Figures are illustrative only, ignore charges and tax, and are not a forecast or personal recommendation.
This calculator projects how an initial investment, combined with regular monthly top-ups, might grow over time at an assumed rate of return. It's designed for anyone building an investment portfolio — inside a Stocks & Shares ISA, a SIPP, or a General Investment Account — who wants a rough sense of where consistent contributions and a plausible return assumption could lead over years or decades.
What counts as a realistic return assumption?
Historically, diversified global equity portfolios have delivered average annual returns in the region of 5-8% above inflation over multi-decade periods, though any individual year — or even decade — can look very different, including sharply negative periods. Lower-risk portfolios with a significant bond allocation have typically produced lower average returns with less volatility. There is no way to know future returns in advance, so treat whatever percentage you enter as a scenario to explore, not a promise, and consider running the numbers at a few different rates (say, a cautious 4%, a central 6%, and an optimistic 8%) to see the range of plausible outcomes rather than anchoring on one figure.
How the projection is built
The calculator compounds your initial investment and monthly contributions together on a monthly basis, at the annual rate you specify divided across twelve months. It then reports the projected total value, how much of that is money you actually put in, and how much is investment growth. This is the same core calculation used across our other growth tools, including the Compound Interest Calculator and Regular Investment Calculator, adapted here to combine both a lump sum and ongoing contributions in one place.
Worked example
£10,000 invested today, with £250 added every month, growing at an assumed 6% a year, produces a substantially larger figure after 25 years than the roughly £85,000 actually contributed. Most of that gap opens up in the later years of the projection — try shortening the term to 10 years in the calculator and you'll see growth contributes proportionally far less, because compounding needs time to build momentum.
Costs will reduce the real outcome
None of the figures here account for platform charges, fund ongoing charges, or dealing costs, all of which reduce your actual return below the headline assumption. A fund with a 1% ongoing charges figure effectively needs to earn 1 percentage point more just to match the same investor in a fund charging 0.2%, and that gap compounds over decades in the same way growth does. Our guide to fee drag over 30 years models this effect directly, and is worth reading alongside any long-term projection like this one.
Frequently asked questions
Should I use nominal or "real" (inflation-adjusted) figures?
This calculator shows nominal growth — it doesn't strip out inflation. If you want to see the effect of inflation on the purchasing power of a projected sum, use our Inflation Calculator on the resulting figure.
What if my monthly contribution will increase over time, e.g. with a pay rise?
The calculator assumes a flat contribution throughout. To model an increasing contribution, you could re-run the projection for separate segments (e.g. current contribution for 5 years, then a higher one for the remainder) and add the results together as an approximation.
Does this apply equally inside an ISA and a taxable account?
The growth maths is identical, but the tax treatment isn't — inside an ISA or SIPP, growth and income are generally free of UK income and capital gains tax, whereas in a General Investment Account, gains and income above your allowances may be taxable, which is not reflected in this calculator.
What term length should I use for portfolio building?
This depends on your goal — a house deposit might be a 5-year horizon, retirement a 20-30 year one. Generally, the longer the horizon, the more risk (and short-term volatility) an investor can typically afford to take, a point explored further in our guide on how much risk to take by age.
Common mistakes when projecting investment growth
A common error is entering an overly optimistic return assumption based on a fund's recent strong performance, rather than a sensible long-run average. Recent returns — especially over 3-5 years — are a poor guide to the future and can easily be well above or below what's sustainable over decades; regulatory risk warnings exist precisely because past performance is not a reliable indicator of future results. It's generally more useful to base assumptions on long-run historical averages for the relevant asset class, adjusted for your own judgement about future conditions, rather than extrapolating a recent good (or bad) run.
A second mistake is projecting a single portfolio return assumption for money that will actually shift between different risk levels over time — for example, being invested mostly in equities in your 30s but gradually shifting toward bonds and cash as a goal approaches. A single static rate is a reasonable simplification for a rough estimate, but for a more accurate multi-decade plan, consider re-running the projection at a lower assumed rate for the final years to reflect a more cautious allocation as you approach your goal.
Should I use the same rate for every year of a long projection?
For simplicity, yes, this calculator does — but be aware that real portfolios are commonly de-risked over time (moving from more equities toward more bonds and cash) as a goal approaches, which would tend to lower the realistic average return in the final years.
How do I account for a portfolio with several different funds?
Use your portfolio's overall weighted-average expected return as the single rate, or run the calculator separately for each major holding and add the projected results together.
How many years of history should I look at before choosing a return assumption?
Longer periods (20-30+ years where available) tend to smooth out the effect of any single unusual decade and give a more representative long-run average than a shorter window, though even very long historical averages are not a guarantee of future returns, particularly for a specific narrower asset class or region.
Should I project growth separately for an ISA and a pension?
It's generally clearer to do so, since the two typically have different contribution patterns, access ages, and tax treatment — running separate projections and viewing them together often gives a more useful overall picture than combining them into one blended figure.