Online Loan Estimate

Imagine your loan balance as a bucket of water with a slow leak. Each month, interest fills the bucket a little from the top while your payment drains it from the bottom. In the early months, the leak is proportionally large because the bucket is full — so most of your payment goes toward interest, not the balance. As the bucket empties, the leak shrinks, and more of each payment goes toward draining the principal. This is amortization: a fixed monthly payment that gradually shifts from mostly-interest to mostly-principal over time.

The math behind the monthly payment is a closed-form formula derived from geometric series. Given a principal P, a monthly rate r (annual rate divided by 12), and n months, the payment is P times r times (1+r) to the power of n, all divided by (1+r)^n minus 1. The exponent is what makes longer-term loans so expensive — compounding over 60 months versus 36 months is not a 67% increase in interest; it is much larger because the unpaid balance earns interest on interest.

What surprises most borrowers is how small the monthly savings are when extending a term, compared to how large the total interest increase is. Dropping a $15,000 loan from 48 to 60 months might reduce your monthly payment by $60 — but add $900 or more in total interest. The payment shrinks linearly with the term; the interest cost does not. Lenders know this arithmetic better than you do, which is why they default to showing you the monthly payment and not the total cost.

Use this tool when you have received a loan offer or pre-approval and need to quickly check whether the monthly payment fits your budget, how much interest you will pay over the full term, and whether your DTI clears the standard lending thresholds. It works well for personal loans, auto loans, and student loans on standard fixed-rate amortizing schedules.

It is also useful for sensitivity testing before you apply: try different term lengths and rates to see how your payment and total cost respond. If you can afford $50 more per month, does cutting the term by 12 months save meaningful interest? This calculator gives you the number to answer that question in under a minute.

Do not use this tool for mortgages — home loans involve property taxes, insurance, PMI, and complex rate structures that require a dedicated mortgage calculator. Do not use it for credit cards, which use revolving balance calculations rather than fixed amortization. And do not use it to compare variable-rate loans: the formula assumes a fixed rate for the entire term, so if your rate can adjust, your actual payments will differ from this estimate.

Mistake 1: Comparing loans by monthly payment alone. A lower monthly payment often signals a longer term or deferred fees — not a better deal. A $200/month loan over 72 months costs dramatically more than a $280/month loan over 48 months at the same rate. Always compare total cost of the loan, not just the monthly number. Lenders advertise low monthly payments specifically because borrowers respond to them.

Mistake 2: Entering the monthly rate instead of the annual rate. If a lender quotes you 0.75% per month, that is 9% annually — not 0.75%. Entering 0.75 into a calculator that expects an annual rate will drastically underestimate your payment and total interest. Always confirm whether a rate is monthly, annual, or APR before entering it. A boundary warning fires when the entered rate looks suspiciously low.

Mistake 3: Ignoring origination fees in the total cost estimate. Many personal and auto loans carry origination fees of 1% to 8% of the loan amount, deducted upfront. A $10,000 loan with a 5% origination fee means you receive $9,500 but repay $10,000 plus interest. The effective cost of the loan is higher than the stated APR. This calculator covers the stated rate only — always ask your lender for the full APR including fees before comparing offers.

The core formula is the present value of an annuity rearranged to solve for payment. If you lend someone money and they repay it in equal monthly installments, the payment that makes the math balance is: M = P * [r(1+r)^n] / [(1+r)^n - 1]. Here, P is the loan principal, r is the monthly interest rate (APR divided by 1,200), and n is the number of monthly payments. For a zero-rate loan, this formula breaks (division by zero), so payment is simply P divided by n.

Total interest is straightforward: multiply the monthly payment by the number of months, then subtract the original principal. That difference is every dollar you paid that did not reduce your balance — it went to the lender as the cost of borrowing. Interest as a percentage of the loan amount tells you the effective premium you paid for access to the funds, expressed as a single number rather than a rate-over-time.

Debt-to-income ratio adds this loan's monthly payment to your existing monthly debt obligations and divides by gross monthly income. The 43% threshold is a general benchmark used across the lending industry. It is not a firm law, but a rule of thumb with substantial real-world weight: lenders use it to assess repayment capacity, and it appears in underwriting guidelines for most consumer loan products.

The amortization formula assumes all payments occur at the end of each period (ordinary annuity). Some lenders structure payments as due at the beginning of the period (annuity due), which reduces total interest slightly — but this is uncommon for consumer loans. More importantly, the formula assumes no prepayments. Because early payments are almost entirely interest, even a single extra payment in month 1 or 2 eliminates far more total interest than the same payment in month 50. The relationship is not linear: the earlier the extra payment, the more compounding periods it short-circuits.