What This Calculator Does
The Investment Growth Calculator projects the future value of an investment given a starting amount, regular monthly contributions, an assumed annual return rate, and a time horizon. It applies compound growth to both the initial lump sum and each monthly contribution separately, summing the result — this is why regular contributions have a dramatically larger effect than people typically expect.
The Power of Regular Contributions
An initial £10,000 at 7% for 20 years grows to approximately £38,700. Add £200/month throughout: the final balance becomes approximately £91,000 — more than double, from contributions that total only £48,000 additional invested (roughly half the extra growth is pure compounding). The later the contributions start, the less time they have to compound — which is why delaying investing by even 5 years has a permanently larger impact than the 5-year delay itself.
Real-Life Example: Pension Contribution Effect
A 30-year-old earning £40,000 contributes 5% of salary (£2,000/year, £167/month) to a pension. Their employer matches 3% (£1,200/year, £100/month). Total contribution: £267/month. Assuming 6% annualised net return over 35 years: projected pension pot at 65 ≈ £341,000. Adding tax relief (at 20% basic rate, contributions cost only £133.50/month net from take-home) makes this an exceptionally efficient way to build long-term wealth.
Real-Life Example: Starting 10 Years Later
Same scenario but starting at 40 instead of 30 — 25 years instead of 35. Same £267/month at 6%: projected pot ≈ £180,000. The 10-year delay costs approximately £161,000 in final balance — far more than the £267 × 120 = £32,040 in missed contributions. The missing growth is £129,000 of pure compounding that 10 more years would have generated.
Why Return Rate Assumptions Matter More Than You Think
Switching from a 5% to a 7% assumed return on £200/month over 30 years changes the projected outcome from approximately £166,000 to £243,000 — a 46% difference in final value from what sounds like a small rate change. Over long periods, 2 percentage points of annual return compounded becomes enormous. This sensitivity is why keeping investment costs low (fund management charges, platform fees) materially improves long-run outcomes.
Using the CalcPro Investment Growth Calculator
Enter an initial investment, monthly contribution, annual return rate, and time period. The calculator returns the projected future balance alongside the total amount you'll have contributed — showing clearly how much of the final balance is your money versus compound growth.