late fee ($25–$50 or 5% of the payment, whichever is greater). After 30 days, the missed payment is typically reported to credit bureaus, damaging your credit score. After 90 days, the loan is often considered in default — the lender may demand the full remaining balance immediately, begin collection proceedings, or (for secured loans like auto loans) repossess the collateral.
How does prepayment work? On most standard amortizing loans, you can make additional payments that apply directly to principal. Because interest is calculated on the remaining balance, any principal reduction saves future interest. Many online lenders and banks apply extra payments immediately; some older-style servicers require you to specifically designate payments as "applied to principal." Always verify with your lender. Check for prepayment penalties — some auto loans and mortgages charge a fee for early payoff.
What is the best order to pay off multiple loans? The mathematically optimal strategy is the avalanche method: pay minimum on all loans, then put extra money toward the highest-interest loan first. This minimizes total interest paid. The motivation method is the snowball: pay off the smallest balance first regardless of rate, for the psychological momentum of eliminating loans. Research shows the avalanche saves more money; the snowball keeps more people on track through behavior change. Choose based on what you will actually stick to.
Can I negotiate a lower interest rate on an existing loan? For personal loans and credit cards, yes — especially if your credit score has improved significantly since you took the loan. Call your lender and request a rate review or hardship reduction. Alternatively, refinancing replaces the existing loan with a new one at a lower rate. For mortgages, refinancing makes sense when the new rate is at least 0.5–1% lower than the current rate and you plan to stay long enough for interest savings to exceed closing costs.
Related Free Tools on RoughTools
- Mortgage Calculator — dedicated calculator for home loan payments with taxes and insurance
- Auto Loan Calculator — car payment calculator with trade-in and down payment options
- Debt Payoff Calculator — avalanche vs. snowball debt payoff planner
- Interest Rate Calculator — calculate the interest rate on a loan given payment and term
Calculate Your Loan Payments Now
The free Loan Payment Calculator at RoughTools calculates monthly payments, total interest, and full amortization for any loan in seconds. Enter loan amount, rate, and term for your complete cost breakdown. No account needed, completely free.