The answer, with the formula
Your debt-to-income ratio, or DTI, is the share of your gross monthly income that goes on repaying debt. The formula is simple: add up your total monthly debt payments, divide by your gross monthly income, then multiply by 100 to get a percentage. As a rule of thumb, under 36 percent is considered healthy, 37 to 43 percent is manageable but worth watching, and over 43 percent is high and tends to make borrowing harder and more expensive.
Gross income means your pay before tax and other deductions. The debt side includes the regular payments you are committed to: personal loans, car finance, credit card minimum payments, student loan deductions and any existing mortgage. Everyday spending like groceries, utilities and a phone contract is not usually counted as debt, though lenders look at that separately when they assess affordability.
What the bands actually mean
The thresholds are guides, not hard cut offs, but they map onto how a lender is likely to view you. Below 36 percent, your debts leave plenty of room in your budget, and you look like a low risk borrower. Between 37 and 43 percent, you are still within reach of most lenders, but you have less slack, so a rate rise or an unexpected bill bites sooner. Above 43 percent, a large slice of your income is already spoken for, and lenders worry about your ability to absorb the new payment, which can mean a smaller loan, a higher rate, or a decline.
- Under 36 percent: healthy, with comfortable headroom.
- 37 to 43 percent: manageable, but limited breathing space.
- Over 43 percent: high, and a likely barrier to new borrowing.
It is worth knowing that some lenders split this into a front end ratio, which looks only at housing costs, and a back end ratio, which includes all debt. The thresholds above describe the back end, all in view, which is the one that matters most for a mortgage decision.
A worked example
Suppose you earn 3,600 pounds a month before tax. You have three debts: a personal loan at 300 pounds a month, car finance at 250 pounds, and a credit card with a minimum payment of 150 pounds. Your total monthly debt is 300 plus 250 plus 150, which is 700 pounds. Divide 700 by 3,600 and multiply by 100, and your DTI is about 19.4 percent. That is comfortably in the healthy band, before any mortgage is added.
Now add a prospective mortgage payment of 900 pounds a month. Your total debt becomes 1,600 pounds, and 1,600 divided by 3,600 is about 44.4 percent. That tips you over the 43 percent line into the high band, which could make a lender hesitate. Here is where clearing a debt helps. Say you use savings to pay off the credit card and remove its 150 pound minimum. Your non mortgage debt drops to 550 pounds, the all in total with the mortgage falls to 1,450 pounds, and 1,450 divided by 3,600 is about 40.3 percent. The same person now sits inside the manageable band, which can be the difference between an approval and a refusal, or between a sharper rate and a worse one.
You can run your own version of this with the debt-to-income ratio calculator, which adds up your commitments and shows the percentage against the thresholds. To see how much a lender might actually offer once your DTI and income are in the picture, the mortgage affordability calculator turns the same inputs into a realistic borrowing figure.
How UK lenders use DTI
In the UK, DTI is one input among several rather than the headline test. Lenders typically cap borrowing at an income multiple, often around 4 to 4.5 times your annual income, and they run a detailed affordability assessment that stress tests whether you could still pay if interest rates rose. A low DTI supports both of these by showing your existing commitments leave room for the new payment, even under stress. A high DTI works against you because the stress test has less to play with.
Lenders also look beyond the ratio at the nature of your debts. A large balance on a credit card you only pay the minimum on is read differently from a structured loan that is steadily reducing. Your credit history, deposit size and job stability all feed in too. So while improving your DTI is one of the clearest ways to present yourself well, it sits alongside a broader picture of how you handle money.
Lowering your ratio before you apply
If your DTI is higher than you would like, you have two levers: shrink the debt payments at the top of the fraction, or grow the income at the bottom. Clearing or consolidating a debt is usually the faster route, because removing a monthly payment entirely takes its full amount out of the calculation. Paying down a credit card to free up its minimum, or settling a near finished loan, can move your ratio meaningfully in a single step.
Timing matters too. Avoid taking on new finance, such as a car loan or a buy now pay later commitment, in the months before a mortgage application, since every new monthly payment pushes your DTI back up just when a lender is looking. If credit card interest is what is keeping a balance stubborn, our credit card payoff calculator shows how quickly a fixed monthly payment clears the debt, which both lifts your DTI and frees up cash. This is general educational information, not financial advice; for a decision as large as a mortgage, it is worth speaking to a qualified mortgage adviser who can look at your full circumstances.