Which will grow £1,000 best over five years: a Cash ISA or Premium Bonds? New savers with no formal finance training must weigh tax-free returns, low risk and easy access while keeping figures simple enough to compare odds, guaranteed interest and how quickly savings can be withdrawn.
For most new savers, a Cash ISA offers guaranteed interest and easy access, while Premium Bonds give tax‑free prize draws with no guaranteed return. Choose a Cash ISA for steady, predictable growth and short‑to‑medium goals; consider Premium Bonds if you prioritise capital security and prize potential. Numeric scenarios for £100, £1,000, £10,000 and £50,000 at 1, 5 and 10 years, plus a simple odds and expected‑return calculator and a step‑by‑step decision flow, help test outcomes and pick an allocation.
Cash ISA vs premium bonds for new savers
A Cash ISA gives predictable, tax‑free interest; Premium Bonds give tax‑free prizes with no guaranteed yield.
How a cash ISA works
A Cash ISA pays interest on your balance, usually shown as an annual rate. Interest compounds by provider rules, often monthly or annually. The money counts against your annual ISA allowance when you pay it in.
How premium bonds work
Premium Bonds are bought in £1 units and enter monthly prize draws run by NS&I. Prizes are tax‑free and capital is guaranteed by HM Treasury. The expected monetary return equals the published prize‑fund rate, but individual outcomes vary widely.
Access and tax differences
Cash ISA withdrawals are usually instant or near instant with most providers. Premium Bonds require cashing in, which can take a few working days. Neither Cash ISA interest nor Premium Bond prizes are subject to income tax under HMRC rules.
When to prefer a cash ISA
Choose a Cash ISA when steady growth and predictable access matter more than lottery‑style upside.
Best uses for a cash ISA
Emergency savings and short‑term goals work well inside a Cash ISA. Regular savers who value stability also benefit because compounding gives reliable gains. The ISA wrapper keeps interest tax‑free regardless of your income.
Typical features and timings
Many Cash ISAs let you withdraw money the same day, though fixed ISAs restrict access for the term. Banks and building societies are normally covered by the Financial Services Compensation Scheme up to £85,000 per institution. ISA transfers protect your allowance if you move funds between providers.
Example numbers to judge speed
A typical Cash ISA example rate is around 3%.00% AER for competitive accounts. The 2024/25 annual ISA allowance is £20,000 per person. These figures help estimate growth and decide whether a Cash ISA meets your target.
When to prefer a Cash ISA: short‑to‑medium goals.
When premium bonds make sense
Consider Premium Bonds for a portion of savings when the chance of a large, tax‑free prize appeals and you accept uncertain returns.
How odds and expected return work
NS&I publishes a prize‑fund rate; currently it is 3.30% (published by NS&I). That rate represents the average return across all bondholders, not a guaranteed outcome for an individual. The expected annual return equals the prize‑fund rate times your holding.
Liquidity and timing for premium bonds
Cashing in Premium Bonds typically takes a few working days to transfer money to your bank. Monthly draws determine prizes, so timing of any needed cash is uncertain. You cannot hold Premium Bonds inside a Cash ISA wrapper, so they do not use that year's ISA allowance.
Limits and protections
The maximum holding in Premium Bonds is £50,000. NS&I is backed by HM Treasury, so capital is secure in the sense government guarantees apply. Prize income is tax‑free and does not need reporting to HMRC.
Feature comparison and decision matrix
Below is a compact comparison to scan quickly and act on.
| Feature |
Cash ISA |
Premium Bonds (NS&I) |
| Return type |
Guaranteed interest (rate varies) |
Tax‑free prizes (random) |
| Predictability |
High |
Low |
| Tax |
Interest is tax‑free inside ISA |
Prizes are tax‑free |
| ISA allowance |
Uses annual allowance (£20,000 in 2024/25) |
Does not use ISA allowance |
| Capital protection |
FSCS up to £85,000 per institution |
HM Treasury backing via NS&I |
| Access time |
Often instant or same day |
Typically several working days to cash in |
| Best for |
Emergency fund, short goals |
Discretionary savings, chance of big win |
Quick citable line
For many new savers, a Cash ISA offers steady, predictable, tax‑free growth, while Premium Bonds offer tax‑free prizes but no guaranteed return.
Practical recommendation: Cash ISAs work well for short and medium goals because they guarantee interest and quick access, but Premium Bonds can be useful for a small, discretionary holding that preserves capital while offering jackpot potential. This approach works well when savers keep an emergency buffer separate from any Premium Bonds holding, and review allocations annually.
Numeric scenarios and win probabilities: examples for £100
Here are worked examples to show how outcomes diverge by product and horizon. All Cash ISA numbers use a 3.00% AER example rate (2024) and prize bonds use the 3.30% prize‑fund rate (2024) as the expected value. Inflation is assumed 2.5% for real returns.
1 year, 5 years and 10 years projections
For £1,000 at 3.00% AER, balance after 1 year = £1,030. After 5 years (annual compounding) = £1,159. After 10 years = £1,344.
For £1,000 in prize bonds with a 3.30% expected return, expected balance after 1 year ≈ £1,033. After 5 years ≈ £1,177. After 10 years ≈ £1,386 (these are statistical averages, not guarantees).
For £50,000 the same maths scale linearly for expected value, but variance matters: a single large prize can swing outcomes. The probability of at least one prize increases with more bonds and with time.
Odds maths and probability of winning
Monthly per‑bond win probability p ≈ (annual prize‑fund rate as a decimal) / 12. For a 3.30% prize‑fund rate (0.033) this gives p ≈ 0.033 / 12 ≈ 0.00275 per £1 bond per month.
Probability of at least one win in N months with B £1 bonds = 1 − (1 − p)^(B×N). For small p, the Poisson approximation 1 − e^(−pBN) is convenient.
Example probability numbers
- With £1,000 (1,000 bonds) and p = 0.00275, chance of at least one prize in 12 months ≈ 1 − (1 − 0.00275)^(12,000) ≈ 1 − e^(−33), which is effectively >99% (most prizes will be small).
- With £100 (100 bonds) and p = 0.00275, chance of at least one prize in 12 months ≈ 1 − (1 − 0.00275)^(1,200) ≈ 1 − e^(−3.3) ≈ 0.963 (≈96–97%).
Step‑by‑step decision flow for new savers
Follow these steps to decide where to put your first savings. Each step is short and actionable.
Step 1: define your goal and horizon
Say what the money is for and when it is needed. Short horizon under 2 years favours Cash ISA or easy‑access accounts. Long horizon opens paths to Stocks & Shares ISAs if accepting volatility.
Step 2: build an emergency buffer
Aim for 3 months of essential spending as a starting target. Keep that buffer in a Cash ISA or similarly accessible, FSCS‑protected account.
Step 3: decide discretionary allocation
If you want a small chance at tax‑free prizes, allocate 5–20% of spare savings to Premium Bonds. Keep the rest in a Cash ISA for reliability.
Step 4: check ISA allowance
Putting money in a Cash ISA uses that tax year’s allowance (£20,000 in 2024/25). Transfer existing ISAs using the provider transfer route to preserve allowance.
Step 5: revisit annually
Compare current Cash ISA rates and NS&I prize‑fund rates once a year. Move funds if another safe option offers better guaranteed returns.
Four practical beginner allocations
- Emergency saver: 100% Cash ISA until buffer reaches 3 months of spending.
- Small fun saver: 80% Cash ISA, 20% Premium Bonds for enjoyment.
- Medium‑term goal (£10k): 70% Cash ISA, 30% Premium Bonds or fixed‑term deposits.
- Large sum (£50k): Keep 3–6 months expenses in Cash ISA; consider splitting remainder between higher‑rate cash accounts, fixed ISAs, or Stocks & Shares ISA after advice.
How to open and manage accounts
This section lists practical steps and timings for first‑time savers.
Opening a Cash ISA
Compare provider rates, check FSCS cover, then open online or in branch. Funding can use the current tax year allowance and transfers preserve allowance. Fixed ISAs require holding for the term or you may face notice periods.
Buying premium bonds with NS&I
Open an NS&I account online or by post and buy Premium Bonds in £1 units. The maximum holding is £50,000 (2024). Monthly draws determine prizes; cashing in takes a few working days.
Transfers, withdrawals and documentation
To move a Cash ISA, request a transfer rather than withdraw and re‑deposit, or you may lose allowance. Keep ID documents ready; most providers clear applications within 3–10 working days. For Premium Bonds, expect 2–5 working days to receive funds after cash‑in.
Do not use Premium Bonds if you need guaranteed returns above inflation in the short term, or if you cannot accept long gaps without meaningful prizes. They are not a substitute for a taxed, regular interest account or investments suited to long‑term growth.
If decisions still feel unclear, compare your personal numbers above and consider speaking to a regulated financial adviser to match product choice to goals and tax position.
Frequently asked questions
Is it better to put money in a Cash ISA or Premium Bonds?
It depends on goal and horizon. Use a Cash ISA for emergency savings and predictable growth, and Premium Bonds for a small discretionary holding that preserves capital and offers prize upside.
Do premium bonds count towards ISA allowance?
No. Premium Bonds do not use your ISA allowance unless you sell them and put proceeds into an ISA. The ISA allowance for 2024/25 is £20,000.
Are premium bond prizes taxable?
Prizes are tax‑free and do not need declaration to HMRC. NS&I prizes are treated as tax‑free winnings rather than interest.
How do I compare expected returns mathematically?
Compare Cash ISA AER with NS&I prize‑fund rate as expected return. Expected value for Premium Bonds equals the published prize‑fund rate times your principal (for 2024, NS&I published 3.30%).
Can I lose my capital with premium bonds?
Capital is guaranteed because NS&I is backed by HM Treasury, so you do not lose nominal capital. Real losses can occur if inflation exceeds your returns for long periods.
How quickly can I access money in each product?
Cash ISAs often allow same‑day withdrawals for easy‑access accounts. Premium Bonds need cash‑in processing, typically taking 2–5 working days to reach your bank.
The practical plan
Start by naming the goal, setting the horizon, and building a 3‑month emergency buffer in a Cash ISA. Allocate a small, clearly limited amount to Premium Bonds only if you enjoy the prize element and accept uncertainty. Review choices annually, watch ISA allowance limits, and move funds via transfer routes to protect tax status.
NS&I publishes prize‑fund rates and odds; check provider pages for current Cash ISA rates before you commit.
Odds maths and probability of winning
The published prize‑fund rate gives an expected return across all bondholders, but it does not by itself equal the per‑bond monthly win probability unless you know the average prize amount won when a win occurs.
- Formally, if r is the annual prize‑fund rate (decimal) then the expected monthly return per £1 is r/12
- if A is the average prize amount paid when a bond wins, the per‑bond monthly win probability p ≈ (r/12) / A. As an illustrative bound, if A were £25 (the smallest prize denomination and the most common prize tier), then with r = 0.033 you get p ≈ (0.033/12)/25 ≈ 0.00011 per bond per month. For 1,000 bonds over 12 months the Poisson mean λ = p×B×N ≈ 1.32 so the chance of at least one win is ≈ 1 − e^(−1.32) ≈ 73%
- for £100 (100 bonds) the chance ≈ 12%
Using the raw prize‑fund rate divided by 12 as if it were the win probability assumes an average prize of £1 and therefore overstates the chance of a win. Always distinguish the expected return (useful for average value comparisons) from the probability distribution of prize sizes (which determines how likely you are to see any meaningful prize).
