DSTI Ratio Calculator: Debt Service to Income

How the calculation works

DSTI Ratio: How Lenders Judge Borrowing Capacity

Before agreeing a loan, lenders in the UK need to be satisfied that a borrower can keep up repayments without strain. The debt service to income ratio, usually shortened to DSTI, is one of the simplest ways to express that test as a single figure. It compares the total of a household's monthly credit repayments with the net income that household receives, and gives a quick view of how much of the budget is already committed to debt. Knowing this figure before an application lets you prepare the case, reduce avoidable debt and avoid a refusal that could have been predicted.

What the ratio measures and how it is calculated

The DSTI ratio expresses monthly repayments as a percentage of net income. The formula is DSTI = (Σ P / R) × 100, where Σ P is the sum of every active credit repayment and R is the household's net monthly income after income tax and National Insurance contributions. The result is a percentage. The higher it is, the less income remains for food, bills, savings and unexpected costs.

The numerator includes all active credit agreements that require a regular payment: a mortgage, car finance, personal loans, the monthly repayments on credit cards and any other borrowing still being paid off. What matters is the payment that must be made each month, not the total amount owed. For a credit card, enter the monthly payment that the card agreement requires rather than the full balance, because the balance can be cleared in other ways.

The denominator is net income, not gross pay. A salary on a contract is a gross figure, but a household budget is funded from what arrives in the bank account. When two people apply together, their net incomes are added. Self-employed profits, bonuses and overtime may be treated differently by each lender, so the figure you enter should reflect income you can rely on every month.

How UK lenders assess affordability

The UK does not have a single published debt-to-income ceiling that applies to every mortgage. Lenders set their own affordability criteria, and these must satisfy the rules that the Financial Conduct Authority sets for mortgage lending in its Mortgage Conduct of Business rules, known as MCOB 11. Those rules require a lender to assess whether the borrower can afford the repayments, to take account of committed expenditure such as other loans, and to test how the repayments would cope if interest rates rose. Affordability is therefore a judgement that combines income, outgoings and a stress-tested rate, not a fixed percentage applied mechanically.

Lenders also look at income multiples, which compare the size of the loan with annual income. The Bank of England's Financial Policy Committee monitors the share of new mortgages lent at high loan-to-income multiples and has set limits on that share. This is a measure that applies across the whole market rather than an individual test for each borrower. For an applicant, the practical effect is that a lender can decline a loan that passes a DSTI check if the loan is very large compared with income.

Variable and tracker products are usually linked to Bank Rate or, for some products, to SONIA (the Sterling Overnight Index Average), so repayments can move when the official rate changes. A DSTI calculated on today's rate may understate the repayment that a lender will test. When you enter an estimated repayment for a new mortgage, use a figure that reflects the stressed rate the lender is likely to apply, not the headline rate shown in an advertisement.

For personal loans and credit cards, the APR in the agreement gives the annual cost of borrowing, including charges. The DSTI formula itself uses the monthly repayment, so the APR does not change the calculation directly. It does help you check that the repayment figure you enter reflects all the charges attached to the agreement.

Reading the result

A low ratio leaves more room in the budget and usually signals a lower risk of missed payments. A high ratio does not mean an application will be refused automatically, but it can make approval harder or lead to a smaller loan. The composition matters as much as the total. A ratio made up mostly of a long-term mortgage behaves differently from the same ratio made up of short-term personal loans and card balances, which often carry higher interest and can be cleared sooner.

It is useful to run the calculation twice: once with the mortgage on its own, and once with every debt included. Many households look manageable on the mortgage alone but move towards the upper end of the scale once car finance and card repayments are added. Comparing both views shows where the pressure comes from. If the ratio sits close to the level at which a particular lender tends to decline applications, ask a broker to confirm how that lender treats your specific debts and income.

Ways to lower the ratio before applying

Three approaches usually have the largest effect. The first is to clear high-cost debts with short remaining terms, such as credit cards and small personal loans. This reduces the numerator directly, and once a short-term agreement is paid off its repayment drops out of the calculation completely.

The second is a larger deposit. A bigger deposit reduces the amount borrowed and therefore the monthly mortgage repayment. The saving is proportional to the reduction in borrowing, so even a modest increase in deposit can change the ratio meaningfully.

The third is adding a joint applicant with declared income, which increases the denominator. Couples where only one person is named on the application often show a higher ratio than they would as joint applicants, even when the second income is modest. A joint applicant is also jointly liable for the debt, so this option should be weighed carefully.

Consolidation can also help. Combining several debts into one loan with a lower monthly repayment can improve the ratio, but a longer term usually means more interest paid overall. Compare the total cost of each option before choosing, and be cautious about extending borrowing simply to reduce a monthly figure. Avoid taking on new credit in the months before an application, since each new agreement adds a repayment to the calculation and lenders check recent credit activity.

Common mistakes

Several errors appear often. Entering gross pay instead of net income makes the ratio look lower than it really is, which can give a false sense of comfort. Leaving out a repayment because it is taken automatically from a bank account, or because it is a small agreement, understates the commitments a lender will see on a credit file. Using the headline interest rate for a new mortgage instead of a stressed rate gives a repayment figure that is probably too low. Finally, the ratio is a snapshot. A pay change, a new agreement or a repayment that has ended will all move the figure, so it should be recalculated when circumstances change.

A zero net income needs separate treatment. The formula divides by R, so when R is zero the ratio is undefined and cannot be calculated. It should not be read as 0%, because a zero figure would suggest that no income is committed to debt, when in fact there is no earnings base to measure against. In that case the DSTI cannot be used as a measure of affordability, and a lender will look at whatever verified income the household has.

When to use this calculator and when not to

Use the calculator while planning: before looking at properties, before speaking to a lender, or after a change in circumstances such as a new car finance agreement, a change of job or a pay rise. It is a quick way to check whether existing repayments leave room for a new loan.

Do not treat the result as a final answer. The calculator does not include credit score, employment type, deposit size or the lender's own stress test. It does not model future changes in interest rates, and it cannot reflect how a particular lender weights bonuses or self-employed earnings. For a decision of this size, a whole-of-market mortgage broker authorised by the FCA can compare products across lenders and explain how each one treats your income and debts. The Money and Pensions Service, through its MoneyHelper service, also publishes free and impartial guidance on mortgages and borrowing.