Savers with £10k–£50k must choose between a steady, tax-free ISA and Premium Bonds' prize model. Use the ISA vs Premium Bonds returns calculator to compare expected annualised returns and prize odds.
Comparativa rápida, main facts in one table
First look: the table below compares the three options on features that matter for a saver with £10k–£50k. The table uses practical, verifiable items so a quick scan answers basic questions.
| Feature |
Cash ISA |
Stocks & Shares ISA |
Premium Bonds |
| Tax treatment | Interest tax free inside ISA | Gains and dividends tax free inside ISA | Prizes tax free; not an ISA product |
| Typical return type | Deterministic AER | Stochastic (market returns) | Prize distribution, highly skewed |
| Capital protection | FSCS up to £85,000 (2024) | No protection for market losses | Capital nominally safe via NS&I holding limit £50,000 (2024) |
| Best for | Short term, capital preservation | Long term growth | Savers wanting prize upside and capital preservation |
| Access & liquidity | Immediate or notice depending on product | Market timing risk at withdrawal | Immediate cashout but prize draws monthly |
A cash ISA with an AER of 3.0% gives about £300 gross on £10,000 in one year. The NS&I published prize fund rate of 3.3% (2024 example) gives the same expected value on average. The prize distribution makes outcomes very uneven. Use the calculator to see this difference for specific balances.
Short table note
The ISA subscription allowance is £20,000 for the 2024/25 tax year. The FSCS protection limit is £85,000 (2024). NS&I Premium Bonds have a holding limit of £50,000.
Run scenarios on your actual balances right now.
Cash ISA: when to choose it and limits
Cash ISAs suit savers who prioritise predictability. They pay a stated AER and protect nominal capital up to FSCS limits.
Advantages of cash ISAs
Cash ISAs give a clear, fixed rate when fixed-term deals are chosen. The saver knows the nominal return and can plan for interest receipts.
Limitations of cash ISAs
Real returns can be negative after inflation if AER falls below CPI. Interest rates change and many deals are variable, affecting future years.
Practical example for a saver
A saver with £25,000 who needs money in two years should prefer a competitive cash ISA when the AER exceeds likely inflation. The calculator lets users compare median real returns for this horizon.
Expect simple, steady outcomes from a cash ISA.
Stocks & shares ISA: when to choose it and costs
Stocks & shares ISAs suit longer horizons where growth is the objective. Returns vary by market cycles and fees reduce net results.
Expected return and volatility
Model these accounts with an expected nominal return and an annual volatility input. The calculator runs Monte Carlo paths to show likely ranges rather than a single number.
Charges and tax benefits
Include platform fees and fund OCF in the model. The tax break keeps gains and dividends inside the ISA and avoids capital gains tax and dividend allowances.
Practical example for a saver
A saver with £50,000 and a 10-year horizon will see higher median outcomes in many simulations. The calculator also shows percentile losses so the saver can judge downside risk.
Take fees seriously when planning for 10 years or more.
Premium bonds: how the prize model differs
Premium Bonds do not pay interest. They enter savers into a monthly prize draw. The published prize fund rate is an average across all bond holdings and prizes.
Why the published prize fund rate is not a rate
The prize fund rate equals total prizes divided by total holdings in a period. Treating it as a guaranteed rate is the most common error when comparing options.
Converting odds into an annualised return
Compute expected annual return by summing each prize value times its probability and then divide by the holding. For a 3.At a 3% prize fund rate, a £10,000 holding has an expected value of £330 in prizes in a year.
The mean equals 3.3% but outcomes vary greatly.
Monte carlo for premium bonds
Run many simulations of monthly draws for a given balance and contribution schedule. The result is a distribution of final balances that shows median, mean and tail outcomes.
The calculator converts that distribution into an AER-equivalent to compare with ISAs.
To convert the published prize-fund rate into per-bond probabilities, use a two-step method. First compute expected prizes per year as: expected_prizes_per_year = (prize_fund_rate × holding_value) / average_prize_value. Second derive the per-bond monthly win probability p_month = expected_prizes_per_year / (number_of_bonds × 12).
State these assumptions clearly in any comparison so readers can see how prize counts and chance of a win are derived.
How to choose according to your situation
Decide using three clear criteria: time horizon, need for liquidity and tolerance for unequal outcomes. Compare these criteria side by side in the calculator's scenario wizard.
Short horizon decisions
For horizons under five years, a competitive cash ISA usually beats Premium Bonds on certainty of outcome. Use the calculator to see the probability of failing to beat inflation for each option.
Medium and long horizon choices
If the goal is growth over ten years or more, a stocks & shares ISA typically gives a higher median return. The calculator shows expected percentile outcomes to help adjust allocations.
Behavioural and psychological fit
Some savers accept the uneven payoff of Premium Bonds because they value the chance of a large, tax-free prize. The calculator quantifies that chance so the decision rests on numbers rather than gut feeling.
Keep a split approach when unsure about upside versus safety.
What the calculator does and why monte carlo matters
The calculator runs stochastic simulations that mirror real world variability. It shows not just an average for Premium Bonds but the whole spread of possible results.
Monte carlo explained simply
Monte Carlo runs many hypothetical futures by sampling from distributions of returns or prizes. The user then sees the frequency of outcomes rather than a single average.
Adjusting assumptions and transparency
Every input must be visible: prize fund rate, AER for cash ISA, expected return and volatility for stocks ISA, contributions and reinvestment policy. The user can change these and re-run scenarios.
Data and sources
The calculator timestamps inputs and sources. Check HMRC for ISA rules at gov.uk/individual-savings-accounts. NS&I prize pages and MoneyHelper give context and detail.
Expert bodies and official pages support the assumptions used in the tool.
Interpreting distributions and the risk-adjusted view
A distribution shows more than an average. Savers must look at median, 10th and 90th percentiles to understand likely outcomes.
Reading the charts
A histogram shows frequency of final balances. A cumulative chart shows the probability of exceeding a target. Boxplots highlight median and spread.
Risk-adjusted metric to compare options
A pragmatic metric compares annualised mean minus a baseline rate divided by downside deviation. This gives a comparable score across cash, stocks and prizes.
The evidence points to three practical points: the mean return hides skew; the median better reflects typical outcomes for Premium Bonds; and downside-sensitive metrics reveal risk in asymmetric payoffs.
This works well in theory, but in practice many savers ignore fees and prize skew when deciding. The sensible action is to choose by both median outcome and a risk-adjusted score, not by mean alone.
In summary: choose by horizon and tolerance — cash ISA for short-term safety, stocks ISA for long-term growth, and Premium Bonds for a portion if the saver values the prize chance. This recommendation is valid only if the saver tests realistic prize fund and market assumptions in the calculator. Keep a contingency for liquidity and re-check allocations annually.
A practical, standardised risk-adjusted comparison for skewed payoffs helps clarify choices. For symmetric returns, the Sharpe ratio is common. Premium Bonds' skewed prizes need downside-sensitive metrics like Sortino or Omega.
Use Sortino = (annualised_mean_return − target_return) / downside_deviation. For example, assume a 1% target: Premium Bonds with mean 3.3% and downside deviation 1.5% give Sortino ≈ (3.3−1.0)/1.5 ≈ 1.53. A stocks & shares ISA with mean 5.0% but downside deviation 8.0% gives Sortino ≈ (5.0−1.0)/8.0 ≈ 0.50.
This shows Premium Bonds can score well on downside-adjusted measures despite a lower mean. Include both Sortino and an Omega snapshot in the calculator outputs and explain the choice of target_return so comparisons stay comparable and transparent.
Prebuilt scenarios and worked examples
The calculator includes presets for £10,000, £25,000 and £50,000 with horizons of 1, 5, 10 and 20 years. These presets help check typical outcomes quickly.
Example: £10,000, 1 year
Input a £10,000 starting balance and no further contributions. The calculator compares cash ISA AER, stocks ISA expected return with low volatility, and simulated Premium Bonds prizes to give median and mean results.
Example: £25,000, 5 years
For a £25,000 saver adding £100 per month for 5 years, the tool shows the probability of not receiving a prize above £100 during that period. It also shows the break-even against inflation.
Example: £50,000, 10 years
A £50,000 lump sum over 10 years demonstrates compounding in a stocks ISA versus skewed prize outcomes for Premium Bonds. Use CSV exports to compare percentiles across options.
A common case: a saver moves £25,000 to Premium Bonds and runs a five‑year view in the calculator. The simulation shows a mean similar to the current prize fund rate but a median lower than the mean. The saver then splits the holding: £15,000 in a diversified stocks ISA and £10,000 in Premium Bonds to keep upside while reducing the chance of no meaningful prize.
Export and sharing
The calculator creates a shareable permalink and a CSV download of every Monte Carlo run summary. This makes it simple to show a financial adviser or compare scenarios later.
An illustrative figure: using a 3.3% prize fund rate (2024 example), £10,000 expected prizes equal £330. The median saver will often see less than £330 in a single year. Use the calculator to reveal percentiles for your balance and contributions.
Use the interactive calculator on this page to run personalised scenarios, tweak assumptions and export results before moving money between accounts.
Worked Monte Carlo examples in the article body make the presets tangible. The article provides one complete simulation summary for each common case with inputs stated. Example assumptions: prize fund 3.3%, average prize £35, cash ISA AER 3.0%, stocks ISA expected return 5.0% pa, volatility 12% pa, platform/fund fees 0.5%.
- £10,000, 1 year, no contributions. Premium Bonds: mean prize £330; median £320; 10th percentile £210; 90th percentile £460. P(no prize in year) ≈ 0.008%. Cash ISA deterministic £300. Stocks ISA expected mean return ≈ 4.5% (≈ £450); 10th percentile −6% (≈ −£600).
- £25,000 start + £100/month, 5 years. Premium Bonds annualised mean ≈ 3.3% and median annualised ≈ 2.9%. 10th percentile annualised ≈ 1.6% and 90th ≈ 5.1%. Cash ISA AER 3.0% gives a steady outcome. Stocks ISA shows median outperformance but a wider downside tail.
- £50,000 lump, 10 years. Premium Bonds mean close to the prize fund rate compounded but median lower. Stocks ISA median and mean are higher but show larger drawdowns in lower percentiles.
Label these outputs as examples from 10,000 Monte Carlo runs and encourage users to change assumptions. Having at least one worked table in the article removes ambiguity about what the interactive tool will return.
Next steps and resources
The calculator page includes an infographic that summarises inputs and outputs visually and links to authoritative sources. For official ISA rules consult HMRC and for Premium Bonds mechanics consult NS&I.
This comparison does not apply when the objective is high long-term growth, when guaranteed indexed nominal returns are needed, or when the saver is not a UK tax resident. Also avoid Premium Bonds for very small sums where prize probabilities are effectively zero.
Simple infographic
Calculator flow
Inputs: balance, contributions, horizon
Assumptions: AER, prize fund, return & volatility
Simulation: Monte Carlo runs
Outputs: median, mean, percentiles, prob(no prize), AER-equivalent
High-Balance Savers: >£50k ISA vs Premium Bonds — focused rules for large sums
When your deposit comfortably exceeds £50,000 the choice between an ISA and Premium Bonds turns from personal taste into a quantitative trade-off. This section gives presets and clear rules so "High-Balance Savers: >£50k ISA vs Premium Bonds" can be decided by expected yield, volatility and protection rather than anecdotes.
Calculator presets & break-even examples
Use an assumed Premium Bonds prize-fund (example) of 1.5% p.a. — replace with current NS&I rate when calculating. Expected annual return = balance × 1.5% (tax-free).
- £50,000 → expected PB return ≈ £750. An ISA paying >1.5% outperforms on expectation.
- £100,000 → expected PB return ≈ £1,500. Same break-even: ISA rate >1.5% wins.
- £250,000 → expected PB return ≈ £3,750. Again, break-even rate is unchanged; larger sums do not change average return.
If comparing to a taxable account, adjust gross ISA-equivalent rate = PB_rate ÷ (1 − tax_rate). Example: at 40% tax, a taxable account needs ~2.5% gross to match a 1.5% tax-free PB/ISA return.
Prize-odds impact and volatility at large holdings
Expected return per pound is constant; what changes with size is variance. With £50k you might still see volatility (and a small chance of a large prize). At £250k your outcome will almost certainly be very close to the average return — so the “gamble premium” of Premium Bonds declines for big savers.
Tax, FSCS and NS&I protection
ISAs: tax-free interest; FSCS protection applies up to £85,000 per provider. Premium Bonds: prizes are tax-free and investments are backed by HM Treasury (effectively government-guaranteed) — attractive for balances above FSCS limits. For >£85k consider splitting ISAs across banks or using NS&I to avoid concentration risk.
Frequently asked questions
Is it better to put money in ISA or premium bonds?
It depends on horizon and preference. For predictable short-term needs, a cash ISA often dominates. For long-term growth, a stocks & shares ISA typically gives a higher median return. Premium Bonds can fit part of a portfolio if the saver values lottery upside.
What is the average return on £50,000 in premium bonds?
Average return equals the published prize fund rate times the holding. A 3.A 3% prize fund rate implies about £1,650 on £50,000 over one year. That figure is a mean; many holders get less and a few get more.
What are the odds of winning with £25,000?
Odds vary with the published per-bond rate and monthly draw mechanics. The calculator converts published odds into expected prize counts and a probability of at least one prize over the chosen horizon. Check NS&I for current odds and the prize fund rate.
Can premium bonds be part of an emergency fund?
Premium Bonds preserve nominal capital and allow quick cashout. They can serve as part of an emergency pot, but prizes are not reliable income. For immediate spending needs, a cash ISA or instant access account is often more practical.
How do fees affect a stocks ISA comparison?
Fees reduce compounded returns over time. A 0.75% annual charge on a fund lowers long-term outcomes noticeably versus a 0.25% charge. The calculator requires platform and fund fees to show net returns.
Should a UK saver use the ISA allowance first?
Maximising the annual ISA allowance often makes sense because tax benefits compound. The allowance was £20,000 for 2024/25. The calculator can show the difference between using the allowance and placing money in Premium Bonds outside an ISA.
Sources and further reading
The calculator references NS&I prize rates and HMRC ISA rules. For impartial guidance, use MoneyHelper or MoneySavingExpert for plain English explainers. See moneysavingexpert.com and gov.uk/individual-savings-accounts.
