Can I afford this big purchase?
"Can I cover the monthly payment?" is the wrong question. The right one: after the upfront cost leaves my savings, does my buffer survive the real running costs for the next year?
Why "I can cover the payment" fails
Big commitments break budgets through two mechanisms the advertised price hides. First, the upfront cost drains the cushion that was protecting you from everything else — the boiler, the dental bill, the month between jobs. Second, the real monthly cost is almost never the advertised monthly cost: insurance, maintenance, service charges and repairs routinely add 15–30% to the number on the advert.
The honest test is buffer survival: simulate your income (with its real month-to-month swings) against your essential costs plus the full commitment, starting from the buffer you have after the upfront payment leaves, and count what fraction of plausible futures keep you above zero.
A real worked example
Someone on £2,600/month with £12,000 saved and £2,050/month essential costs, buying a £6,000 car with £550/month total running costs (finance + insurance + fuel + maintenance):
buffer £12,000 · upfront £6,000 · commitment £550/mo · income £2,600/mo · essentials £2,050/mo · volatility 8% · horizon 12m
On these stated inputs: 95–100% — comfortably affordable. But the stress scenario (running costs +25%, income −15%) drops to 41% — near a coin flip. The verdict isn't "yes, buy it"; it's "affordable if the £550 is real". Get the insurance quote and a realistic maintenance number in writing before trusting the green light. The engine also reports the largest commitment this budget can carry with the odds still on your side: about £1,050/month — useful when comparing options.
What people get wrong
- Using the advertised monthly figure. The advert shows the finance payment; you'll pay finance + insurance + running costs + the repair in month seven.
- Ignoring what the deposit does to resilience. Spending 60%+ of your savings upfront leaves you exposed to the first emergency, even when the monthly maths works.
- Assuming income is fixed. Even salaried income has bad months — unpaid leave, job changes. Variable earners at 15–40% volatility should never budget at their average.
- Not pricing the wait. Unlike most decisions, a purchase can usually wait. Three more months of saving measurably shifts the odds — the engine will show you by how much (honestly noting it can't predict price rises while you wait).
The honest context
13.1 million UK adults — 24% — had low financial resilience in May 2024, and 42% could not cover three months of living costs if their main income stopped (FCA Financial Lives 2024). Over-committed budgets are the norm, not the exception. The whole point of doing this maths before you sign is staying out of that statistic.
What we won't tell you: whether the purchase is worth it — value, depreciation, joy per pound. There's no defensible statistic for whether big purchases "work out", so LEXUN doesn't invent one. The model forecasts only the resolvable question: does your buffer survive the commitment? It refuses to run on guesses, and shows every answer as an honest range.
Work out yours
Runs entirely in your browser; nothing leaves your device. You'll get a survival band, the stress case, the largest commitment your budget can honestly carry, what waiting would change, and a frozen criterion to come back and score against reality.
Run the affordability analysis →