30 Year Interest Rate Calculator and Mortgage Basics

This 30-year interest rate calculator estimates the monthly principal-and-interest payment and total interest for a fixed-rate loan. Enter the loan amount and annual APR; the results show the payment and the amount paid in interest over 360 monthly payments.

Formula

The calculator uses the standard fixed-payment loan formula. Let P be the amount borrowed in dollars, APR be the annual rate in percent, r = APR / 1200 be the monthly rate as a decimal, and n = 360 be the number of monthly payments. The monthly payment is rounded to cents: payment = round(P × r / [1 − (1 + r)⁻ⁿ], 2). Total interest is the rounded monthly payment multiplied by 360, minus P. The APR is an input, not a claim about an available rate.

Worked Example

Using the default inputs, P = $300,000 and APR = 6.5% per year. The monthly rate is 6.5 / 1200 = 0.0054166667, and n = 360.

Payment = round(300,000 × 0.0054166667 / [1 − (1.0054166667)⁻³⁶⁰], 2) = $1,896.20 per month. Total interest = $1,896.20 × 360 − $300,000 = $382,632.00. These totals use the displayed, cent-rounded payment.

Assumptions

• The interest rate stays fixed for the full 30-year term.
• There are 360 equal monthly payments, made at the end of each monthly period.
• Interest compounds monthly using the annual APR divided by 12.
• The calculation excludes fees, taxes, insurance, mortgage insurance, escrow, and any separate charges or credits.

What a 30-Year Interest Rate Means

A 30-year loan term is the scheduled period for repaying the borrowed principal and interest. The interest rate determines how quickly interest accrues on the remaining balance; it is not the same thing as the monthly payment or the total cost. For a fixed-rate loan, a higher input rate increases the calculated payment and, with the other inputs unchanged, the interest paid over the full term.

This page models only the loan arithmetic. A mortgage payment shown by a lender can also include escrowed property taxes and homeowners insurance, and some loans have fees or other charges. Those figures are not estimated here. Get the actual loan amount, rate, fees, and payment details from the lender’s disclosures before using a quote for budgeting or comparison.

How to Read the Results

The first result is monthly principal and interest, not a complete housing payment. It assumes the same payment for all 360 months and does not account for a changing rate, missed payments, extra principal, or a payoff before the term ends. The total-interest result is the sum of those scheduled payments minus the original principal; it is not the total of every possible homeownership cost.

Because payment is rounded to cents before multiplying by the number of months, the displayed total follows the calculator’s stated convention. A lender’s amortization schedule may differ slightly because of day-count methods, payment dates, rounding at each step, or included charges.

Common Mistakes and Limits

Enter the annual percentage rate as a percentage, such as 6.5 for 6.5%, rather than 0.065. The formula converts it to a monthly decimal by dividing by 1,200. Also check that the loan amount represents the amount financed, not a home’s purchase price or a down payment. Do not treat principal-and-interest as an all-in monthly housing cost.

The formula assumes a constant rate, equal end-of-month payments, and no additional principal. It does not model adjustable rates, temporary introductory rates, fees financed into the balance, payment changes, or a borrower’s eligibility. If any of those apply, use the relevant loan documents and a model that includes those details.

How to Use This List

Use the calculator to examine a clearly specified loan amount and annual APR over a 30-year term. If you have a lender quote, enter its stated rate and principal amount; if you are exploring scenarios, change one input at a time so you can see how the payment responds. Treat results as estimates, and keep taxes, insurance, fees, and other costs separate unless you have figures for them.

Frequently Asked Questions

Is the calculator’s APR a current market rate?
No. The 6.5% default is only a sample input. Replace it with a rate quoted for your circumstances by a lender.

Does the monthly payment include taxes and insurance?
No. It estimates principal and interest only; property taxes, insurance, mortgage insurance, and escrow are excluded.

Why can a 30-year loan have substantial total interest?
Interest accrues over many monthly periods on the unpaid balance, so a long repayment schedule can accumulate significant interest even when each payment is level.

Does the calculation work for an adjustable-rate loan?
Not as a full projection. It assumes one fixed APR for all 360 payments and does not model future rate adjustments.

Can I use the result as a lender’s payoff amount?
No. A payoff figure depends on the balance and timing at a particular date and may include charges not represented here.

Final Thoughts

A 30-year rate is one input in a long-term payment calculation, not a complete measure of a loan’s cost. Use the estimate to understand the effect of principal and APR, then check the lender’s disclosures for the exact payment and included costs. This result is an estimate, and the lender’s disclosure gives the exact figures.

Related

• Escrow vs Down Payment: What Homebuyers Should Know
• Escrow Payments for Taxes: How They Work
• Escrow Reserve Payment Calculator and Explanation

in p | Loan amount | $ | 300000 | 1 | 100000000
in apr | Interest rate (APR) | % | 6.5 | 0.01 | 100
out pmt | Monthly principal-and-interest payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -360), 2) | 2
out int | Total interest over 30 years | $ | pmt * 360 - p | 2