Investment Growth Calculator
Forecast what your investments could be worth. Enter a starting sum, what you add each month and the annual return you expect. You get a year-by-year projection showing what you paid in, the growth on it and the final value. It works for an index fund, a stocks and shares ISA or a whole portfolio.
How it's calculated
Monthly Growth = Balance × (Annual Return ÷ 12)End Balance = Start Balance + Growth + Monthly Contributions
How an investment forecast works
The calculator compounds monthly. Each month it adds one twelfth of the annual return to the balance, then adds your contribution, and repeats for every month of the period. Early contributions therefore earn growth for longer than later ones, and the growth itself earns growth. That is compounding, and it is why the final value pulls away from the amount you paid in as the years pass.
What £10,000 plus £500 a month could grow to
No one knows the return in advance, so run a forecast at more than one rate. The table shows the same plan at 4%, 6% and 8% a year. The spread between the columns is the honest picture: over 30 years the difference between 4% and 8% is more than the whole amount paid in.
| After | Paid in | At 4% a year | At 6% a year | At 8% a year |
|---|---|---|---|---|
| 10 years | £70,000 | £88,533 | £100,134 | £113,669 |
| 20 years | £130,000 | £205,613 | £264,122 | £343,778 |
| 30 years | £190,000 | £380,160 | £562,483 | £854,537 |
Three things the forecast leaves out
- Fees. Platform and fund charges come out of your return every year. A 0.5% annual charge on a 7% return leaves 6.5%, and over 30 years that gap is large. Enter the return after charges.
- Inflation. The figures are in today's money only if you enter a real return. At 3% inflation, a nominal 7% is a real 4%, and the 4% column is the one to read.
- Volatility. Markets do not deliver 7% every year. They deliver +20% one year and −15% the next, and the order matters, especially near the end when the pot is largest. The Sharpe ratio calculator shows how to weigh a return against the swings it comes with.
Frequently Asked Questions
- What annual return should I use?
- Broad global equity indices such as the FTSE All-World have historically returned around 5–7% a year after inflation over long periods, though past returns are no guide to the future. Use 5–7% for planning. Higher figures produce projections that look impressive and rarely survive contact with reality, particularly over shorter horizons where a single bad year matters.
- Does this account for inflation?
- No. This calculator shows nominal returns, meaning before inflation. To estimate the real return, subtract your expected inflation rate — typically 2–3% — from the annual rate you enter. The distinction matters over long periods. At 3% inflation, money loses roughly half its purchasing power over 25 years, so a nominal projection overstates what the pot will actually buy.
- How important are regular contributions?
- Very important. Contributing regularly means you buy at a range of prices rather than betting everything on one entry point, which is known as pound-cost averaging. It reduces the damage a badly timed lump sum can do. Even modest monthly amounts compound substantially over 20–30 years, because each contribution starts earning from the day it lands.
- What would £10,000 plus £500 a month grow to at 7% over 20 years?
- You would pay in £130,000 in total: the £10,000 lump sum plus 240 monthly contributions of £500. At 7% a year the pot grows to about £300,851, so £170,851 of it is growth. That is 2.31 times what you put in. Growth is compounded monthly and each contribution is added at the end of the month, which is why the answer differs slightly from a simple annual formula.
- Can I use this for an index fund or a stocks and shares ISA?
- Yes. The maths is the same for any investment with a steady average return. For an index fund, enter its long-run return after the ongoing charge, because fees come out of your return every year. Inside an ISA there is no tax on the growth, so the figures shown are what you keep. Outside an ISA, dividends and gains above your allowances may be taxed, which the tax-efficient investment calculator linked below can show.