Investment Calculator: Project Your Portfolio Growth

Quick answer

Investing £250 a month at a 5% average annual return grows to about £102,758 in 20 years, of which £60,000 is your own contributions and the rest is compound growth. Time invested matters more than timing: the last decade of a 30-year plan typically adds more than the first two combined.

Use the investment calculator

Enter your starting lump sum, your monthly contribution, an assumed annual growth rate and the number of years you plan to stay invested. The investment calculator above returns a projected end value and shows how much of that is your own money versus investment growth. Change one figure at a time to see how much a higher contribution or a longer horizon actually moves the needle.

How investment growth compounds

The engine behind any honest projection is compounding: each year your returns earn returns of their own. A pot that grows 6% adds 6% to a bigger base every year, so the curve steepens the longer you leave it alone. That is why two people paying in the same amount can end up thousands apart purely because one started earlier.

The calculation has two moving parts. The lump sum grows on its own, and each monthly contribution starts compounding from the moment it lands. In plain words:

End value = (lump sum grown over the period) + (each monthly payment grown over its remaining months).

For the lump sum alone, the formula is the standard compound growth one: future value equals present value multiplied by (1 + annual rate) raised to the number of years. Monthly contributions use the same idea but applied month by month, because money paid in year one has far longer to grow than money paid in the final year. If you want the underlying arithmetic broken down step by step, our compound interest calculator walks through the raw formula and the difference between annual and monthly compounding.

One thing worth setting straight early: investment returns are not a fixed, guaranteed rate the way a savings bond is. A 6% figure is an assumption you supply, not a promise the market makes. Real returns arrive lumpy, with good years and falling years, and the order they come in matters. This investment calculator smooths all of that into one steady rate so you can compare scenarios, not predict the future.

Lump sum versus regular investing

Most UK investors do a bit of both: a starting amount, then a standing order every payday. Each behaves differently inside the projection.

• A lump sum puts your whole stake to work immediately, so it captures the maximum compounding. It also takes the full hit if markets drop the week after you invest.
• Regular monthly investing spreads your buying across high and low prices, which is often called pound-cost averaging. You give up some early compounding, but you remove the stress of trying to time your entry.

The calculator lets you model both at once, which is realistic. A first-time investor might start with £2,000 and add £200 a month; someone rolling over an old workplace pension might have a £40,000 lump sum and add nothing further.

How the investment calculator works

Behind the scenes the tool does three things. First, it grows your opening lump sum across the full term at your chosen rate. Second, it adds each monthly contribution and compounds it only for the months remaining until the end date. Third, it sums the two and subtracts your total contributions to show the growth portion separately.

The headline outputs are:

• Projected end value - the total pot at the end of your time horizon.
• Total invested - your lump sum plus every monthly payment added together.
• Investment growth - the end value minus what you put in. This is the part compounding created.

Because returns are assumed rather than known, treat the output as a planning estimate. A sensible habit is to run three versions - a cautious rate, a middle rate and an optimistic rate - so you see a range rather than a single false-precision number.

Worked example: Priya's stocks and shares ISA

Priya is 32, lives in Manchester and wants to invest for the long term inside a stocks and shares ISA. She starts with a £5,000 lump sum and sets up a £300 monthly standing order. She assumes a 6% average annual return and plans to stay invested for 20 years. Here is roughly how the investment calculator builds her result.

Step 1 - grow the lump sum. £5,000 left to compound at 6% for 20 years grows to about £5,000 × (1.06)²⁰ ≈ £16,036.

Step 2 - grow the monthly contributions. Priya pays in £300 a month, which is £3,600 a year. Over 20 years she contributes £72,000. Because each payment compounds for the months it is invested, that £72,000 grows to roughly £138,600 at a 6% annual rate compounded monthly.

Step 3 - add them together. £16,036 + £138,600 ≈ £154,600 projected end value.

Step 4 - split out the growth. Her total invested is £5,000 + £72,000 = £77,000. So investment growth is roughly £154,600 − £77,000 = £77,600 - meaning more than half her final pot came from compounding, not from her own deposits.

Now change one input. If Priya raises her monthly contribution to £400 but keeps everything else the same, the end value climbs to around £185,000. If instead she keeps £300 a month but invests for 25 years rather than 20, she clears roughly £225,000. Time on the market does even more heavy lifting than a bigger monthly payment - which is the single most useful thing this tool can show you.

Choosing a growth rate you can defend

The growth rate is the input that swings everything, so be honest with it. There is no official UK investment return to plug in - unlike a tax band, it is not set by HMRC. A few anchors help you pick a figure rather than guess:

• Lower-risk, bond-heavy portfolios have historically produced more modest long-run returns than higher-risk equity ones.
• A broad global shares fund has historically returned mid-single-digit to higher-single-digit percentages a year on average over long periods, but with sharp ups and downs along the way.
• Always think in real terms. If your pot grows 6% but prices rise 3%, your spending power only grows about 3%. Our inflation calculator shows what a future pot is actually worth in today's money.

The Money and Pensions Service guidance on investing basics from MoneyHelper is a good, impartial place to read up on risk and return before you settle on an assumption.

Fees and tax: the two leaks this calculator does not see

A clean projection ignores two real-world drags. Build them into your thinking even if the tool keeps the maths simple.

Platform and fund charges

An annual platform fee plus a fund's ongoing charge might total somewhere between a fraction of a percent and well over 1% a year. That sounds small, but it compounds against you in exactly the way growth compounds for you. If you assume a 6% return and pay 1% in total charges, your real working rate is closer to 5% - so model the net figure, not the headline one.

Tax on investments held outside a wrapper

How are investments taxed in the UK? It depends entirely on the account they sit in:

• Inside a stocks and shares ISA: no UK Capital Gains Tax and no further tax on dividends. The ISA allowance is £20,000 per tax year. See our ISA calculator for how the wrapper affects returns.
• Inside a pension: growth is tax-free within the pension; tax is generally handled when you draw it.
• In a general investment account: gains above the Capital Gains Tax annual exempt amount of £3,000 are taxable, and dividends above the £500 dividend allowance are taxed too.

Capital Gains Tax on investments is charged at 18% for gains within your basic-rate band and 24% above it. Dividends above the allowance are taxed at 10.75% (basic rate), 35.75% (higher rate) or 39.35% (additional rate) for 2026/27. You can model a realised gain with our capital gains tax calculator if you invest outside an ISA. These tax rules are UK-wide for savings and dividends - Scotland sets different income tax bands, but the dividend and capital gains rates above apply across the whole UK.

Tax-efficient investing: use the wrapper first

The cheapest way to improve a projected return is rarely a punchier growth rate - it is sheltering your money from tax in the first place. For most UK investors the running order is: use the ISA allowance, use pension contributions for the tax relief, and only then invest in a general account. Doing this means more of the compounding in the calculator above actually reaches you rather than HMRC. If your aim is a specific retirement number rather than open-ended growth, our FIRE calculator works backwards from the pot you will need.

Common mistakes to avoid

• Using an optimistic return as your base case. Plug in a rate you would be comfortable with in a bad decade, then treat anything above it as a bonus.
• Forgetting inflation. A £200,000 projection in 25 years is not £200,000 of today's spending power. Always sanity-check against inflation.
• Ignoring fees. A 1% charge can quietly cost you tens of thousands over a long horizon. Use a net-of-fees return in the calculator.
• Assuming the projection is a straight line. Real returns bounce around. The single steady rate is a planning tool, not a forecast of any given year.
• Investing outside an ISA when you have allowance left. Paying avoidable Capital Gains Tax or dividend tax on a general account drags your real return below the headline.
• Stopping contributions in a market dip. Falling prices are when your monthly payments buy the most units - pausing then is usually the worst time.

One more practical note: this is a projection of potential investment growth, not a savings tool. If you might need the money within roughly five years, the up-and-down nature of investing makes a cash savings account safer for that portion. Compare with our savings calculator for shorter horizons.

These figures are estimates for guidance only and are not personal financial or tax advice. Investments can fall as well as rise and you may get back less than you put in.

Who should use this calculator

This projects an investment portfolio rather than a savings account, and the distinction matters: investment returns are volatile, arrive unevenly, and are not guaranteed. The number it produces is a scenario, not a forecast, and it is most useful for comparing scenarios rather than believing any one of them.

Run it twice — once at an optimistic return and once at a pessimistic one — and the gap between the two is the honest picture. Use the fee field: an ongoing charge of 1% versus 0.2% sounds trivial and compounds into a very large difference across decades, because it is levied on the whole balance every year.

What this calculator assumes

• A constant annual return, compounded. Real portfolios do not deliver this, and the sequence of good and bad years changes the outcome.
• All income — dividends and interest — is reinvested.
• Contributions continue at the rate set for the whole period.
• Inflation, where entered, restates the result in today’s purchasing power.

Limitations — what it does not cover

• Volatility and sequence risk. The same average return produces very different results depending on when the losses fall.
• Tax on dividends and gains outside an ISA or pension.
• Platform, fund and transaction charges beyond the single fee field.
• Currency risk on overseas holdings.
• Your own behaviour — selling during a fall is the most common reason real returns undershoot projected ones.
• The possibility of loss. Investments can and do fall below what you paid in.

Related calculators

To go deeper, try our compound interest calculator for the raw maths, the ISA calculator for tax-free investing, the dividend yield calculator if you invest for income, and the FIRE calculator if you are aiming for a specific retirement target.

What monthly investing grows to at 5% a year

Returns are not guaranteed and capital is at risk; 5% is an illustration, not a promise. Shelter growth from tax inside an ISA (£20,000 a year) or pension. Impartial guidance: MoneyHelper investing.