The big four numbers
Buying a home comes down to four numbers: monthly payment (principal, interest, taxes, insurance, plus PMI below 20% down), affordability (what your income and debt-to-income ratio support), down payment (cash needed up front), and equity (the slice of the home you actually own as principal pays down and the home appreciates). Each calculator on this hub focuses on one of those four — together they cover the full pre-purchase picture.
How much house can you afford
Start with Home Affordability to find your ceiling. It applies the standard 28/36 rule (housing under 28% of gross income, total debt under 36%) plus the actual debt-to-income limits lenders use, so the number you get is closer to what an underwriter would actually approve than a back-of-envelope multiple of your salary.
Down payment, PMI, and equity
The Down Payment Calculator shows how the size of your down payment affects loan amount, monthly cost, and whether you need to pay private mortgage insurance. Below 20% down, PMI typically adds 0.3–1.5% of the loan balance per year. The Home Equity Calculator tracks the other side: how equity grows from principal payments and home appreciation over time, including a year-by-year split between the two.
Rent vs. buy
Rent vs. Buy answers the longer-horizon question. It compares the full lifetime cost of buying (down payment, closing costs, mortgage payments, taxes, insurance, maintenance, opportunity cost of the down payment) against renting (rent + opportunity cost on the avoided down payment). It's the right tool for deciding whether to buy at all, whereas the mortgage payment calculator answers "given that I'm buying, what does it cost per month?" via the Mortgage Payment Calculator.
Once you have a mortgage
Two calculators take over after closing. The Mortgage Refinance Calculator compares your current loan to a new one, surfacing monthly savings, the closing-cost break-even month, and lifetime interest difference — the three numbers that determine whether refinancing actually pays off. The Mortgage Extra Payment Calculator models the other path to lower lifetime interest: paying extra principal — recurring monthly, an annual lump sum, or a one-time payment — and reports exactly how much time and interest you save.
15-year vs. 30-year: the numbers
This is the most consequential choice in the whole mortgage, and it is usually argued badly. Here is a $400,000 loan at illustrative rates — 15-year mortgages typically price about three-quarters of a point below 30-year:
| 30-year @ 6.50% | 15-year @ 5.75% | |
|---|---|---|
| Monthly payment | $2,528 | $3,322 |
| Total interest | $510,178 | $197,895 |
| Interest saved | — | $312,283 |
The standard rebuttal is that you should take the 30-year and invest the $793 monthly difference. Run that honestly — identical total outlay in both cases — and it still loses. Investing the difference for 30 years at a 7% return produces about $967,900. Taking the 15-year and then investing the entire $3,322 payment for the 15 years after payoff produces about $1,052,800 — roughly $85,000 more. The lower locked rate compounds too, and that is easy to forget.
That said, the result flips if the rate gap between 15- and 30-year loans is narrow, or if investment returns run well above 7%. And the genuine case for the 30-year is not the math at all — it is that a $793 lower required payment is much easier to sustain if your income drops. You can always pay a 30-year down faster; you cannot make a 15-year payment smaller. Model both with the Mortgage Payment Calculator and the Extra Payment Calculator.
Related categories
Auto and personal loans use the same amortization math — see Loan Calculators for non-mortgage debt, and Amortization Schedule for a full month-by-month breakdown of any loan, including a mortgage. For long-term wealth-building tools, see Savings & Investing.
Frequently Asked Questions
Should I choose a 15-year or 30-year mortgage?
The 15-year costs far less interest and usually carries a lower rate, but the payment is much higher. On a $400,000 loan, a 30-year at 6.5% runs about $2,528 a month and $510,000 in lifetime interest; a 15-year at 5.75% runs about $3,322 a month and $198,000 in interest. The common counter-argument is that you could take the 30-year and invest the $793 monthly difference. Run that fairly — same total outlay in both cases — and the 15-year still comes out ahead by roughly $85,000 after 30 years at a 7% return, because the guaranteed savings from the lower rate compound too. The real argument for the 30-year is not returns, it is flexibility: the lower required payment is easier to sustain if income drops.
How much house can I afford?
A common guideline is that total housing costs stay under 28% of gross monthly income, and all debt payments under 36%. Lenders often approve more than that, which is why affordability and comfort are different questions. Remember the payment is not just principal and interest — property tax, homeowners insurance, PMI, and HOA dues can add hundreds a month, and property tax and insurance both tend to rise over time.
When does refinancing make sense?
Refinancing makes sense when the monthly savings recover the closing costs before you expect to sell or refinance again. If closing costs are $6,000 and the new rate saves $200 a month, the break-even is 30 months — refinancing is worth it if you will stay past that, and not if you will not. Watch the term reset too: refinancing a loan you are 8 years into back to a fresh 30-year can lower the payment while increasing total interest paid.
What is PMI and when does it go away?
Private mortgage insurance is typically required when your down payment is under 20%. It protects the lender, not you. Under the Homeowners Protection Act, PMI terminates automatically once the balance reaches 78% of the original property value, and you can request cancellation at 80%. Requesting it at 80% rather than waiting for automatic termination usually saves several months of premiums.
Should I make extra mortgage payments or invest instead?
Compare your mortgage rate to your realistic after-tax investment return. Extra principal payments earn a guaranteed return equal to your mortgage rate, which is attractive when rates are high and less compelling when they are low. Extra payments are also illiquid — money put into the house is hard to get back out without a sale or a HELOC. Most guidance suggests funding an emergency fund and any employer 401(k) match before making extra mortgage payments, because both have a better risk-adjusted payoff.