Test whether your mortgage is comfortable now and whether it would still be comfortable if interest rates rose meaningfully. The calculator gives you three numbers that matter: your current affordability ratio (mortgage as % of net income), your stressed ratio at higher rates, and the buffer remaining after all essential outgoings. A clear red/amber/green indicator tells you whether to act now or wait.

How to read the affordability ratio

The affordability ratio is mortgage payment ÷ net income, expressed as a percentage. The wider "committed ratio" includes other monthly debt commitments too. Both ratios matter to lenders and to you.

Mortgage/income ratio	Interpretation
Under 25%	Comfortable — room for rate rises and life events
25-35%	Standard — most UK homeowners
35-45%	Stretched — typical lender upper limit
45-55%	Exposed — rate rise of 1-2pp would push into distress
Over 55%	Already in distress at current rates

Why the stress test matters

UK mortgages are typically fixed for 2, 3, 5, or 10 years. When the fix expires, you re-fix at then-current rates. If rates have risen, your payment rises. The stress test asks "what happens if my next fix is at a meaningfully higher rate?"

Lenders are required by the FCA to stress-test affordability at higher rates before lending. The mandatory 3pp stress was withdrawn in 2022 but most lenders still apply 2-3pp. You should personally stress at the same level — or higher if your fix has many years to run.

Worked example 1 — comfortable household

Net income £4,200/month. Mortgage £1,200/month at 4.99%. Other commitments £400 (credit card minimums + car finance). Stress at +3pp.

• Current ratio: 1,200 ÷ 4,200 = 29% ✓ comfortable
• Stressed payment: £1,200 × (1 + 3/4.99) ≈ £1,920/month
• Stressed ratio: 1,920 ÷ 4,200 = 46% ⚠ stretched
• Current buffer: 4,200 − 1,200 − 400 = £2,600/month
• Stressed buffer: 4,200 − 1,920 − 400 = £1,880/month (still healthy)
• Risk: GREEN/AMBER — comfortable today, stretched but still positive buffer at +3pp.

Worked example 2 — exposed household

Net income £3,200/month. Mortgage £1,400/month at 4.99%. Other commitments £600. Stress at +3pp.

• Current ratio: 1,400 ÷ 3,200 = 44% ⚠ stretched at current rates
• Stressed payment: £1,400 × (1 + 3/4.99) ≈ £2,240/month
• Stressed ratio: 2,240 ÷ 3,200 = 70% ✗ severe distress
• Current buffer: 3,200 − 1,400 − 600 = £1,200/month
• Stressed buffer: 3,200 − 2,240 − 600 = £360/month (essentially no buffer)
• Risk: RED — rate rise would push into hard distress. Act now.

Worked example 3 — high earner with big mortgage

Net income £9,000/month. Mortgage £3,500/month at 4.5% on £750,000 loan. Other commitments £1,200 (private school, childcare). Stress at +3pp.

• Current ratio: 3,500 ÷ 9,000 = 39% ✓ standard for high earners
• Stressed payment: £3,500 × (1 + 3/4.5) ≈ £5,830/month
• Stressed ratio: 5,830 ÷ 9,000 = 65% ✗ severe distress
• Current buffer: 9,000 − 3,500 − 1,200 = £4,300/month
• Stressed buffer: 9,000 − 5,830 − 1,200 = £1,970/month (still positive but materially tighter)
• Risk: AMBER — manageable today; reduce other commitments or build cash reserve before next remortgage.

Worked example 4 — first-time buyer at the limit

Net income £2,800/month. Mortgage £900/month at 5.49% (FTB 95% LTV). Other commitments £350. Stress at +3pp.

• Current ratio: 900 ÷ 2,800 = 32% ✓ standard
• Stressed payment: £900 × (1 + 3/5.49) ≈ £1,392/month
• Stressed ratio: 1,392 ÷ 2,800 = 50% ✗ approaching distress
• Current buffer: 2,800 − 900 − 350 = £1,550/month
• Stressed buffer: 2,800 − 1,392 − 350 = £1,058/month (still positive but slim)
• Risk: AMBER — manageable today; income growth before next remortgage critical.

Levers to improve your buffer

1. Increase income

The most direct lever. A £200/month income lift improves both ratios and grows the buffer pound-for-pound. Promotion, side income, or partner returning to work.

2. Reduce other commitments

Clearing credit cards eliminates minimum payments. Settling car finance early frees up £200-£400/month. Reducing other commitments by £300 has the same buffer effect as increasing income by £300 — often easier to achieve.

3. Overpay the mortgage

Reducing the balance reduces the future payment when you remortgage. £200/month overpayment over 2 years reduces balance by £4,800+ interest savings, lowering next fix's monthly cost. Most fixes allow 10% overpayment per year without ERC.

4. Extend the term

At remortgage, extending the term from 22 to 30 years drops the monthly payment meaningfully. Trade-off: pays more total interest. Useful for short-term affordability stretch with plan to overpay later.

5. Interest-only temporarily

Some lenders allow part-and-part (some capital, some interest-only) structures that reduce monthly payment. Pure interest-only requires repayment plan. Discuss with lender at remortgage.

6. Reduce LTV

A capital lump sum at remortgage (e.g. from family gift, inheritance, bonus) reduces LTV band — often unlocks better rates and lower monthly payment.

7. Downsize

The ultimate lever. Selling and buying a smaller property releases equity and reduces the mortgage. Last resort but sometimes the right call if other levers exhausted.

How lenders calculate the same numbers differently

Lender affordability assessments are stricter than this calculator in 2-3 ways:

• Gross vs net income: Lenders use gross income for their multiples (4-5×). Conversion to monthly net (your real take-home) gives a different denominator.
• Conservative outgoings: Lenders use ONS Family Spending data as a floor for essential outgoings, even if your actual spending is lower. Their assessment of your "available" income is usually lower than yours.
• Stress test mandatory: Lenders stress-test affordability at +2-3pp before lending. They use the stressed payment, not the current pay rate.
• Wider commitments definition: Lenders include childcare, travel-to-work, food, utilities. The calculator uses a narrower "essential debts and bills" definition.

Pass this calculator's stress test and you'll usually pass the lender's. Fail this calculator's stress test and you almost certainly will fail the lender's. See mortgage stress test explained for the lender-side framework.

When to act vs when to hold

Act now if:

• Stressed ratio above 50%
• Stressed buffer below £200/month
• Other commitments above 20% of net income (focus on debt reduction)
• Fix expires within 12 months

Build over 6-12 months if:

• Stressed ratio 40-50%
• Stressed buffer £200-£800/month
• Fix expires within 24 months

Hold and monitor if:

• Stressed ratio below 40%
• Stressed buffer above £800/month
• Fix expires beyond 24 months

Common stress-test mistakes

Using gross income

Affordability ratios use net (take-home) income, not gross. A £60k gross salary is roughly £3,500 net monthly after tax, NI, pension, student loan. Using £5,000 (gross divided by 12) overstates affordability by 30%+.

Ignoring credit card minimums

A £20,000 credit card balance has a typical £400/month minimum payment. Lenders include this; you should too. Other commitments includes ALL debt service, not just visible loans.

Forgetting childcare

Lenders include childcare in commitments. £800/month childcare can swing affordability materially. Include it if you have it.

Stressing only at +1pp

A 1pp rate rise sounds large but actually only adds 15-25% to a typical mortgage payment. Real distress scenarios are +2-3pp+. Stress at FCA legacy 3pp as a baseline.

Comparing to gross household budget

"I have £4,000 free after the mortgage" sometimes means gross. Strip out tax, NI, pension contributions, then debt service, then essentials. The remaining buffer is what actually absorbs rate rises.

Three buffer levels every household needs

Operating buffer

The monthly income surplus after all essential outgoings. This is what the calculator shows. Target 25-35% of net income.

Emergency reserve

Instant-access cash savings equal to 3-6 months of total essential outgoings. Covers job loss, large unexpected repairs, family emergencies.

Rate-rise reserve

Additional cash savings equal to 12 months of the stressed payment increase. Lets you absorb a remortgage rate shock while finding ways to reduce other commitments.

How this differs from the standard stress test calculator

The site's mortgage stress test calculator shows your stressed mortgage payment in pounds — what you'd actually pay at higher rates. This affordability buffer calculator goes one step further and asks: can you sustainably afford that stressed payment given your income and other commitments? It's the household-level stress test, not just the payment-level stress test.

Use both together: stress test calculator for the payment figure, affordability buffer calculator for the sustainability assessment.

What to do with the risk indicator

Green indicator

Your buffer is healthy at both current and stressed rates. Use the headroom: consider overpaying the mortgage (most fixes allow 10% overpayment per year without ERC), building cash reserves, or contributing more to pension.

Amber indicator

You're comfortable today but exposed to a meaningful rate rise. 12-24 month action plan: build cash reserves, reduce other commitments, consider overpaying to lower next-fix balance. Don't take on any new debt.

Red indicator

You're already stretched at current rates and would be in distress at +3pp. Act immediately: clear high-interest debt first; consider extending mortgage term at next remortgage; consider partial interest-only structure; consider downsizing. Most household financial crises start in the red zone — don't ignore it.

Buffer trajectory: how the picture shifts over a fix

Your buffer evolves across the fix period. Typical 5-year fix:

• Year 1: Tightest. Just-paid completion costs, new bills setup, perhaps furnishing. Buffer feels small even if numbers say healthy.
• Years 2-3: Stabilises. Income usually rises faster than fixed mortgage. Buffer grows in real and nominal terms.
• Year 4: Peak buffer typically. Fixed mortgage feels increasingly cheap vs current income.
• Year 5 (fix end): Anticipation phase. Plan remortgage at month -6. New fix may reset buffer down depending on rate environment.

Run this calculator at month -6 of every fix end to test whether the next remortgage rate environment is sustainable before you commit to a new fix.

Stress test maths in detail

The calculator uses a proportional adjustment to the current payment rather than a full re-amortisation. This is accurate enough for stress-test purposes and matches how most lenders conceptualise the test. Formula:

stressedPayment = currentPayment × (1 + rateRise ÷ currentRate)

Worked example: current rate 5%, current payment £1,000. Stress at +3pp: new rate 8%, stressed payment £1,000 × (1 + 3/5) = £1,600. The proportional adjustment slightly overstates the new payment (full amortisation would give around £1,540) — a conservative bias that's right for stress testing.

What lenders look at beyond the simple ratio

A lender's affordability assessment is more detailed than this calculator:

• Family size: More dependants = higher cost-of-living floor
• Childcare costs: Specifically included in lender models
• Pension contributions: Treated as non-discretionary
• Travel to work: Lender model includes regional commuting costs
• ONS Family Spending floor: Lenders use ONS data as a minimum for essentials, even if you spend less
• Variable income discount: Bonuses typically counted at 50-100% depending on track record

Pass this calculator and you're in a strong position; you'll usually pass the lender's. Fail this calculator and the lender will almost certainly fail you. See how much mortgage can I afford for the lender-side framework.

Frequently asked questions

What is an affordability buffer?

How much room you have between essential monthly outgoings and net income. A healthy buffer absorbs unexpected costs and rate rises. Less than 15-20% of net income as buffer = exposed.

What's a healthy affordability ratio?

Below 35% is comfortable; 35-45% standard; 45-55% exposed; over 55% distress. Most UK lenders cap around 45%.

How much rate rise should I stress against?

Stress at +3pp as a minimum (FCA legacy standard), +5pp if your fix has 4+ years to run.

What does the risk indicator mean?

Green = comfortable; Amber = adequate but exposed to rate rise; Red = stretched even at current rates.

What counts as 'other commitments'?

Anything that takes a regular monthly amount: credit card minimums, loans, car finance, student loan, child maintenance, gym, subscriptions.

How is the stressed payment calculated?

The calculator increases your current payment by the same proportional amount as the rate rise. At 5% rate, £1,200/month becomes ~£1,440 at 7% (+2pp, ~40% increase).

What can I do if I'm in the red zone?

Increase income; reduce other debts; reduce mortgage exposure (longer term, interest-only temporarily, downsize). Don't ignore — most household financial stress comes from over-committed mortgage at rate-rise events.

Should I overpay if I have a green buffer?

A strong buffer is a good place to consider overpaying — most fixes allow 10% overpayment per year without ERC. Balance against keeping cash reserves and alternative uses of money.

Action sequence by risk colour

A quick checklist for what to do at each risk indicator level:

• Green: Maintain emergency reserve at 3-6 months of outgoings; consider overpaying within ERC-free 10%; review again at next fix end.
• Amber: Build cash reserve to 6 months of outgoings; identify £100-£300/month of discretionary spending to redirect to savings or debt reduction; lock the longest fix you can comfortably afford at next remortgage.
• Red: Stop taking on new debt immediately; clear highest-interest debts first; reduce mortgage exposure via product transfer (avoid affordability fail); consider speaking to a debt charity (StepChange, Citizens Advice) if buffer is negative.

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Last reviewed: 21 June 2026.