Stock Investment Calculator UK 2026/27: ROI, Annualised Return & CGT

The Stock Investment Calculator→ models a portfolio using a starting lump sum, monthly contributions, an assumed annual price-growth rate, a dividend assumption and a holding period. It can show projected value, total contributions, return on investment, a contribution-timed money-weighted annual return and a simplified Capital Gains Tax estimate for a taxable account.

The tool is useful because it makes the assumptions visible. It is not a forecast of what a share, fund or portfolio will earn. Real returns are uneven, fees reduce outcomes, dividends can change, and a tax calculation depends on the actual asset, wrapper, disposal history and investor’s wider position. Use the result to ask better questions rather than treating the largest displayed balance as a recommendation.

What the calculator is designed to show

The calculator answers a narrow modelling question: what would the portfolio look like if these contributions, growth assumptions and dividend treatment continued for this period? It can compare the money paid in with the modelled ending value and show how much of the result is attributed to growth under the chosen assumptions.

It does not answer whether a particular stock is good value, whether your portfolio is diversified, whether the investment is suitable for your goal or whether the chosen return is realistic. MoneyHelper’s investing guidance stresses the possibility of losing money and the importance of understanding risk, charges and the product. Those suitability questions sit outside a compound-growth formula.

Before entering figures, write down:

• the goal and the date when the money may be needed;
• whether the starting amount is genuinely available to invest;
• the asset or portfolio being modelled;
• the account wrapper: taxable GIA, ISA, pension or another arrangement;
• the contribution pattern and whether cash is held before monthly investment;
• the fees, tax assumptions and risks the model leaves out.

That context prevents a numerical output from being mistaken for a complete financial plan.

Lump sum and monthly contributions

A lump sum is invested at the beginning of the scenario. Every pound has the full modelled horizon. Monthly contributions enter gradually, so later payments have less time to compound. If the same amount is available on day one, a monthly plan also leaves part of the money outside the investment for longer.

The comparison changes when the money is earned monthly. A salary earner who invests each month is not necessarily delaying an available lump sum; they are investing when the cash arrives. The calculator can show the arithmetic difference, but it cannot tell you whether holding a large cash balance while waiting is appropriate.

Compare like with like. A £10,000 lump sum and £500 a month for five years are not equal contributions. If the scenarios have different total payments, say so clearly. A higher ending value may simply reflect more money invested, not a better strategy.

A mixed scenario can model an initial amount plus monthly contributions. It may help an investor who wants some immediate exposure while following a repeatable process. It is not automatically safer or more profitable. In a steadily rising market, delaying part of the money can reduce the modelled result; in a market that falls early, later contributions may buy at lower prices.

Understanding ROI

Return on investment is a simple total-return measure. A basic version compares the gain with the amount contributed:

ROI = (ending value − total contributions) ÷ total contributions

If £10,000 becomes £12,000 before costs, the simple gain is £2,000 and the simple ROI is 20%. If monthly contributions are added, the timing of each payment matters. A simple ROI does not say how long each pound was invested and should not be compared with a savings rate without considering the different risks and cash flows.

ROI can also be affected by whether dividends are shown as cash, reinvested or included in the projected value. Check the calculator setting before interpreting the result. A portfolio with a high dividend input may show more growth, but the dividend is not free money: the company or fund’s value, tax treatment and reinvestment terms matter.

Understanding the money-weighted annual return

The annualised return solves for the rate that equates the timing of the initial investment and monthly contributions (plus any dividends held as cash) with the projected ending portfolio value. Unlike a start-to-end CAGR based on total contributions as if they were invested on day one, it accounts for contribution timing. It is still an estimate from smooth assumptions—not a forecast, a time-weighted fund return or a record of actual performance—and it excludes fees, inflation and personal tax.

When the selected cash-flow pattern does not yield a single unambiguous rate, the calculator reports the metric as unavailable instead of choosing one result silently.

Growth, dividends and total return

The calculator separates price growth and dividend yield inputs. In real markets, the two are connected to the company, fund, valuation and economic conditions. A dividend can be reduced or cancelled. A fund can distribute income or reinvest it. A share-price fall can outweigh a dividend payment.

If reinvestment is enabled, the model treats dividends as additional money in the portfolio. That can increase compounding in the projection. It does not calculate every payment date, reinvestment price, dealing fee or tax deduction. If dividends are taken as cash, the ending portfolio value and total income tell a different story.

For a UK investor, GOV.UK’s dividend guidance explains the relevant dividend tax framework. The calculator’s dividend-tax estimate is simplified and does not know your total dividend income, taxable income, other allowances, employer benefits or tax return. It should not be used as the amount to set aside for HMRC.

When comparing funds or shares, use a consistent total-return basis. A price-only chart for one asset and a dividend-reinvested result for another can create a false comparison.

The simplified CGT estimate

The calculator can show an illustrative CGT estimate on a taxable General Investment Account. For 2026/27, GOV.UK lists an individual annual exempt amount of £3,000 and CGT rates of 18% or 24% for individuals, depending on how the taxable gain fits with taxable income. These are official planning facts for the tax year, but the calculator’s selected-band estimate does not reproduce every rule in a real return.

The actual liability can depend on:

• the asset and whether the disposal is within the relevant CGT rules;
• acquisition and disposal dates;
• allowable costs and transaction expenses;
• losses brought forward or arising in the year;
• other disposals and gains;
• the taxpayer’s taxable income and rate band;
• spouse or civil-partner transfers where relevant;
• reliefs, exemptions or special share rules;
• whether the investment is held inside an ISA or pension.

The annual exempt amount is not a tax-free allowance for the entire portfolio value. CGT is generally considered on taxable gains when assets are disposed of, not simply because a calculator shows an unrealised projection. A model that subtracts estimated CGT from an ending value is therefore a planning illustration, not a statement of tax due.

GIA, ISA and pension boundaries

The calculator’s simplified CGT estimate assumes a taxable account. It should not be copied into an ISA calculation. GOV.UK explains that income and gains within an ISA are tax-free under the ISA rules, subject to the account and subscription conditions. The overall ISA allowance and provider rules still matter, and investment losses are not transformed into guaranteed returns by the wrapper.

A SIPP or workplace pension is also not the same as a GIA. Contributions, tax relief, investment choices, access and withdrawals have their own rules. A pension projection should show the contribution and withdrawal assumptions separately. Do not treat the calculator’s “after CGT” number as a pension outcome.

The wrapper can change the practical answer more than a small difference in the assumed growth rate. Compare fees, access, employer contributions, tax relief, future withdrawals and the purpose of the money. The tool is intentionally not a full ISA or pension planner.

Fees, inflation and purchasing power

The headline growth assumption is normally before some or all fees unless you include a fee reduction in the input. Check the calculator’s exact behaviour and model platform charges, fund charges, dealing fees, spreads and currency conversion separately where necessary.

Inflation is also separate. A nominal ending balance is not the same as today’s purchasing power. If a model projects £100,000 in 20 years, that does not mean it will buy what £100,000 buys today. Run a lower nominal scenario and discuss the future cost of the goal rather than reporting one nominal number as a target.

A fee can affect a long horizon twice: it reduces the balance charged today and removes money that might have compounded. A product with a lower fee can still be unsuitable if its risk, access or tax structure does not fit. A product with a higher fee may provide a service, but the investor should understand what it pays for and whether the service is needed.

A sensible scenario set

Do not use one precise rate. Create a table with the same contributions and horizon:

Change one input at a time. If the outcome changes dramatically when the growth rate moves by a small amount, the plan is sensitive and should not rely on the central case. Test contribution timing, fees, dividend reinvestment, inflation and tax wrapper separately.

Common mistakes

Treating the projected balance as a forecast

A calculator multiplies assumptions. It does not know the future path of prices, the next dividend decision or the exact exchange rate. Label every output as a projection.

Applying GIA tax to an ISA or pension

The wrapper changes the rules. Keep the simplified CGT estimate in its stated taxable-account boundary.

Forgetting contribution timing

The same total amount can produce different results when it enters on different dates. State whether monthly cash is invested immediately or held in cash first.

Ignoring a fall before the goal date

An average return can hide a severe fall near the planned withdrawal. Match the asset risk to the goal and keep near-term money accessible.

Confusing dividend yield with guaranteed income

Dividend yield changes, and dividends can be reduced or cancelled. Model income separately from capital value and check the UK tax treatment.

FAQs

Bottom line

The UK Stock Investment Calculator is a transparent projection tool for testing contribution patterns and assumptions. Use ROI to understand total gain, the money-weighted annual return to account for contribution timing, and the CGT section only as a simplified taxable-account illustration. Keep dividends, fees, inflation and wrapper rules visible rather than hiding them inside one optimistic return number.

For 2026/27, verify the current GOV.UK CGT, dividend and ISA rules before making a real calculation. If the result depends on a named asset, provider, disposal history or pension arrangement, the calculator cannot replace the relevant documents or professional advice. A robust plan is one that remains understandable and workable when returns are lower, fees are higher or the market falls.

Official sources

• GOV.UK — Capital Gains Tax rates and allowances
• GOV.UK — Tax on dividends
• GOV.UK — How ISAs work
• GOV.UK — Income Tax rates and allowances
• MoneyHelper — Investing