The Money Maths

Home/Investment Growth Calculator

Investment Growth Calculator

Project what regular investing could grow into as a range of outcomes, not one smooth line. 1,500 simulations show your odds of hitting a target.

Your plan
£
£
%
yrs
£
Projected outcome
Median value after 20 years 0
Total you put in—
Median growth on top—
Likely range (5th–95th)—
Chance of reaching target—
Predicted outcomes
Median 5th–95th range Paid in

These outcomes come from running 1,500 simulations that let returns vary month to month around the expected rate, so instead of one smooth line you see the spread of where you might land — the riskier the mix, the wider the band. It assumes future ups and downs resemble the past, which markets regularly break, so treat this as modelled uncertainty, not a forecast. Figures are before inflation, fees, and tax; a UK ISA or pension can shelter much of the growth.

How the maths works

The formula behind this calculator, with a worked example built from your own inputs.

Compound growth & Monte Carlo simulationInvest

A steady projection compounds the pot and adds each contribution:

Vₘ = Vₘ₋₁ × (1 + r) + c

r = monthly return = annual ÷ 12  ·  c = monthly contribution

But real returns aren't steady, so the tool runs 1,500 simulations where each month's return is drawn at random around the expected rate:

rₘ = μ + σ·Z   where   μ = annual ÷ 12,   σ = volatility ⁄ √12

Z is a standard normal random draw (generated by the Box–Muller transform). Dividing volatility by √12 converts an annual figure to monthly — variance scales with time, so standard deviation scales with its square root.
Sorting all 1,500 outcomes each month gives the percentile bands; the median is the middle outcome, and the 5th–95th range holds 90% of them. The probability of hitting your target is simply the share of runs finishing at or above it.

Note: the simulation is seeded, so identical inputs always give identical results. That's for consistency between visits — not because the future is predictable.
About the currency selectorCurrency

Changing the currency relabels the figures — it does not convert them. There are no exchange rates involved, and nothing is fetched from the internet.

If you enter 280,000 and switch from pounds to dollars, the tool reads it as 280,000 dollars, not as a converted amount. The maths is identical in every currency because percentages, ratios and time don't care about the unit — a 25-year loan at 4.8% has the same payment-to-principal relationship whether it's in yen or rand.

What does change is formatting, which follows each region's own conventions: where the symbol sits (€ goes after the number in German), which separators are used (Switzerland uses apostrophes, South Africa uses spaces), how digits are grouped (India groups in lakhs — 1,71,120 rather than 171,120), and whether decimals exist at all (yen and won have no minor unit).

So pick the currency you're actually working in and enter local amounts. Don't enter pounds and switch to dollars expecting a conversion.

Assumptions & what these formulas ignoreImportant

Interest compounds monthly on the outstanding balance — standard for UK repayment mortgages, though some lenders calculate daily, giving slightly different totals.

Rates are assumed fixed for the whole term. In reality a fixed period ends and you revert or remortgage, so long-term totals are illustrative.

Not included anywhere: arrangement and broker fees, stamp duty, early repayment charges, void periods, maintenance, letting agent fees, income or capital gains tax, and inflation. Figures are in today's money.

Overpayment caps: many fixed deals limit penalty-free overpayments to around 10% of the balance a year.

Investment returns assume future variability resembles the past — which markets regularly break. Depreciation curves are typical patterns, not valuations; real resale depends on mileage, condition, spec and demand.

These tools are for estimating and comparing options. They aren't financial advice, and for a decision that matters it's worth speaking to a qualified adviser or broker.

Other calculators

All free, all showing their working.

Mortgage Repayment CalculatorWork out your monthly mortgage payment, total interest over the term and loan-to-value ratio.Mortgage Overpayment CalculatorSee how much interest you save and how many years you cut off your mortgage by overpaying each month.Buy-to-Let Rental Yield CalculatorCalculate gross yield, net yield, monthly cash flow and cash-on-cash return on a rental property.EMI Calculator for Personal & Car LoansCalculate the equated monthly instalment on a personal or car loan, total interest, and what it costs as a percentage of the amount borrowed.Step-up SIP CalculatorWork out what a monthly investment grows into when you raise it each year with pay rises, compared with keeping the amount flat.Car Depreciation & True Cost of Ownership CalculatorFind what your car will be worth in 5 years and what owning it really costs per month once depreciation and running costs are combined.Savings Goal TrackerSet a savings target and date, log contributions as you make them, and see whether your pace is on track.Inflation & Purchasing Power CalculatorSee what money is really worth over time.

About these calculators

How the numbers are produced, what they are not, and where to get regulated advice.

How the numbers are produced

Every figure comes from a published formula shown openly in the How the maths works section, alongside a worked example built from your own inputs. Nothing is estimated, weighted or adjusted behind the scenes.

The calculations run entirely in your browser. Nothing is stored, no accounts, no tracking, no adverts, and no affiliate links to mortgage or insurance products. Nobody pays to appear here, so there is no incentive to nudge a result in any direction. The one time anything leaves your device is if you click Update to latest figures on the inflation tab — that makes a single request to the World Bank's open data API for newer price indices, and sends nothing about you. Every other figure, including the bundled inflation history, is already in the page.

Default rates and depreciation curves are typical illustrative figures, not live market data. The tools do not fetch interest rates, house prices, exchange rates or resale values from anywhere.

Limitations — read before relying on this

These are estimating tools for comparing options, not a substitute for professional advice. In particular:

· Figures exclude fees, stamp duty, early repayment charges, tax, void periods, maintenance and inflation.
· Rates are assumed fixed for the whole term; in reality fixed periods end and you revert or remortgage.
· Investment simulations assume future variability resembles the past — which markets regularly break. They show modelled uncertainty, not a forecast.
· Depreciation curves are typical patterns, not valuations. Real resale depends on mileage, condition, spec and demand.
· Changing currency relabels figures; it does not convert them.

For a decision that matters, speak to a qualified mortgage broker, accountant or FCA-regulated financial adviser. Nothing here is financial advice.

Sources & further reading

For impartial, regulated guidance in the UK:
· MoneyHelper — the government-backed free money guidance service
· MoneySavingExpert — mortgage guides and comparison tools
· Bank of England — current Bank Rate
· GOV.UK — Stamp Duty Land Tax
· FCA — checking an adviser is regulated

The amortisation, yield and depreciation formulas used here are standard published finance formulas, not proprietary methods — you are encouraged to check them against any other source.