How much can I borrow

What actually decides the number

Two ratios do almost all the work.

Back-end DTI — every monthly debt payment, including the new mortgage, divided by your gross monthly income. This is the binding constraint for most borrowers, and it is why paying off a car loan can raise your buying power more than a raise would.

Front-end DTI — housing costs alone against income. Conventional lending mostly ignores it; FHA and USDA do not.

Both count the whole housing payment, not just principal and interest: property tax, home insurance, HOA fees and mortgage insurance all sit inside the ratio. That is why two buyers with identical incomes qualify for very different loans depending on where they buy — a high-tax county uses up the same share of income that an extra car payment would.

Mortgage insurance is the one figure you do not type in. Your down payment sets the loan-to-value, the loan-to-value sets the premium: at 20% down there is none, and below that it scales with how little you put down. FHA charges it regardless of how much you put down. You know your down payment; nobody knows the insurer’s rate card offhand.

Why the programs disagree

Each has its own cap, and the differences are not small:

• VA allows the highest ratios and asks no down payment, but is limited to eligible veterans and service members.
• Conventional is the default and generally works to around 45%, stretching toward 50% with strong compensating factors.
• FHA is stricter on the housing ratio but far more forgiving on credit score, which is usually why people choose it.
• USDA is the tightest on ratios and adds geographic and income tests this page does not model.

Above a cap a loan is not automatically refused. It moves from routine to case-by-case: reserves, a bigger down payment or a strong score can carry it. Below the cap, the loan is ordinary.

What this does not do

It is not a pre-approval. A lender runs your actual credit file through an automated system that weighs things no calculator sees — payment history, employment stability, reserves, the property itself.

Income has its own rules. Self-employment, bonuses, commission and rental income are averaged and documented in ways that often differ from what you consider your income. Salaried W-2 income is the simple case; everything else gets discounted.

The caps here are the ordinary ones. Programs publish maximums that exceed them, and lenders apply overlays that tighten them. Treat the output as the boundary of “routine”, not as a rule.

It says nothing about what you should borrow. The largest loan you can get approved for and the largest loan you should take are different numbers, and only one of them is arithmetic.

Next questions

Once you know the size, the cost is set by two other things: the limit in your county, which decides whose rules apply, and what your credit score and down payment cost, which is published and can be worked on before you apply.