Loan and Mortgage Calculator

Guide by Kepler Oates · Updated Oct 22, 2026

What this calculator is for

Before you sign for a car, a personal loan, a student loan or a home, you want three answers: what will this cost me each month, what will it cost me in total, and when will it be gone? This calculator gives you all three, plus a picture of how fast the balance falls.

It has two modes:

• Loan (car, personal, student): enter the loan amount, the yearly interest rate and the term in years.
• Mortgage: enter the home price and your down payment percentage instead. The tool works out the amount you borrow, and adds property tax (as a percentage of the price per year), home insurance per year and HOA dues per month to show your total monthly with costs.

In both modes you can add an extra amount paid each month to see how much interest and time it saves.

How the math works

Most fixed-rate loans use the standard amortization formula:

M = P × r ÷ (1 − (1 + r)^−n)

• P is the amount you borrow.
• r is the monthly interest rate: the yearly rate divided by 100, then by 12.
• n is the number of monthly payments: years × 12.
• M is the fixed monthly payment for principal and interest.

In plain words: the formula finds the one payment that, made every month, covers that month's interest and chips away just enough principal to reach zero on the last payment. Early on, most of each payment is interest because the balance is large. As the balance shrinks, the interest portion shrinks and more of the same payment goes to principal.

When you add an extra payment, the tool simulates the loan month by month: interest is charged on the current balance, then your regular payment plus the extra comes off. It compares that against the plain schedule to show the interest and months you save.

Worked example: a $25,000 car loan

Example numbers only: you borrow $25,000 at 7% for 5 years.

1. Monthly rate: r = 7 ÷ 100 ÷ 12 = 0.0058333.
2. Number of payments: n = 5 × 12 = 60.
3. (1 + r)^−60 = 0.70541, so 1 − 0.70541 = 0.29459.
4. P × r = 25,000 × 0.0058333 = 145.83.
5. M = 145.83 ÷ 0.29459 = $495.03 a month.

Over 60 payments you pay about $29,702 in total, so the interest is about $4,702. In the first month, $145.83 of your payment is interest and $349.20 goes to principal.

Now add $100 extra each month. The loan is paid off in 4 years 1 month instead of 5 years, and total interest drops to about $3,763. That is roughly $939 saved and 11 months sooner.

A mortgage example

Example numbers only: a $400,000 home, 20% down, 6.5% for 30 years, property tax 1.1% of the price per year, insurance $1,800 a year, no HOA.

• Borrowed: 400,000 × 0.80 = $320,000.
• Principal and interest: $2,022.62 a month.
• Tax: 400,000 × 1.1% ÷ 12 = $366.67. Insurance: 1,800 ÷ 12 = $150.
• Total monthly with costs: $2,539.28.
• Total interest over 30 years: about $408,142, more than the amount borrowed.

Adding $200 a month cuts the term to about 23 years 5 months and saves roughly $105,429 in interest. (With an extra payment, the total monthly figure includes the extra too.)

How to read your results

• Monthly payment: principal and interest only.
• Total monthly with costs (mortgage mode): payment plus any extra, tax, insurance and HOA.
• Total interest: what borrowing costs you, including the effect of any extra payment.
• Paid off: how long until the balance hits zero. The note below gives an approximate finish month.
• The chart: the balance left at the end of each year. A gentle slope early and a steep drop later is normal.

Common mistakes

• Comparing loans by monthly payment alone. A longer term lowers the payment but usually raises total interest a lot. Compare total interest too.
• Forgetting the other housing costs. Tax, insurance and HOA can add hundreds a month. Use mortgage mode, not loan mode, for a home.
• Ignoring mortgage insurance. With less than 20% down, lenders usually add PMI. The tool flags this but doesn't include it, so ask your lender for the figure.
• Using the wrong rate. Enter the interest rate on your offer. Fees rolled into the loan raise the real cost, so check the loan estimate.
• Assuming extra payments go to principal automatically. Some lenders apply them to future payments instead. Ask how to mark them as principal-only.

Practical tips

• Run the same loan at two or three terms and write down the total interest for each. The difference is often eye-opening.
• Try small extras like $25, $50 or $100. Even modest amounts can shorten a loan by months.
• Keep your emergency fund in place before throwing every spare dollar at a low-rate loan.
• Use Print / save PDF to keep a copy of the scenario you compared.

Doing the work

A number on a screen is a start, not a plan. If you decide to pay extra, set it up as an automatic transfer and record each payment, the same way the workbook's daily Receipt logs what you did and what it produced. To make sure the payment fits your month, map it out in the Budget Planner, and if you are juggling several debts, read Debt Snowball vs Avalanche.

This is general information, not financial advice.

Frequently asked questions