Loan Calculator

A fixed-rate loan is easy to sign and surprisingly hard to reason about, because the payment stays the same while its makeup shifts every month. This calculator takes a principal, an annual rate, and a term in years and returns the monthly payment, the total repaid, and the total interest across the life of the loan. It is intended for comparing offers on mortgages, auto loans, student loans, and personal loans before you commit.

How to Use

1. Enter the loan amount: the principal you plan to borrow, in dollars.
2. Enter the annual interest rate: the yearly rate your lender quotes, as a percentage (6.5, not 0.065).
3. Enter the term in years: a whole number between 1 and 50.
4. Read the three results: monthly payment, total payment over the term, and total interest, all updating as you type.

The Amortization Formula

Every fixed-rate, fully amortizing loan uses the same equation:

M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ]

• P is the principal.
• r is the monthly interest rate — the annual rate divided by 100 and then by 12.
• n is the total number of monthly payments — years × 12.
• M is the fixed monthly payment.

For $250,000 at 6.5% over 30 years: r = 0.065 / 12 = 0.0054167 and n = 360. The factor (1 + r)^n works out to about 6.992, so M = 250,000 × (0.0054167 × 6.992) / (6.992 − 1) ≈ $1,580.17. Total payment is $1,580.17 × 360 = $568,861.20, and total interest is that minus the principal: about $318,861.20.

The same equation can be rearranged to answer other questions. Solving for n tells you how long a given payment will take to clear a balance; solving for P tells you how much house or car a target payment supports. This calculator exposes only the forward direction — principal, rate, and term in; payment and totals out — but knowing the form helps you sanity-check a lender’s figure. Because r is the monthly rate, forgetting to divide the annual rate by 12 is the most common arithmetic error, and it inflates the payment dramatically rather than slightly.

Why Early Payments Are Mostly Interest

Interest is charged on the balance that is still outstanding, not on the original loan. In month one the balance is the entire principal, so at 6.5% the interest charge is 250,000 × 0.0054167 ≈ $1,354.17. Subtracting that from the $1,580.17 payment leaves only about $226 toward principal. Next month the balance is $249,774, the interest is marginally smaller, and a few more dollars reach the principal. This is why the tool reports so little equity in the first years and why an extra $100 paid early can erase hundreds of dollars of future interest: every dollar of principal removed stops attracting interest for the remaining term.

A comparison across the life of the loan makes the shift obvious. In year one, interest accounts for roughly $16,200 of the $18,960 paid and principal for only about $2,800. Around the midpoint — roughly year 19 of a 30-year schedule — the two components cross, and more of each payment starts reducing the balance than paying interest. By the final year, almost the entire payment is principal. Extra payments made during the first few years therefore eliminate far more future interest than the same amount paid near the end, which is why prepayment conversations so often focus on the early years.

Worked Examples: The Term Trade-Off

The term moves the payment and the total interest in opposite directions. Using $250,000 at 6.5%:

Term	Monthly payment	Total repaid	Total interest
30 years	~$1,580	~$568,861	~$318,861
15 years	~$2,177	~$391,950	~$141,950
30 years at 5.0%	~$1,342	~$483,138	~$233,138

Doubling the payment roughly halves the interest, while cutting the rate from 6.5% to 5.0% saves about $85,700 over 30 years without changing the monthly figure as dramatically. Running both dials through the calculator makes the trade-offs concrete.

A 0% example behaves differently: $12,000 over 2 years becomes $12,000 / 24 = $500 per month with zero interest, because every payment is principal.

What the Calculator Does Not Include

The output is principal and interest only. It does not model adjustable rates, interest-only periods, balloon payments, or biweekly schedules. It also omits taxes, insurance, PMI, HOA fees, and lender fees, all of which raise the true cost of a mortgage. For a fuller picture, add those to the monthly payment yourself and remember that fees increase the effective annual cost above the quoted rate.

The calculator also assumes you hold the loan to maturity. Most mortgages are refinanced or paid off early through a sale, which changes the realized interest considerably and is not captured by a full-term total. If you expect to move in five years, the right comparison is the interest paid over those five years plus the closing costs, not the 30-year figure.

Common Mistakes

• Entering the rate as a decimal. Type 6.5 for 6.5%; typing 0.065 tells the calculator your rate is 0.065%, which is not what most lenders quote.
• Comparing monthly payment alone. A longer term lowers the payment but raises total interest; look at both columns.
• Forgetting the down payment. The principal is the amount borrowed, not the purchase price. A $300,000 home with 20% down finances $240,000.
• Assuming the last payment is normal. Rounding can make the final payment differ slightly, though the displayed totals already account for that at the cent level.
• Ignoring fees. A lower rate with high closing costs can cost more over a short holding period; compare total interest plus fees, not the rate alone.
• Mixing up home price and loan amount. Down payment, closing costs rolled in, and seller concessions all change the principal. Enter the amount the lender actually disburses.

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