Buying Your First Home: A Complete Numbers Walkthrough
How to figure out what you can afford, what your payment will actually be, and what refinancing might look like later — with real numbers.
Buying a first home usually starts with the wrong question — "how much house can I afford?" — before answering an earlier one: what payment actually fits your budget without straining it.
Step 1: work out your affordability ceiling first
Start from income, not from a listing price. With $7,000 in monthly gross income, $500 in existing debts, and a lender targeting a 36% debt-to-income ratio, the maximum total housing payment is $7,000 × 0.36 = $2,520, minus the existing $500 debt, leaving $2,020 available for principal and interest. At 6.5% over 30 years, that supports a maximum loan of roughly $319,600 — this is the ceiling, not necessarily the target.
Step 2: price a specific home against that ceiling
Say you find a $350,000 home and put 6% down ($21,000), financing $329,000 at 6.75% over 30 years. Running the mortgage formula: monthly principal and interest comes to approximately $2,134 — above the $2,020 ceiling calculated in step 1. That gap is exactly the kind of thing this two-step process is designed to catch before you're emotionally attached to a specific house: either the down payment needs to increase, the rate needs to improve, or the price needs to come down.
Step 3: sanity-check your existing debt load
Debt-to-income ratio isn't just a lender formality — it's a genuine measure of how much monthly obligation you're carrying relative to income. Recalculating DTI with the new mortgage payment included (not just existing debts) shows the real picture a lender will actually underwrite against, which is often the number that determines loan approval more than the home price itself.
Step 4: know that refinancing later is a real, separate option
Rates fluctuate over the life of a 30-year loan. If rates drop meaningfully in a few years, refinancing that same $329,000 balance from 6.75% down to, say, 5.75% could save well over $200 a month — the exact same amortization math, just re-run against a new rate on the remaining balance. This isn't a reason to accept a worse rate today, but it's worth knowing the door isn't permanently closed on today's number.
Putting it together
The order matters: affordability ceiling first, specific home price second, DTI reality-check third. Skipping straight to "can I get approved for this house" without first knowing your own ceiling is how many buyers end up house-poor even with a technically approved loan.
Work through your own numbers with the home affordability calculator, mortgage calculator, and debt-to-income ratio calculator.