Here's a question most UK retirement calculators duck:
If markets do what they've done historically — including the bad bits — what are the actual odds my money lasts to age 95?
Your pension provider can't really answer it. The FCA requires them to use three standardised growth rates — 2%, 5% and 8% — applied flatly every year. That gives you three single-line projections. It's good for comparing products. It's terrible for planning a retirement, because real markets don't deliver a flat 5% every year. Some years they're up 25%, others they're down 30%, and the order those returns arrive matters enormously when you're drawing down.
The thing nobody tells you about retirement maths
Take two retirees. Both have £500,000. Both retire on £25,000 a year. Both earn an average 5% real return over 30 years. They should end up in the same place, right?
No. If the first retiree's first three years are down −15%, −10% and −5%, but then markets recover to deliver that 5% average, her pot is dead by age 85. The second retiree gets the same returns but in the opposite order — three good years up front, the bad ones later — and finishes with £600,000 left over.
This is sequence-of-returns risk. It's the single biggest reason "you'll have £620k at 67" projections lie to you. The average is the same; the outcome is night and day.
A Monte Carlo simulation captures this by replaying your plan thousands of times, each with a different random sequence of yearly returns. Then it counts: in how many of those replays did your money last? If 4,100 out of 5,000 succeeded, you've got an 82% chance.
How to read the answer
The probability number is what most people fixate on, and rightly so:
- Above 90% — you're in solid shape. Some advisers would say you're saving more than you need to.
- 80–90% — the sweet spot most planners target. Comfortable margin without over-saving.
- 65–80% — workable, but worth tightening — small changes have big effects in this range.
- Below 65% — your plan needs attention. Not panic — attention. Retire later, spend less, save more, or some combination.
The other thing to look at is the fan chart — the spread of pot values year by year. The bold line is the median (half of futures landed above, half below). The bands either side show the 25–75% likely range and the 10–90% wider range. If the bottom edge bottoms out before your target age, that's where the worst-case 1-in-10 future runs out of money.
A free UK Monte Carlo retirement calculator
I built one. It's free, anonymous, and takes five inputs. It applies real UK tax — Personal Allowance, basic and higher rate, plus the 25% SIPP tax-free bit. It handles State Pension at 67. It runs 5,000 simulations and gives you the probability number plus the fan chart in about a second.
Will my money last? — UK Monte Carlo Calculator
Five inputs. 5,000 simulated futures. Real UK tax. Built in Edinburgh. Free, no signup.
Run my simulation →A worked example
Sarah is 45, has £200,000 invested across her ISA and SIPP, contributes £18,000 a year (including her employer's pension match), wants to retire at 65 on £30,000 a year in today's money, and will qualify for the full State Pension at 67.
The simulator gives her a 97% probability of success. Median ending balance at age 95: £2.1 million. Even in the worst 10% of simulated futures she finishes with £370k. She's in great shape.
Now consider David, also 45, but with £80,000 invested, contributing £6,000 a year, wanting to spend £35,000 a year, retiring at 60. The probability drops to 28%. Median depletion age: 73. In most simulated futures his money runs out 22 years before age 95.
What can David do? The simulator's "what if" buttons show the effect:
- Retire 2 years later → probability jumps to 42%.
- Spend £200 a month less (£32.6k/yr instead of £35k) → 46%.
- Save £200 a month more → 43%.
- All three together → 73%. Now we're talking.
The point isn't to terrify David. The point is to show him the levers. Each lever moves the number a measurable amount. Small changes, compounded over 20 years, are extraordinary.
What this isn't
This is a planning aid, not regulated financial advice. The free tool deliberately keeps it simple — five inputs, default assumptions. It assumes a 5% real return with 12% volatility, which matches global equity history but might be too optimistic if you hold a 60/40 portfolio. It doesn't model Scottish rates, HICBC, dividend allowances or variable spending in retirement. Want the full breakdown — every formula, every simplification, every "we don't model this" caveat? Read the full methodology in plain English →
The full WealthR app uses the same simulation engine but on your real numbers — actual ISA balances, every SIPP and DB pension, partner pensions, BTL income, Scottish tax if applicable, all six ISA wrappers, top-slicing relief on offshore bonds. If you want the honest answer with your actual data, that's where to go.
If your plan is complex (large estate, business owner, divorce in the picture, IHT planning above the nil-rate band, defined benefit transfer decisions), see a regulated UK financial adviser.
The bottom line
If your retirement plan is based on a single "your pension might be £X at 67" projection from a provider, you're planning with one eye closed. Run a Monte Carlo simulation. See the spread. Find the levers. Most retirement decisions get better when you can see how the probability moves, not just what the average might be.
The free tool takes about a minute. Run your simulation here →