How Long Will My Savings Last?

The tool simulates your savings month by month. Each month it adds interest at your stated rate divided by 12, increases your spending by the monthly equivalent of inflation, then subtracts the net spending (spending minus any income) from the balance. It runs that loop until the balance hits zero or 100 years pass, whichever comes first. The output is the date your money runs out, plus milestones along the way (50% gone, 25% gone, emergency fund reached).

Two assumptions matter here. First, inflation compounds monthly even though most people think of it annually; the calculator handles this so a stated 3% annual inflation behaves correctly month-to-month. Second, interest is paid on the balance before that month's withdrawal, which is how most savings accounts and money-market funds actually work. Together these two details mean the burndown date is usually a few months later than a naive 'savings divided by monthly spending' estimate would suggest.

The classic use case is planning a career break: you have £40,000 saved, you spend £2,500 a month, you want to know how long that lasts. The honest answer is around 16 months at zero inflation and zero interest, but realistic numbers stretch that to 18-19 months once you factor in even a modest 4% return on the cash sitting in an easy-access ISA. If you bring in some income (freelance work, a part-time job, rental income), the burndown extends further: £500 a month income against £2,500 spending means you only burn £2,000 net, pushing the same pot to closer to 23 months.

For early retirement, the burndown answers the harder question of whether your pot is genuinely sustainable. Most FIRE planning uses the [4% withdrawal rule](/uk-tax-calculator) but real lives have lumpy spending - a new boiler at year 3, a wedding at year 7, a knee replacement at year 12. The month-by-month view lets you stress-test the plan against single big withdrawals you'd struggle to model with a simple percentage rule. If the pot survives the first 10 years with extra £5,000 expenses every other year, it's probably durable.

Holding savings flat for 20 years feels safe, but inflation eats them slowly. At 3% annual inflation, £100 of spending today costs £180 in 20 years; the same pot of money buys roughly half what it does now. The simulator increases your spending each month to match inflation, which is why the burndown date is usually closer than people expect. A pot that 'lasts 30 years' at today's prices may only last 22 once the spending side rises with inflation.

The way to push back against this: your interest rate needs to beat inflation, ideally by 1-3%. UK easy-access savings accounts paying 4.5% in a 2.5% inflation environment give you about 2% real growth, which extends the runway. Cash in a current account paying 0% loses 2.5% of purchasing power a year and accelerates the burndown noticeably. The [compound interest calculator](/compound-interest-calculator) shows how that gap compounds across decades.