People spend a year working on their credit score and then get turned down for something that had nothing to do with it.
Debt-to-income is what your monthly obligations add up to as a share of what you make before tax. It is arithmetic, not a judgement about you, and it is the thing that most often decides whether a file works.
Two numbers, not one
Front-end is just the housing payment against your income. Back-end is housing plus everything else with a monthly payment: cars, student loans, credit card minimums, child support, anything you co-signed.
Back-end is the one that usually matters.
What the guidelines actually say
Fannie Mae's published standard for a manually underwritten loan is 36 percent, which can go to 45 if you meet their credit and reserve requirements. Run through Desktop Underwriter, the maximum allowable is 50 percent.
VA has no hard cap at all. Their 41 percent figure triggers a supervisor review, not a decline, and the governing test is residual income, which is what you have left over each month after everything including estimated utilities and taxes.
We do not publish FHA's numbers on this site, because we could not verify them from a primary HUD source and putting an unverified underwriting limit in front of you would be worse than leaving it out. Ask a lender who can pull the handbook.
Which debt to pay off
This is the part worth doing before you pay anything off.
A car loan with four payments left often does nothing for you, because lenders frequently exclude debts with fewer than ten payments remaining. You just spent eight thousand dollars and moved your ratio by zero.
A credit card with a two hundred dollar minimum and a four thousand dollar balance moves your ratio a great deal, and it costs you a fifth as much.
Same money. Very different result. That is the whole reason to calculate this before you act rather than after.
The honest caveat
None of this is an approval, and no calculator on this site or any other one is. Every lender adds its own overlays on top of the published guidelines. What this gets you is the ability to walk into that conversation knowing your own number instead of finding it out from someone else.