# What is my total mortgage interest?

Computes the total interest on a fixed-rate mortgage, the interest in the first payment and the first year, the interest paid each year, and the interest an extra monthly payment saves.

- Page: https://www.acalculator.org/finance/mortgage-interest-calculator
- JSON spec: https://www.acalculator.org/finance/mortgage-interest-calculator.json
- Version: 3506cbb49d23

## Default answer

Example with the default inputs (Loan amount $320,000.00, Interest rate 6.5%, Loan term (years) 30, Extra each month $0.00, Loan start date September 30, 2026) on the example date Wednesday, September 30, 2026: A $320,000.00 mortgage at 6.5% over 30 years charges $408,142.36 of interest in total.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| amount | Loan amount | The amount borrowed: the home price minus the down payment. |
| rate | Interest rate | The fixed yearly rate. The monthly rate is this ÷ 12. |
| years | Loan term (years) | The length of the mortgage, for example 30 or 15 years. |
| extra | Extra each month | An extra amount paid toward principal with every payment. |
| start | Loan start date | The day the loan starts. The first payment is one month later. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| interest | Total interest | All the interest paid over the life of the loan. |
| payment | Monthly payment | The level principal and interest payment, before any extra. |
| firstMonth | Interest in the first payment | The loan amount × the rate ÷ 12. |
| firstYear | Interest in the first year | The interest in the first 12 payments. |
| share | Interest share of all payments | Total interest as a percent of everything paid. |
| principal | Principal repaid | The loan amount, repaid in full. |
| total | Total paid | The loan amount plus all the interest. |
| payoff | Paid off in | The month of the last payment, when a start date is given. |
| saved | Interest saved by paying extra | The interest without the extra payment minus the interest with it. |
| sooner | Months saved by paying extra | How many fewer payments the extra monthly amount takes. |

## Method

payment = L × r ÷ (1 − (1 + r)^−n), with L the loan, r the rate ÷ 12, and n the months; each month, interest = balance × r and the rest of the payment lowers the balance; total interest is the sum.

## Assumptions

- The rate is fixed and interest is charged monthly at the yearly rate ÷ 12.
- The first payment is one month after the start date; the last payment clears the balance.
- Nothing is rounded to the cent between months; only the display is rounded.
- Property tax, insurance, mortgage insurance, and fees are not included.

## Worked examples

1. amount = $320,000.00, rate = 6.5%, years = 30 gives payment = $2,022.62, interest = $408,142.36, firstMonth = $1,733.33, firstYear = $20,694.69, total = $728,142.36. Source: CFPB, How does paying down a mortgage work? (interest on the balance each month).
2. amount = $320,000.00, rate = 6.5%, years = 15 gives payment = $2,787.54, interest = $181,757.84.
3. amount = $120,000.00, rate = 0%, years = 10 gives payment = $1,000.00, interest = $0.00, share = 0%, firstYear = $0.00.
4. amount = $320,000.00, rate = 6.5%, years = 30, extra = $300.00 gives interest = $269,696.46, saved = $138,445.90, sooner = 106.

## FAQ

### How is mortgage interest calculated each month?

Each month the lender charges the balance you still owe times the yearly rate ÷ 12. On a $320,000 loan at 6.5%, the first month's interest is 320,000 × 0.065 ÷ 12 = $1,733.33. The rest of the payment lowers the balance, so the next month's interest is a little smaller.

### Why is most of my early payment interest?

The payment stays the same, but interest is charged on the balance, which is largest at the start. On a 30-year loan at 6.5%, about 86% of the first payment is interest. As the balance falls, more of each payment goes to principal.

### How much interest does a 15-year mortgage save?

A shorter term means a higher payment but far less interest. $320,000 at 6.5% costs $408,142 of interest over 30 years and $181,758 over 15 years. In real life a 15-year loan often has a lower rate too, which saves more.

### Does paying extra each month reduce interest?

Yes. Extra money goes straight to principal, so every later month charges interest on a smaller balance. $300 extra a month on $320,000 at 6.5% over 30 years saves $138,446 of interest and ends the loan 106 months sooner. Check that your loan has no prepayment penalty first.

### Is mortgage interest tax deductible?

Mortgage interest can be deducted if you itemize, for up to $750,000 of mortgage debt on loans taken after December 15, 2017 (IRS Publication 936). Most people take the standard deduction instead, so check whether itemizing helps you. This page shows the interest, not a tax saving.

### Why does my lender's number differ a little?

Lenders round each payment to the cent, and some loans charge interest by the day. This page keeps full precision and charges a month's interest as the balance × rate ÷ 12, so totals can differ by a few dollars.

## Sources

- Consumer Financial Protection Bureau, How does paying down a mortgage work? https://www.consumerfinance.gov/ask-cfpb/how-does-paying-down-a-mortgage-work-en-1943/
- Consumer Financial Protection Bureau, What is a prepayment penalty? https://www.consumerfinance.gov/ask-cfpb/what-is-a-prepayment-penalty-en-1957/
- IRS Publication 936, Home Mortgage Interest Deduction ($750,000 limit). https://www.irs.gov/publications/p936
- Amortized loan payment (annuity) formula: S. A. Broverman, Mathematics of Investment and Credit, chapter 3.