Savings vs Investing Calculator: should you save or invest?
Your plan
Use the expected withdrawal date, not how long you hope to hold it.
Rates, risk and inflation
Illustrative investment presets
Planning examples only: 4%/8%, 6%/12% and 8%/20% nominal return/volatility. They are not historical market figures and do not represent a named fund or forecast. The 7% return, 15% volatility and 0.2% fee defaults are round stress-testing inputs chosen to make both growth and downside visible.
Your comparison
Please correct the highlighted inputs to calculate results.
For these assumptions, expected investing finishes £2,640 ahead of savings, but the simulated downside is £4,687 below savings.
This result is caused by the entered 5-year horizon, 3.5% APY, 7% nominal return, 15% volatility, 0.2% fee and 2% inflation assumption. Short-term or essential money generally prioritises stability; longer, flexible horizons can accommodate more volatility.
Suitability context: your answers allow more time and some capacity for loss, but that does not make investing automatically suitable or guarantee a better result.
All investing expected balance
£5,191 net growthYour save-and-invest split
–Total money paid in
£5,000 starting principal + £18,000 later contributionsSavings interest earned
After month-end contributionsInvestment market growth
Expected gross growth before feesInvestment fees deducted
£23,000 + £5,350 − £159 = £28,191Inflation-adjusted balances
£23,142 / £25,533
All savings / all investing in today’s moneyInvesting minus savings
+10.3% versus savingsBreakeven
First month investing is strictly greaterInvestment uncertainty
Downside (10th percentile)£20,864
Middle (50th percentile)£27,492
Upside (90th percentile)£36,471
In the model, investing finished below savings in 37.3% of paths and below the £23,000 paid in in 20.8% of paths.
Across 5,000 seeded simulations, the 10th percentile means about 10% finished lower; the 50th is the median; and the 90th means about 10% finished higher. These are model outputs, not confidence guarantees.
Year-by-year comparison
Text alternative to the chart. Amount paid in includes the starting amount and month-end contributions.
| Time | Paid in | All savings | All investing | Chosen split | Investing p10 | Investing p50 | Investing p90 |
|---|---|---|---|---|---|---|---|
| Start | £5,000 | £5,000 | £5,000 | £5,000 | £5,000 | £5,000 | £5,000 |
| 1y | £8,600 | £8,832 | £9,050 | £8,941 | £7,732 | £8,982 | £10,466 |
| 2y | £12,200 | £12,799 | £13,375 | £13,087 | £10,889 | £13,200 | £16,065 |
| 3y | £15,800 | £16,904 | £17,993 | £17,449 | £14,074 | £17,609 | £22,197 |
| 4y | £19,400 | £21,153 | £22,925 | £22,039 | £17,416 | £22,417 | £29,062 |
| 5y | £23,000 | £25,551 | £28,191 | £26,871 | £20,864 | £27,492 | £36,471 |
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Worked example using the default inputs
Start with £5,000, add £300 at each month-end for 5 years, compare 3.5% savings APY with a 7% nominal investment return, 15% volatility, 0.2% annual fee and 2% inflation.
Total paid in
£23,000£5,000 start + £18,000 later contributionsSavings ending balance
£25,551£2,551 interest earnedExpected investment balance
£28,191£5,350 gross growth − £159 feesToday’s-money balances
£23,142 / £25,533Savings / investingSimulated downside
£20,864Seeded 10th-percentile investment resultUnder the constant-return assumption, investing ends £2,640 ahead of savings. The seeded downside is £4,687 below savings and below the £23,000 paid in, showing why the higher expected balance is not guaranteed. Inflation reduces the purchasing power of both nominal totals.
Save or invest? A practical decision guide
| Factor | Cash savings | Investing |
|---|---|---|
| Purpose | Emergency buffer and known near-term spending | Flexible, longer-term growth goals |
| Typical timeframe | Often under five years | Often at least five years; longer gives more recovery time |
| Access | Can be immediate, depending on account terms | Sale and settlement take time; selling during a fall can lock in a loss |
| Capital stability | Balance does not fluctuate with markets | Market value moves and can fall below the amount paid in |
| Return certainty | Rate may be fixed or variable; variable rates can change | No guaranteed return |
| Inflation risk | Purchasing power falls if APY is below inflation | Can outpace inflation, but may not |
| Fees | Often no explicit fee, but check terms | Platform, fund, advice and dealing fees may apply |
| Tax treatment | Depends on account and allowances | Depends on account, income, gains and allowances |
| Potential losses | Eligible UK deposits may have FSCS protection; inflation can still reduce real value | Capital losses are possible; protection does not cover ordinary market performance |
- Deal with priority or expensive debt where appropriate and establish accessible emergency savings.
- Reserve cash for essential goals due in the near term.
- Consider investing only money that can remain untouched and whose temporary loss would not derail the goal.
- Use a mixed strategy when different portions of the money have different jobs.
MoneyHelper suggests a rule of thumb of three to six months of essential outgoings in instant-access savings. The FCA notes that investing for at least five years can help offset short-term fluctuations; neither is a guarantee or a personalised recommendation.
Calculation methodology
- P₀
- Starting principal.
- PMT
- Contribution added at the end of each month.
- n
- Number of months, rounded from the entered years × 12.
- A
- Savings APY as a decimal.
- R
- Expected nominal annual investment return as a decimal.
- F
- Annual investment fee as a decimal.
- i
- Annual inflation assumption.
- σ
- Annual investment volatility.
Exact rates and timing
Savings use the effective monthly rate rₛ = (1 + A)^(1/12) − 1. The savings step is Bₜ₊₁ = Bₜ(1 + rₛ) + PMT. This makes the entered APY mathematically consistent: twelve monthly periods reproduce the annual yield.
For the expected investment line, gross monthly return is rᵢ = (1 + R)^(1/12) − 1 and the effective monthly fee is f = 1 − (1 − F)^(1/12). Each month, the code calculates gross market movement Bₜ × rᵢ, deducts (Bₜ + growth) × f, then adds PMT. Gross growth and fees are accumulated separately so ending balance = starting principal + later contributions + market growth − fees.
When the combined monthly net rate is exactly zero, the balance is simply principal plus contributions. Negative expected net returns remain negative rather than being clamped to zero; the same monthly sequence is used, so zero and negative cases reconcile without dividing by a rate.
Inflation and simulation
Today’s-money value at month m is nominal balance ÷ (1 + i)^(m/12). Inflation does not change the nominal account balances.
The investment range runs 5,000 geometric-Brownian-motion paths. Each monthly gross factor is exp(([ln(1+R) − σ²/2] ÷ 12) + σz ÷ √12), where z is a seeded standard-normal draw. It then uses the same fee timing and month-end contributions. A fixed pseudo-random seed makes input comparisons stable; with σ=0, the simulated path equals the constant-return line. Percentiles and probabilities describe only this simplified distribution: real returns can be serially correlated, discontinuous, taxed, or outside it. A constant return is a planning assumption, not a forecast.
The mixed result applies the chosen percentages separately to starting principal and later contributions, calculates the cash and investment portions with the same rules, and adds them together.
Frequently asked questions
Should I save or invest?
Money needed for emergencies or essential goals within about five years generally benefits from cash stability. Money that can stay invested longer may have more time to recover from market falls, but returns are not guaranteed.
How much emergency cash should I keep?
MoneyHelper gives a rule of thumb of three to six months of essential outgoings in an instant-access savings account. Your appropriate amount depends on income security, insurance, dependants and essential costs.
What investment return should I assume?
Use a cautious nominal planning assumption that matches the assets and fees you expect, then test lower returns too. The presets are illustrations, not forecasts or claims about what a portfolio will earn.
Can investments finish below savings?
Yes. Investments can fall and may finish below savings or below the amount paid in. The simulation estimates how often that happens under the entered return and volatility assumptions; it cannot predict the future.
How are inflation and fees applied?
Inflation converts future balances into today's-money values. The annual investment fee is converted to an effective monthly deduction and applied after each month's simulated or assumed market movement, before the month-end contribution.
Are taxes and ISA allowances included?
No. The calculator excludes tax, tax relief, dealing costs and account-specific rules. ISA eligibility and allowances depend on current UK rules and personal circumstances, so check GOV.UK before acting.
Can I combine saving and investing?
Yes. Choose Save and invest, then set separate percentages for the starting balance and monthly contribution. The results compare that split with the two all-or-nothing alternatives.
Does this upload my data?
No. The calculations run locally in your browser and the page does not send the values you enter to Starlight Tools.
Review, limitations and sources
Reviewed by: Starlight Tools Financial Calculations Team (model design and numerical QA). Methodology reviewed: 16 July 2026.
Important: This educational model is not financial advice. Savings rates can change and investment returns are not guaranteed. Taxes, tax relief, trading costs and account rules are excluded. UK protection and ISA rules depend on eligibility, provider authorisation and current law.
Sources checked 16 July 2026: MoneyHelper: saving vs investing; FCA: risk and returns; Bank of England: 2% inflation target; FSCS protection checker; GOV.UK: current ISA rules.
The FSCS states that eligible deposits with UK-authorised banks, building societies and credit unions are protected up to £120,000 per eligible person, per authorised firm from 1 December 2025. Always check the provider and shared banking licences.
Change log: 16 July 2026 — corrected APY conversion and breakeven logic; added fee reconciliation, seeded risk outcomes, goal checks, mixed allocation, current source notes and visible FAQs.