How to Calculate Your Mortgage in 2027
Rates move, but the maths doesn't. In six steps you can work out what a mortgage really costs, whether you can afford it, and how much a few extra dollars a month can save you — with real numbers and charts to show it.
The short version: a $320,000 loan at 6.5% over 30 years costs $2,022.62 a month and $408,142 in interest. Adding just $200 a month cuts about 6 years 7 months off the loan and saves $105,429. (Rates in this article are illustrative — use today's quote when you run your own numbers.)
Step 1 – Know the formula
Almost every fixed-rate mortgage uses the same standard formula for the monthly principal-and-interest payment:
- P – the loan amount (home price minus your down payment)
- r – the monthly interest rate (annual rate ÷ 12)
- n – the number of monthly payments (years × 12)
Example: a $400,000 home with 20% down means a $320,000 loan. At 6.5% (r = 0.065 ÷ 12) over 30 years (n = 360), the payment is $2,022.62 a month. You don't need to do this by hand — the free FinanceKit mortgage calculator does it instantly.
Step 2 – Add the rest of the monthly cost
The formula gives principal and interest only. Your real monthly housing cost usually also includes property tax, home insurance and, if your down payment is under 20%, mortgage insurance. Ask your lender for estimates of each and add them on top before you decide what you can afford.
Step 3 – Check it against your income
A common rule of thumb is that housing costs should stay under about 28% of your gross monthly income, and total debt payments under about 36%. These are guidelines, not laws, but they are a useful sanity check. For the $2,022.62 payment above, 28% means a gross income of roughly $7,224 a month (about $86,684 a year) — before taxes and insurance are added.
Step 4 – See where each payment goes
Early in the loan, most of each payment is interest. On the loan above, the first payment is $1,733.33 interest and only $289.28 principal. That balance flips slowly over the years:
Interest vs principal in each year's payments
$320,000 at 6.5% over 30 years. Gold = interest, green = principal.
Step 5 – Test extra payments
Because interest is charged on the remaining balance, every extra dollar early on works harder than it would later. Here is the same loan with and without an extra $200 each month:
Loan balance over time
Illustrative: $320,000 at 6.5%. With $200 extra per month the loan is paid off in 23 years 5 months.
Case study 1 – Maya, first-time buyer
Maya buys a $400,000 home with 20% down and a 30-year loan at 6.5%. Her payment is $2,022.62. She wonders if rounding up to a $2,222.62 payment is worth it.
| Standard | + $200 / month | |
|---|---|---|
| Monthly payment | $2,022.62 | $2,222.62 |
| Time to pay off | 30 years | 23 yrs 5 mo |
| Total interest | $408,142 | $302,714 |
| Interest saved | — | $105,429 |
Maya enters the extra $200 as the extra monthly payment in the Mortgage Planner and sees the new payoff date, interest saved and time saved update straight away.
Step 6 – Compare your loan options
Rate and term change the picture dramatically. A shorter term means a higher payment but far less interest:
Total interest paid on a $320,000 loan
The 15-year loan assumes a lower illustrative rate of 5.75%.
Case study 2 – Daniel & Priya, 15 years or 30?
Daniel and Priya compare two offers on the same $320,000 loan: a 30-year loan at 6.5% and a 15-year loan at 5.75%.
| 30-year | 15-year | |
|---|---|---|
| Monthly payment | $2,022.62 | $2,657.31 |
| Total interest | $408,142 | $158,316 |
The 15-year loan costs $249,826 less in interest, but the payment is $634.69 a month higher. They use the planner's loan comparison and the 28% affordability check to see whether it fits their income before deciding.
Run your own numbers in the Mortgage Amortization & Payoff Planner
Everything in this article — the schedule, the extra-payment test and the comparison — is built into one spreadsheet that works in Excel and Google Sheets:
- Enter your loan amount, rate, term (1–30 years) and start date on the Loan Setup sheet
- See a full month-by-month amortization schedule
- Add an extra monthly payment and see the interest and time you save
- Compare two loan scenarios side by side
- Check your payment against the 28% affordability guideline
Make the numbers work with the rest of your finances
A mortgage doesn't sit on its own. These planners help with the pieces around it:
50/30/20 Budget Planner
Fit your new payment into a budget with Needs, Wants and Savings targets, and see where you're over.
See the Budget PlannerDebt Freedom & Payoff Planner
Pay down high-interest debts first with avalanche or snowball plans, then put the freed-up cash toward your mortgage.
See the Debt PlannerNet Worth & Wealth Tracker
Track your home as an asset and your mortgage as a liability, and watch your net worth climb month by month.
See the Net Worth TrackerWant them all? Browse the FinanceKit Spreadsheet Store for bundles.
Quick answers
Will 2027 rates change these numbers?
Yes — the formula stays the same but your payment moves with the rate. Re-run your numbers with the rate you are actually quoted.
Is an extra payment always better than investing?
Not always. It depends on your mortgage rate, your other debts and your goals. Compare the interest you save against what that money could earn elsewhere, and clear higher-interest debt first.
This article is for general education only and is not financial advice. All figures are illustrative examples calculated with the standard fixed-rate mortgage formula; actual rates, taxes, insurance and lender requirements vary. The case studies are hypothetical scenarios, not customer stories.