How much house can you afford? Ask the 28/36 rule. Form MC-10
Lenders don’t approve vibes; they approve ratios. Two of them: housing costs at or under 28% of gross monthly income, and all debts including housing at or under 36%. Type your income, debts, and down payment and the ledger solves the lower cap backwards into a purchase price — seeded below with $120,000 of income, $600 in monthly debts, and $80,000 down, which the ledger approves up to $401,821.
Tool № 10 · Monday, September 28, 2026 · free, always
The 28% rule binds first for most buyers — debts decide the rest.
Input slip · Form MC-10
Two tabs, one ledger. Affordability solves your income backwards into a max purchase price; Monthly payment prices a specific house down to the dollar. Everything recalculates as you type, and everything stays in your browser — nothing is sent anywhere.
Prefilled at the Freddie Mac 30-year average for the week of Sept 24, 2026 — replace it with your actual quote.
Set to 6.5% for the worked example below (the Freddie Mac 30-year average for the week of Sept 24, 2026 was 7.03%) — replace it with your actual quote.
Posted entry · Form MC-10-R
More doors into the same ledger Form MC-10
Every angle on the mortgage, same arithmetic underneath:
The two ratios that decide your price Form MC-10
The front-end ratio caps housing — P&I, tax, insurance, HOA, PMI — at 28% of gross monthly income. On $120,000 a year that’s $2,800 a month. The back-end ratio caps housing plus all other debts at 36%: $3,600 minus $600 in debts is $3,000. The lower number wins, so $2,800 is the cap.
The price is that cap divided by what a dollar of house costs per month. At the prefilled 7.03% rate the stack works out to $2,147.57 in principal and interest, $368.34 in tax, $150 in insurance, and $134.09 in PMI on the approved $401,821 — the $80,000 down payment is 19.91% of the price, just under the 20% line, so PMI applies and drops the month the balance crosses below 80% of the price.
Change the rate and watch the price move: at 6.5% the same $2,800 cap approves $416,643. The rate is the single biggest lever on what you can buy — bigger than the down payment. Type your actual quote, not the prefill.
Common questions Form MC-10
What is the 28/36 rule?
The two ratios lenders use: housing costs at or under 28% of gross monthly income (front-end), and housing plus all other debts at or under 36% (back-end). The lower of the two caps your payment, and this page solves that cap backwards into a price.
Which rule usually binds?
The 28% front-end rule, for most buyers with ordinary debts. Big car payments or student loans can make the 36% back-end rule bind instead — the page tells you which one won, right under the stamp.
Does a bigger down payment raise my max price?
Two ways: it shrinks the loan, so the same monthly cap buys more house, and crossing 20% kills PMI, which frees monthly budget for price. The seeded $80,000 lands at 19.91% — nudge it to 20% and watch the approval move.
Are property tax and insurance included?
Yes. The price is solved so that principal and interest, tax, insurance, HOA, and PMI land exactly on your monthly cap — not just the loan payment.
Is this the same as a pre-approval?
No. Pre-approval is a lender checking your credit and documents. This is the arithmetic underneath it — what the ratios allow before anyone looks at your paperwork.
Why does the interest rate matter so much?
Because the approved price is your cap divided by the monthly cost per dollar borrowed, and the rate sets that cost. A quarter point moves the approval by thousands.