Investment Growth Calculator

What this calculator does

• Projects an investment forward from a lump sum plus regular monthly contributions.
• Applies an annual platform or fund charge to the growth.
• Shows the effect of a horizon on the final value.

How the calculation works

The projection compounds an opening balance forward while adding contributions each month, so money invested early has longer to grow than money invested late. That timing effect is why the final figure is dominated by the early years of a long horizon even though the contributions are identical throughout. The annual charge is deducted from the return, and the important point about it is that it is charged on the whole balance rather than on the growth, so its cash cost rises every year as the balance rises. On a twenty-year horizon that turns a fraction of a percent into a meaningful share of the final value. Nothing here models tax, so the figure represents a wrapper-free view: inside an ISA or a pension it is the full amount, while in a general investment account dividends and gains would be taxable along the way and on disposal.

Step by step

1. Start from the initial investment.
2. Compound it at the expected return, net of the annual charge.
3. Add the monthly contribution each month.
4. Repeat for every month of the horizon.

Worked example

£10,000 invested with £500 a month added for twenty years, assuming 6% growth and a 0.25% annual charge.

What was entered

The arithmetic

1. £500 a month over twenty years is £120,000 of contributions.
2. Adding the initial £10,000 gives £130,000 of money actually paid in.
3. Compounded at 6% less the 0.25% charge, the projected value is about £250,616.
4. So roughly £120,616 — about 48% of the final value — is growth rather than contributions.
5. Extending the horizon lengthens the period the early contributions compound over, which is where most of the additional value would come from.

What the calculator returns

Key assumptions

• Returns are steady at the rate entered, with no volatility.
• Contributions are made every month without fail and are never increased.
• The annual charge is applied to the balance each year.

Limitations

• Real returns are volatile. Two portfolios averaging 6% can end up materially apart depending on when the good and bad years fall, particularly if money is being withdrawn.
• Inflation is not applied unless the return you entered is a real rate, so the figure is in future pounds.
• Tax is not modelled. Outside an ISA or pension, dividends and gains are taxable, which reduces the outcome.
• Charges are often layered across a platform fee, a fund charge and transaction costs, so one figure may understate the drag.
• Investments can fall as well as rise, and past returns are not a reliable indicator of future ones.

Common questions

Related calculators

• Stocks & Shares ISA Growth Calculator — Run the same contributions inside a tax-free ISA wrapper.
• Compound Interest Calculator — See the underlying compounding mechanics on a single lump sum.
• Monte Carlo Investment Simulator — Replace the single average return with a range of possible outcomes.
• General Investment Account Tax Calculator — See what the tax would be if this were held outside a wrapper.

Official sources

Every figure in this guide was checked against the sources below. Where a source could not confirm a figure, it is marked as requiring verification rather than presented as settled.

• Investing beginner's guide — MoneyHelperInvestment risk, charges and time horizon
• Individual Savings Accounts (ISAs) — GOV.UK (2026 to 2027 tax year)Overall ISA subscription limit £20,000, shared across cash, stocks and shares, innovative finance and Lifetime ISAs