What this projection is estimating
This calculator projects the potential value of an investment portfolio built from an initial lump sum plus ongoing monthly investing, at a single assumed average annual return. It's a useful way to sanity-check whether a given combination of starting capital, monthly contribution and time horizon is broadly heading towards a goal you have in mind, whether that's a house deposit, a specific age, or simply building long-term wealth.
Why a single "expected return" is a simplification
Real investment returns don't arrive as a smooth, constant percentage every year — markets rise and fall, sometimes sharply, and the actual sequence of returns you experience can differ enormously from a tidy average even if the long-run average turns out similar. A portfolio that falls 20% in year one and then recovers strongly behaves very differently, especially if you're withdrawing money during that period, from one that grows steadily by the same average amount every year. This calculator's straight-line assumption is a planning tool, not a promise, which is exactly why the disclaimer above matters more here than almost anywhere else on this site.
How your assumed return should relate to your portfolio
The rate you plug in should roughly reflect the type of investment you're actually holding, not a generic optimistic figure. A globally diversified index fund has historically returned quite different average figures over different multi-decade periods depending on the exact mix of assets and the era studied, and a more cautious, bond-heavy portfolio would typically assume a lower rate than one weighted towards equities. Your own risk tolerance should shape both the return you assume and, more importantly, the portfolio you actually hold.
Fees quietly work against every projection
Platform charges, fund ongoing charges and any advice fees all reduce your effective return, and because they're taken as a percentage, they compound against you in exactly the same way growth compounds in your favour. A fund charging 1% more per year than an equivalent alternative isn't just costing you 1% of your current balance annually — over a 20 or 30 year horizon that difference can meaningfully change your final outcome. It's worth checking your platform and fund charges using our investment platforms and fees guide before assuming a headline return will translate directly into your own results.
Lump sum, regular contributions, or both
This calculator combines a starting lump sum with ongoing monthly investing because that's how most people actually build a portfolio over time — an initial sum to get started, then regular contributions from income afterwards. If you're specifically trying to decide whether to invest a lump sum immediately or spread it in gradually, our lump sum versus regular investment calculator looks at that decision directly, and our regular investment calculator is useful if you're starting from nothing and building purely through monthly contributions.
A worked example
With the calculator's defaults — £10,000 to start, £250 invested every month, at an assumed 6% annual return, over 20 years — total contributions come to £70,000 (£10,000 plus £250 × 240 months). The projected value comes out at roughly £132,700, meaning growth has contributed around £62,700, slightly more than the contributions themselves. This is a good illustration of how, over a sufficiently long horizon at a reasonable assumed return, growth can eventually overtake the money you actually put in.
Drop the assumed return to a more cautious 4% with everything else unchanged, and the projected value falls to roughly £103,300 — nearly £30,000 lower on exactly the same contributions, purely from a two percentage point difference in assumed return compounding over two decades. Testing a range of return assumptions like this, rather than relying on one figure, gives a more honest sense of how sensitive any long-term projection really is.
Diversification and why it matters here
This calculator treats "the investment" as a single assumed return, but a real portfolio is usually made up of many underlying holdings spread across different companies, sectors and sometimes countries, precisely so that a poor result from any one holding doesn't derail the whole plan. A well-diversified, low-cost index fund is one of the most common ways investors achieve this without needing to pick individual shares themselves, and it's worth understanding roughly what you're actually invested in before treating any single assumed return as a reliable planning figure for your specific portfolio.
Frequently asked questions
What return should I assume for a stocks and shares ISA?
This depends entirely on what the ISA is invested in, since a stocks and shares ISA is just a tax wrapper, not an investment itself — a cautious multi-asset fund and an aggressive all-equity fund held in the same ISA would justify very different assumed returns.
Should I assume a return net of inflation or before it?
Either is valid as long as you're consistent and clear about which you're using. A return quoted "in real terms" already strips out inflation, giving you an answer in today's spending power; a nominal return needs a separate step (see our inflation calculator) to translate the final figure into today's money.
Is monthly investing better than investing once a year?
The difference is usually small over a long horizon, but investing as soon as money is available (rather than letting it sit in cash first) generally gives it more time in the market to compound, which is the main reason monthly investing from income tends to be recommended over saving up for a single annual contribution.
How often should I check my portfolio's performance?
Checking too frequently can encourage reacting to short-term noise rather than long-term trends — many long-term investors deliberately review holdings only once or twice a year, outside of confirming contributions are being invested as expected.
What if I need to reduce my monthly contribution for a while?
Pausing or lowering contributions temporarily during a genuinely tight period is far better than stopping investing altogether — resume at your previous level, or higher, as soon as you reasonably can, since the gap's cost grows the longer it's left unaddressed.