Investing / Planning Tool

UK Investment Return Calculator

Model long-term portfolio growth, compare what comes from contributions versus returns, or work backwards from a target amount.

ProjectionMonthly compounding
ControlsFees + inflation

Planning assumptions

£
£
%

A planning assumption you choose — real returns vary and can be negative.

%

Deducted from the annual return before compounding.

%

Set a rate to also see the result in today's money.

25 years
No sign-up · No tracking · Runs in your browser
Base case / Estimated future value Growth —
£—
Research view

Total invested£—
Growth£—
Based on the annual return you entered, compounded monthly. Not a forecast or guarantee.
Risk note: This is a mathematical projection, not a market forecast. Investment values can fall as well as rise, and actual returns, fees and tax treatment will differ by product and personal circumstances.
Methodology

Separate contributions, growth and the drag from fees.

The planning engine compounds monthly, deducts any annual fee from the assumed return and can translate the future result into today's money when you provide an inflation rate.

Read Casvia methodology →
Planning context

Use assumptions as scenarios, not promises.

For UK investors, tax wrappers such as ISAs and pensions can materially change the after-tax outcome. The calculator focuses on the growth mathematics and leaves product selection to you.

Review data sources →

How it works

In Estimate growth mode, the calculator compounds your initial investment and monthly contributions at the annual return you choose (monthly compounding). It shows the future value split into what you put in and growth. In Reach a target mode it works backwards — given a target amount, it finds the monthly investment needed to get there, assuming the same initial lump sum and return.

Example calculation

For an initial £10,000 plus £250/month at 7% over 25 years:

  • Total invested: £85,000
  • Estimated future value: ≈ £259,800
  • Growth: ≈ £174,800 — about 3.1× your own money

To reach £250,000 from a £10,000 start at 7% over 25 years you would need about £238/month.

Example uses a chosen return for illustration only — returns vary, can be negative, and are not guaranteed.

Methodology & formula

Future value is calculated with monthly compounding:

FV = P × (1 + r/12)12t + M × [ (1 + r/12)12t − 1 ] / (r/12)
  • P = initial investment
  • M = monthly investment
  • r = annual return (as a decimal)
  • t = number of years

Growth = FV − total invested. In target mode, the monthly amount is solved numerically (bisection) so that FV equals the target. Inflation is included only when you enter an inflation rate. Tax and changing market returns are not modelled.

If you set an annual fee, the fee is deducted from the return each year (net rate = return − fee, clamped at 0) before compounding. The fees drag shown is the difference between the fee-free and fee-adjusted future values. If you set an inflation rate, the result is also discounted to today's money.

Assumptions: a constant annual return with monthly compounding, no withdrawals and no charges (unless you set a fee). Fees and inflation are included only when entered; tax and market volatility are not modelled. Real-world returns fluctuate, so treat results as a rough guide, not a promise.

UK-specific information

  • ISAs: investing inside a Stocks & Shares ISA means growth is free of UK Income Tax and Capital Gains Tax — up to your £20,000 annual allowance (2026/27). GOV.UK · 2026/27
  • Pensions: workplace and personal pensions give tax relief on contributions (20% basic rate and more for higher earners) — see the Pension calculator for a starting point.
  • Past performance: historical UK stock market returns are commonly used as a planning assumption, but they are not a guide to the future and you can lose money.
  • Fees: any annual fees you enter are deducted from the return each year before compounding — the fees drag shows the long-run impact. Ongoing charges of 0.5–1% a year can still reduce returns by a meaningful amount.

Frequently asked questions

What return should I use?
It depends on what you invest in. A cash ISA pays a fixed rate; a diversified portfolio has historically returned more over long periods but with risk. Pick a figure you understand — this calculator shows the maths, not a promise.
Is this a forecast?
No. It is a mathematical projection based on the single rate you choose. Real returns vary year to year and can be negative, especially over short horizons.
How do I reach a target amount?
Switch to 'Reach a target' mode, enter the target, and the calculator works out the monthly amount needed given your start and assumed return. Starting earlier or investing more both reduce the monthly amount.
Do I pay tax on investment growth?
Inside an ISA, no. Outside, you may owe Capital Gains Tax on profits and tax on dividends and interest, within your annual exemptions. Tax rules can change.
What about fees?
Fees you enter are deducted from the annual return before compounding, so the result is net of the fee you set. Ongoing charges of 0.5–1% a year can reduce long-run returns by a meaningful amount, so check fund charges when comparing.
Can I invest in one go or monthly?
Both work. Monthly investing spreads your entry across time and is easy to automate inside an ISA; a lump sum gets more time in the market. This calculator shows the difference.

Related calculators

Compound Interest Calculator · ISA Calculator · UK Savings Calculator · Home

Sources

Casvia provides educational calculators and general information. It does not provide personal financial, tax or investment advice. Rules and thresholds can change — always check the latest official guidance.