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Loan Calculator
The loan calculator turns three numbers — how much you borrow, the annual interest rate, and how long you take to repay — into the figure that matters most on any loan agreement: the fixed monthly payment. It also shows the total interest you'll hand over by the time the balance reaches zero, so you can compare the true cost of two offers instead of just their monthly stickers. It's built for anyone weighing a personal loan, auto finance, debt consolidation, or a small-business term loan, and it informs the decision that follows the math: whether the payment fits your monthly cash flow, and whether a cheaper rate or shorter term is worth shopping for.
How the calculation works
M = P × [ i(1 + i)^n ] / [ (1 + i)^n − 1 ]
The formula above is the standard amortizing-loan payment equation. Every letter has a specific meaning:
- M is the fixed monthly payment you're solving for.
- P is the principal — the amount you actually receive and owe on day one.
- i is the monthly interest rate. Take the annual rate as a percentage, divide by 12, then divide by 100 to get a decimal. A 6% APR becomes 0.06 / 12 = 0.005.
- n is the total number of monthly payments — the term in years multiplied by 12.
The equation is built this way because an amortizing loan has two demands that have to balance exactly: every payment is identical, and the balance must hit zero on the last payment. Interest is charged each month on whatever principal remains, so the interest portion of each payment shrinks as the principal does. The (1 + i)^n terms compound the principal forward to the end of the term; dividing by them spreads that future value back across n equal installments. The result is the one payment size that, applied every month, covers the interest and chips away at principal so the balance lands on exactly zero when n runs out.
Worked example
You're consolidating $12,000 of credit-card debt into a 3-year personal loan at 7.9% APR.
Inputs: P = 12,000, annual rate = 7.9%, term = 3 years.
Monthly rate: i = 0.079 / 12 = 0.006583. Number of payments: n = 36.
Compound factor: (1 + i)^n = (1.006583)^36 ≈ 1.2686.
Numerator: i × (1 + i)^n = 0.006583 × 1.2686 ≈ 0.008351.
Denominator: (1 + i)^n − 1 = 1.2686 − 1 = 0.2686.
Fraction: 0.008351 / 0.2686 ≈ 0.031095.
Monthly payment: M = 12,000 × 0.031095 ≈ $373.14.
Over 36 payments that's roughly $13,433 total, of which about $1,433 is interest. Before you sign, check whether $373 fits the budget slot your old minimum payments occupied — if it's lower, consolidation is saving you money each month and puts a fixed end date on debt that used to revolve forever. The $1,433 interest figure is the real price of borrowing; weigh it against the interest you'd have paid on the cards to judge whether the move was worth it.
Second worked example
Same $12,000, same 7.9% APR, but now a 6-year term.
Inputs: P = 12,000, i = 0.006583 (unchanged), n = 72.
Compound factor: (1.006583)^72 ≈ 1.6094.
Numerator: 0.006583 × 1.6094 ≈ 0.010596.
Denominator: 1.6094 − 1 = 0.6094.
Fraction: 0.010596 / 0.6094 ≈ 0.017389.
Monthly payment: M = 12,000 × 0.017389 ≈ $208.67.
Over 72 payments that's about $15,024 total, with roughly $3,024 of it as interest. Doubling the term cut the monthly payment nearly in half but more than doubled the total interest. The rate never moved — the extra cost is purely the price of holding the lender's money for twice as long. That's the trade-off the calculator exposes: a longer term buys breathing room in the monthly budget at a steep premium in lifetime cost. If cash flow is the real constraint, the 6-year loan may still be the right call — but go in knowing the roughly $1,591 gap between the two scenarios.
When this calculator is the wrong tool
It assumes a fixed rate and a fixed payment. Variable-rate credit — most credit cards, some private student loans, HELOCs — can reprice at any time, so the number here is a snapshot, not a guarantee. For those, rely on the lender's amortizing disclosure or model best- and worst-case rates by hand.
It ignores fees. Origination fees, closing costs, and required insurance fold into the real APR but not into this calculation. A loan with a 5% origination fee is meaningfully more expensive than the rate alone suggests; compare offers using each lender's APR, which bundles certain fees, rather than the nominal rate.
It doesn't handle interest-only periods, balloon payments, or graduated schedules. Those break the "equal payment, zero balance" assumption the formula depends on. For mortgages with PMI or property-tax escrow, use a dedicated mortgage calculator that adds those line items to the payment.
Common mistakes
Entering the annual rate where a monthly decimal belongs. Typing "6" into a field that expects 0.005, or "0.5" thinking it means 5%, throws the payment off by an order of magnitude. Always confirm whether a field wants the annual percentage or the monthly decimal before you type.
Mixing up years and months on the term. The formula needs n in months. Enter a 5-year term as "5" where months are expected and you'll get a nonsensical payment. Pick the unit the field asks for and convert yourself only if needed.
Comparing loans on monthly payment alone. A lower payment often just signals a longer term and more total interest. Always read the total-interest figure next to the monthly one before choosing — the cheaper month can cost thousands more over the life of the loan.
Treating the result as tax advice. For mortgages, some interest may be deductible, which lowers the effective after-tax cost. This calculator shows the pre-tax cash outflow, not what borrowing actually costs after deductions.
Frequently Asked Questions
Does the loan calculator include origination fees?
No. It computes principal and interest only, so origination fees, closing costs, and required insurance are excluded and the real cost of borrowing is usually higher than the total interest shown. Compare loans using each lender's APR, which bundles certain fees, rather than the nominal rate.
Why is my actual payment higher than the calculator shows?
Usually because of escrowed items this tool doesn't model — property taxes and insurance on a mortgage, PMI, or HOA dues. Lenders collect these alongside your payment and pass them on. The calculator captures only the principal-and-interest slice of the check you'll actually write.
What's the difference between APR and interest rate here?
The interest rate is the pure cost of borrowing the principal. APR wraps in certain fees and expresses the true yearly cost. This calculator runs on the interest rate, so for fee-heavy loans it understates reality. Enter the APR for a more conservative estimate when fees apply.
Can I use this for a car loan?
Yes. Auto loans are standard amortizing loans, so the principal-rate-term inputs map directly. Enter the amount financed (price minus down payment and trade-in), the annual rate, and the term in years. Skip it for leases, which use a money-factor structure this tool doesn't model.
How do extra payments change the result?
They don't appear here, because the calculator assumes exactly n equal payments. In reality, extra principal payments shrink the balance faster, cutting future interest and shortening the term. A loan-payoff or amortization calculator that accepts extra payments will show the savings — as a rule, even one extra payment a year can remove years from a long term.
Is the monthly payment really the same every month?
For a fixed-rate amortizing loan, yes — that's what the formula guarantees. What shifts is the split inside each payment: interest is highest in month one and falls as principal is paid down, while the principal portion rises. The check you write stays constant; only the internal accounting changes.