Why a range beats a single number
Type your savings into almost any net worth calculator and it will hand back one clean figure: in thirty years you will have $1,847,203. It looks authoritative. It is closer to fiction than forecast, because it assumes the market hands you exactly 7% every single year for thirty years running. It never has, and it never will.
Real returns are lumpy. A portfolio might gain 22% one year and lose 18% the next. The long-run average survives that, but the path does not, and the path is what determines where you actually end up. Two people who save identical amounts for thirty years can finish hundreds of thousands apart purely on the order the good and bad years arrived in.
So this calculator gives you the shape of the outcome instead. The shaded band is where your net worth lands in roughly eight cases out of ten. The solid line through the middle is the most likely single result. The dashed line is what you would have if the money simply sat there earning nothing — the gap between the two is what compounding is doing for you.
How this calculator works
Each month, your balance is grown by an expected return and your contribution is added on top. The expected return comes from the portfolio mix you choose, de-annualised properly — a 7% year is treated as 1.07 raised to the power of one twelfth, not 7% divided by twelve, which quietly overstates growth.
Alongside the balance, the calculator tracks how uncertain that balance is, using the volatility of the mix you picked. Volatility is the standard measure of how much returns bounce around: roughly 6% a year for a bond-heavy portfolio, around 18% for one that is mostly equities. That uncertainty compounds month on month, which is exactly why the band fans out as the horizon lengthens.
The band is then drawn as a lognormal distribution rather than a symmetric one. This matters. A portfolio is a product of returns, so its outcomes are skewed: you can never lose more than everything, but the upside has no ceiling. A symmetric band would overstate how bad the bad case is and understate how good the good case can be.
There is no random simulation involved, which is why the same inputs always produce the same chart. That is deliberate — it means the link you share shows the other person precisely what you saw.
Reading your result honestly
Look at the bottom of the range first. That is the number worth planning around, because a plan that only works if markets cooperate is not a plan. If the pessimistic edge still gets you where you need to be, you have a genuine margin of safety. If only the optimistic edge does, you are relying on luck.
Then check how much of the final figure is money you contributed versus money that growth added. Over short horizons, contributions dominate and your saving rate is almost the whole story. Somewhere around the fifteen to twenty year mark that flips, and growth starts doing more work than you do. Seeing where that crossover falls for your own numbers is usually more motivating than the headline figure.
Finally, keep the inflation adjustment switched on. Future money is a poor unit of account — it flatters every long projection and makes thirty-year figures feel far more comfortable than they should. In today's purchasing power, the same result is soberer and far more useful for deciding what to actually do this month.
What this calculator cannot know
It assumes your contribution stays flat, which it will not — pay rises, career breaks, children and mortgages all move it. It assumes you leave the money invested through the bad years, which is the single hardest part of investing and the one most people fail at. It knows nothing about tax, fees, or the account wrappers available where you live, all of which can move the outcome substantially.
It is a model of the shape of your future, not a prediction of it. The most useful thing you can do with it is change one input at a time and watch what actually moves the band. For most people, an extra hundred a month shifts the picture more than any amount of agonising over which fund to pick.
This is general information, not financial advice. If the numbers here are load-bearing for a real decision, talk to someone qualified and regulated where you live.
Common questions
How accurate is a net worth projection?
No projection is accurate as a single number, which is why this one does not give you one. Markets do not deliver the same return every year, so the honest output is a range. This calculator shows the band your net worth lands in roughly eight years out of ten, based on the return and volatility of the portfolio mix you pick. Treat the width of that band as the real answer.
What return should I assume?
The three presets use 5%, 7% and 9% a year for bond-heavy, balanced and equity-heavy portfolios. These are rounded long-run averages for globally diversified portfolios, not forecasts. If anything, use the lower end: the cost of assuming too much is discovering the shortfall when you no longer have time to fix it.
Why does the range get wider the further out I look?
Because uncertainty compounds along with the money. One year of market variation is small. Thirty years of it stacks up, and the gap between a good run and a bad run becomes enormous. A calculator that shows a narrow range at thirty years is hiding something.
Should I show the result in today's money?
Usually yes. A million pounds or dollars in thirty years does not buy what a million buys now. The inflation adjustment discounts by 2.5% a year so the figure is one you can actually picture in terms of rent, food and holidays today.
Do I need an account to use this?
No. Nothing is stored, nothing is sent anywhere, and there is no email box. The numbers live in the page URL, which is why you can copy the link and send your projection to someone else.
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