Loan Calculator — see your real cost before you sign

How the loan calculator works

Every fixed-rate installment loan — whether it’s a $5,000 personal loan, a $35,000 car loan, or a $200,000 mortgage — uses the same underlying math. The bank lends you a lump sum, you agree to pay it back in equal installments over a set period, and each installment is split between principal (paying down what you borrowed) and interest (the cost of borrowing).

The formula is called the amortization formula, and it computes your fixed periodic payment so the loan is exactly paid off by the last installment:

M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]

Where M is your payment, P is the loan amount, r is the periodic interest rate (annual rate divided by payments per year), and n is the total number of payments. It looks intimidating, but the calculator above handles the arithmetic — what matters is knowing what’s happening.

The key insight is that early in the loan, most of each payment goes to interest, not principal. As the balance shrinks, the interest portion of each payment shrinks too, and more goes to principal. The amortization schedule shows exactly how this plays out year by year.

Why payment frequency matters

This calculator lets you choose monthly, biweekly, or weekly payments. Switching from monthly to biweekly isn’t just cosmetic — biweekly payments mean you make 26 payments per year instead of 24 (two monthly payments split in half). That extra month of payments per year shaves significant interest off the loan and shortens the actual payoff time. The calculator shows the exact difference.

When to use this calculator

You’re shopping for a personal loan

Personal loan rates vary widely — from around 6% for excellent credit to 36% for subprime offers. Before you accept any offer, plug the loan amount, the quoted APR, and the term into this calculator. The total interest figure is the real cost of the loan, and it’s often dramatically higher than people expect on longer terms.

You’re comparing loan offers side by side

Two lenders might offer you the same amount with similar monthly payments, but if one has a longer term, you’ll pay much more in total interest. Run both offers through the calculator and compare the “Total paid” figures — that’s the apples-to-apples comparison that matters.

You’re deciding between a short and long term

A 3-year loan has higher monthly payments but dramatically less total interest than a 7-year loan for the same amount. Run both scenarios and see the trade-off. If the higher monthly payment is affordable, the shorter term almost always wins financially.

You’re checking whether you can afford to borrow

Before you apply for a loan, calculate the monthly payment. Then check it against the standard rule of thumb: total debt payments (this loan plus any others) should stay under about 36% of your gross monthly income. If the new payment pushes you over that line, the loan is probably too much.

How to interpret the results

The calculator gives you four numbers. Here’s what each one tells you about the loan:

Monthly payment (or biweekly / weekly): This is the fixed amount you’ll pay every period for the entire term. It doesn’t change — unless your loan has a variable rate, which this calculator doesn’t model.

Total interest: The sum of every interest charge across the life of the loan. This is the actual cost of borrowing. On a $25,000 loan at 6.5% for 5 years, you’ll pay around $4,349 in interest. On the same loan stretched to 10 years, the interest more than doubles even though the rate is identical.

Total paid: Principal plus interest — the total amount of money that leaves your bank account over the life of the loan. This is what you actually pay to borrow.

Interest as % of total: A quick sanity check. On a healthy short-term loan, this might be 10–15%. On a long-term loan at a high rate, it can climb above 50% — meaning more than half of what you pay goes to the lender’s profit, not paying down what you actually borrowed.

What this calculator doesn’t account for

This calculator gives you a clean view of the core loan math, but real-world loans involve costs that aren’t in the basic formula. The numbers below the calculator are accurate for the interest portion of the loan, but the actual total you pay can be higher because of:

Origination fees — many personal loans charge 1–8% of the loan amount as an upfront fee, deducted from what you receive. A $10,000 loan with a 5% origination fee actually delivers $9,500 to your bank account, but you still owe interest on the full $10,000.

Prepayment penalties — some loans charge a fee if you pay them off early. Most modern personal loans don’t, but mortgages and some auto loans still do.

Late fees — missed or late payments typically trigger a fee, often $25–$50, plus possibly a higher interest rate going forward.

Insurance requirements — auto loans typically require full-coverage insurance; mortgages require homeowner’s insurance and sometimes private mortgage insurance (PMI).

Variable rates — this calculator assumes a fixed rate for the entire term. If your loan has a variable rate (common on home equity lines of credit and some private student loans), the actual payment can change over time.

Tax effects — some loan interest is tax-deductible (mortgage interest on a primary residence, student loan interest within limits) but this calculator shows pre-tax numbers.

5 ways to pay less interest on any loan

The calculator shows you exactly how much interest you’ll pay if you accept the loan as quoted. Here are five proven ways to pay less:

1. Shorten the term

A 3-year loan vs a 5-year loan for the same amount, at the same rate, can cut total interest by 40% or more. The monthly payment is higher, but every payment includes more principal and less interest from day one.

2. Make extra payments

Most modern personal loans, auto loans, and mortgages let you pay extra without penalty. An extra $50 a month on a $20,000 5-year loan at 7% saves around $400 in interest and pays the loan off about 8 months earlier.

3. Switch to biweekly payments

Use the calculator’s biweekly option to see the savings. Paying half your monthly payment every two weeks instead of one full payment per month means you make 26 half-payments per year — the equivalent of 13 monthly payments. That extra payment a year pays the loan off years earlier on a long-term loan.

4. Refinance when rates drop

If interest rates fall by 1% or more after you take out a loan, refinancing can save thousands. Run the original loan and the refinance offer through the calculator separately, and compare total interest. Factor in any refinance fees before deciding.

5. Negotiate the rate

Loan rates aren’t always fixed — if you have a strong credit score and shop multiple lenders, you can often negotiate. Even 0.5% off the rate on a large loan saves hundreds or thousands in total interest.

Loan calculator FAQ

What’s the difference between APR and interest rate?

The interest rate is the cost of borrowing the money itself. The APR (Annual Percentage Rate) includes the interest rate plus certain fees (like origination fees), expressed as an annual percentage. For an apples-to-apples comparison between loans, use APR. This calculator uses APR — enter the APR from your loan offer for the most accurate result.

Why is my actual monthly payment slightly different from this calculator?

Lenders sometimes round payments to the nearest cent or dollar, and some lenders use slightly different day-count conventions (30/360 vs actual/365) for the daily interest calculation. The calculator uses the standard amortization formula and produces results within a few cents of what most lenders quote. For loans with origination fees added to the principal, enter the total financed amount, not the cash you receive.

Does this calculator work for mortgages?

Yes — the underlying math is identical. However, mortgages also involve property taxes, homeowner’s insurance, and potentially PMI, which this calculator doesn’t model. We’ll publish a dedicated mortgage calculator (with PITI breakdown) soon. In the meantime, this calculator gives you the principal-and-interest portion of the payment, which is the biggest piece.

Does this work for student loans?

It works for fixed-rate student loans in the standard repayment plan. Federal student loans with income-driven repayment plans (IBR, PAYE, REPAYE) have variable payments based on income, which this calculator can’t model.

What if my loan has a variable interest rate?

This calculator assumes a fixed rate throughout the loan. For variable-rate loans, you can estimate the worst-case scenario by entering the rate cap (the maximum the rate can climb to) — but be aware that the actual cost depends on how rates move over the loan’s life.

Can I model extra payments?

Not in this version. The calculator gives you the standard amortization for the loan as quoted. To see the effect of extra payments, compare two scenarios: the original loan term, and a shorter term that approximates what an extra payment would do. We’re adding extra-payment modeling in a future version.