The formula
How to calculate debt to income ratio
Debt to income ratio is the share of your gross monthly income already committed to debt payments. It is the first affordability test most lenders apply, and a high ratio blocks approval regardless of credit score.
Count contractual payments only: the mortgage or rent, personal loans, car finance, student loan deductions and credit card minimums. Utilities, groceries, insurance and subscriptions are living costs, not debt.
What to enter:
- Monthly debt payments — mortgage or rent, loans, credit card minimums, car finance — not utilities or food
- Gross monthly income — before tax, as lenders use
Results appear immediately — there is nothing to submit. Changing a field rewrites the link, so you can share the exact scenario you are looking at.
Where a figure is not immediately to hand — a precise interest rate, an exact balance — a reasonable estimate is a perfectly good starting point. Because every result updates instantly, refining a rough guess into the real figure once you have it takes a moment, and nothing about the calculation depends on getting it exactly right on the first attempt.
Why debt to income ratio matters
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
Beyond a one-off check, the same calculation is worth revisiting whenever the underlying numbers change — a new interest rate, a change in income, a different term. Because the figures live in the page's own web address, coming back to update just one field and compare the new result against the old one takes seconds rather than starting again from a blank page.
It is also worth being clear about what a single figure like this can and cannot settle on its own. It answers the specific question the formula was built to answer, and nothing more — a favourable debt to income ratio result does not automatically mean a decision is a good one overall, since plenty of other factors that a formula cannot capture, from personal circumstances to how comfortable a commitment feels, usually matter just as much as the arithmetic. Use the number as one solid input among several rather than the whole of the decision.
Where the same calculation needs to be run for several different scenarios side by side — three loan offers, two savings plans — the fastest approach is usually to open the calculator in a second browser tab for each one, so that the results can be compared directly rather than overwriting each other in a single set of fields.
Worked example
Here is the calculation with the starting values:
- Monthly debt payments: 1,150
- Gross monthly income: 4,200
That gives:
- Debt to income ratio: 27.38 %
- Room before 36%: 362
- Gross income left after debt: 3,050
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
Under 28% is comfortable. Between 28% and 36% is acceptable to most lenders. Above 43% you fall outside standard criteria at many lenders, and above 50% the application is likely to fail affordability regardless of the deposit.
Where this goes wrong. Using net pay. DTI is calculated on gross income, so working from take-home pay produces a ratio several points too high and can make a perfectly viable application look marginal.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
Most UK lenders work to a total commitment ratio under about 36%, though the binding constraint is usually the income multiple — typically 4 to 4.5 times salary — combined with a stress test at a higher interest rate.
Clear the smallest balances first, since removing a payment entirely helps more than reducing several. Consolidating over a longer term lowers the monthly figure and the ratio, but raises the total interest paid.
The answer it gives you is debt to income ratio. With 1,150 monthly debt payments and 4,200 gross monthly income, that comes to 27.38 %. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.