UK Investment Return Calculator

See how your lump sum and monthly investing could grow over time — or work out what to invest each month to reach a target amount.

Your plan

Mode

Initial investment

Monthly investment

Expected annual return

A long-run figure you choose — real returns vary and can be negative.

Number of years

25 years

Estimated future value

£—

Total invested£—

Growth£—

Based on the annual return you entered, compounded monthly. Not a forecast or guarantee.

Pure maths — no UK thresholds. Investments can fall in value. Your actual return depends on what you invest in and when. See MoneyHelper — investments.

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. No fees, inflation, tax or market volatility are modelled.

Assumptions: a constant annual return with monthly compounding, no withdrawals and no charges. 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.
This calculator ignores fees, which can significantly reduce long-run returns — check the ongoing charges of any fund.

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 are not included. Ongoing charges of 0.5–1% a year can reduce long-run returns by a meaningful amount, so factor them in when comparing funds.

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

MoneyHelper — Investments: moneyhelper.org.uk
GOV.UK — ISAs: gov.uk (2026/27 allowance)

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.