Updated in July 2026
Could converting foreign income to sterling quietly shave 0.5–3% off your returns in total comparison terms, depending on frequency of conversions and how you annualise the loss? A single one‑off conversion is a transaction cost that reduces capital (for example a 1% spread plus a £10 receiving fee on £10,000 equals about a 1.1% one‑off hit), but if you convert repeatedly the same per‑conversion hit can be treated as an ongoing cost and approximated on an annualised basis; figures below indicate whether they are presented as a single‑event capital erosion or an annualised recurring cost.
Many UK residents who hold sterling but occasionally handle foreign cash underestimate how those conversion and transfer costs eat into interest or prize odds when choosing where to park £1k–£100k+.
Comparative quick view
The table below summarises typical gross and net outcomes for sterling holders who convert foreign currency before saving. Read the row that matches the product being considered.
| Product |
Typical gross return (examples) |
Net after FX 0.5%/1%/2% + fee |
Liquidity |
Tax & protection |
| Cash ISA (Cash) |
Rates vary by provider, account terms and market conditions |
Example: ISA 3% net = 3% if no FX; if conversion needed subtract spread + fee/pot |
Immediate access often same day to 30 days |
Tax free in the UK; FSCS protection is available up to the applicable limit per eligible person and authorised institution |
| Bonds (NS&I) |
Expected value equals the NS&I prize-fund rate, which can change over time |
Example: prize 1% → £10/yr per £1k; convert costs reduce expected value same as ISA |
Cashable in 2–5 working days; prizes paid monthly |
Prizes tax free in UK; check local tax for non‑residents; NS&I is government backed |
| Multi‑currency accounts (Wise, Revolut) |
Near zero interest; FX at close to mid‑market minus small fee |
Use for cheaper conversions; net depends on provider spread (≈0.3%–1%) |
Immediate transfers between currencies possible; withdraw to bank in days |
Not ISA eligible; check FSCS coverage for underlying banks |
When you convert EUR or USD to GBP, a 1% FX spread on a £10,000 conversion costs roughly £100. A £10 receiving fee adds 0.1% to the cost. These two items together can exceed the gap between a headline ISA rate and Premium Bonds' expected value.
Illustration: one‑off conversion hit on a £10k pot
FX spread 1% = £100
Receiving fee £10 = 0.1%
Combined immediate cost ≈1.1% of the pot
Cash ISAs for multi‑currency savers
A cash ISA keeps interest tax free in the UK and may provide FSCS protection up to the applicable limit per eligible person and authorised institution. If the saver already holds sterling, a cash ISA yields the full headline rate. If the saver must convert foreign currency, conversion costs reduce the effective return.
Pros for FX holders
Interest in a cash ISA is guaranteed and paid as a rate. The ISA wrapper shields interest from UK income tax for UK residents. Cash ISAs avoid the probability risk of prize draws.
Limits and rules
The ISA annual subscription limit is set for each tax year, so check the current HMRC allowance before subscribing. Transferring and re‑subscribing matters: withdrawing and replacing funds in the same tax year can break the subscription record. Use the ISA transfer process to keep the tax wrapper.
Elige esto si: the saver keeps most funds in sterling and values a predictable net return.
Premium Bonds explained: odds and return
Premium Bonds pay prizes via a monthly draw rather than interest. The expected annual cash return equals the NS&I prize‑fund rate multiplied by the pot size. Use prize‑fund scenarios when comparing to ISAs.
Expected £ per year equals pot × prize‑fund rate. For example, at 1% a £10,000 pot gives £100 expected per year. At 4% the same pot gives £400 per year.
Probability of at least one prize
The chance of winning at least once uses the per‑bond odds from NS&I and the binomial complement 1−(1−q)^n. For large pots, a Poisson approximation gives practical estimates. NS&I publishes odds and prize distribution tables for accurate calculation. NS&I Bonds
Elige esto si: the saver accepts probabilistic returns and values capital security plus the upside of prizes.
FX costs: convert and lose
Converting foreign currency to sterling creates two main costs: a percentage spread off the mid‑market exchange rate and any fixed receiving or correspondent fees. These costs reduce the initial capital and lower the effective annual return.
Where costs arise
Providers charge a markup on the mid‑market rate called a spread; typical spreads range between 0.3% and 2% depending on provider. Banks may also apply a flat incoming fee or correspondent bank charges, often between £0 and £25. Fintech providers often show mid‑market rates but add a small percentage fee.
Worked conversion examples
Convert a €10,000 holding with a 1% spread and a £10 receiving fee. The spread costs roughly €100; the fee is a fixed deduction after conversion. For a small pot, the fee amplifies the percentage hit. For example, a £10 fee on a £1,000 pot is 1% extra loss.
The most frequent error is comparing headline ISA interest to the expected Premium Bonds value without subtracting FX and receiving fees. That mistake flips the better option for many savers.
Elige esto si: the saver often receives income in foreign currency and needs a clear conversion plan before saving.
Net return scenarios for £1k, £10k, £50k and £100k
Scenario examples make decisions concrete. The numbers below show how a one‑off conversion at time of deposit changes net annual returns for each pot. Use them as templates and replace inputs with live figures.
Scenario assumptions
Assume a cash ISA gross rate of 3%. Use two Bonds prize scenarios: 1% (low) and 4% (high). Use a conversion spread of 1% and a receiving fee of £10. For each pot, convert the £10 fee into a percentage and subtract spread plus fee/pot from gross returns.
Net outcome table
| Pot |
Net Cash ISA (3%) |
Net Premium (1%) |
Net Premium (4%) |
| £1,000 |
3% − 1% spread − 1% fee = 1.0% |
1% − 2% total cost = −1.0% |
4% − 2% total cost = 2.0% |
| £10,000 |
3% − 1% − 0.1% = 1.9% |
1% − 1.1% = −0.1% |
4% − 1.1% = 2.9% |
| £50,000 |
3% − 1% − 0.02% = 1.98% |
1% − 1.02% = −0.02% |
4% − 1.02% = 2.98% |
| £100,000 |
3% − 1% − 0.01% = 1.99% |
1% − 1.01% = −0.01% |
4% − 1.01% = 2.99% |
This works well in theory but in practice timing matters. If conversions happen repeatedly, recurring spreads add up. For one‑off conversions the fee/pot effect fades with larger balances.
Elige esto si: the saver wants to test concrete numbers for specific pot sizes before choosing a product.
A simple Premium Bonds expectation simulator can make comparisons tangible. Start with the NS&I prize‑fund rate to get the Premium Bonds expected value: Expected annual £ = pot × NS&I prize‑fund rate. To turn that into probabilities, assume an average prize size (for illustration only) and compute the expected number of prizes per year = (pot × prize‑fund rate) ÷ average prize size; approximate the chance of at least one prize with 1 − exp(−expected_number). For example, at a 1% prize‑fund and an assumed average prize of £50, a £1,000 pot has expected annual winnings £10 → expected_number = 10/50 = 0.2, so probability of at least one prize ≈ 1 − e^(−0.2) ≈ 18%.
The same method gives a £10,000 pot expected £100 → expected_number = 2 → chance ≈ 86.5%. Use these outputs alongside the Premium Bonds expected value to compare against cash ISA rates after deducting currency conversion costs, FX spreads and any receiving fee; this yields a practical, numeric view of downside, upside and the typical wait time to a first win.
How to buy, transfer and cash out
The operational steps and timings change net outcomes. Experienced savers plan the flow of funds to avoid multiple conversions and protect the ISA wrapper.
How to buy or subscribe
Open the chosen Cash ISA or Premium Bonds account online or by post. Fund it from a sterling account to avoid extra conversions. If funding from foreign currency, convert via a low‑spread provider or a multi‑currency account first.
Transferring and cashing out
An ISA transfer must be done by instructing the new provider to move funds; expect up to 30 days. NS&I cash encashment usually reaches a bank account in 2–5 working days. Keep records for HMRC and for proof of source under money laundering checks.
Elige esto si: the saver cares about preserving the ISA tax wrapper and wants minimal operational surprises.
What nobody tells you
Premium Bonds are often described as risk free because capital is secure. The missing point is the long‑run expected value for many savers will be lower than simple cash ISAs after conversion costs. The prize draw is attractive for its upside, not for consistent yield.
The case most often seen is that a saver converts small foreign amounts repeatedly and loses more in FX spreads than any prize‑fund advantage. The data point is simple: a 1% spread on repeated conversions erodes capital over time.
Elige esto si: the saver wants the upside of prizes but accepts the low probability of matching ISA returns.
This guidance is less relevant for savers who deal only in sterling, those seeking high growth via stocks and shares, or when sums are so small that transfer fees dominate. Check live ISA rates and the current NS&I prize‑fund before moving funds.
If uncertain after these steps, compare live ISA rates and the current NS&I prize fund on official sites before transferring funds, and consider a quick calculation using your provider's FX spread and any receiving fee.
Nominal returns in sterling are only part of the story for multi‑currency savers: the other half is purchasing power after inflation and FX moves. To convert a sterling nominal return into real return in another currency, combine three effects: the nominal cash ISA or Premium Bonds return in GBP, the change in the GBP/other‑currency exchange rate over your holding period, and inflation in the currency you want to spend in. A simple formula is: real return ≈ (1 + nominal_GBP_return) × (1 + FX_change_against_GBP) ÷ (1 + local_inflation) − 1, where FX_change is positive if GBP strengthens.
Example: a 3% cash ISA, GBP depreciates 2% versus EUR (FX_change = −2%) and eurozone inflation is 3%. This gives (1.03×0.98)÷1.03 −1 ≈ −0.98% real return in EUR terms. That shows how a seemingly attractive cash ISA rate can still leave a sterling holder worse off in real foreign‑currency purchasing power once currency conversion costs, FX moves and domestic inflation are modelled.
Frequently asked questions
Is it better to buy Premium Bonds or an ISA?
It depends on conversion costs and pot size. If conversion spread plus fixed fees exceed the gap between the ISA rate and expected prize rate, choose a cash ISA. For pots under £10k and a £10 receiving fee, conversion makes Premium Bonds less attractive unless prize‑fund rates are high.
Do Premium Bond prizes count as taxable income in the UK?
No, prizes are tax free in the UK. UK residents do not pay income tax on Premium Bond prizes, though non‑residents should check local tax rules where they live.
Do Premium Bonds count towards ISA allowance?
No, Premium Bonds held with NS&I do not use the ISA annual subscription limit. ISA allowances are separate; for 2024/25 the allowance is £20,000. Use an ISA and Premium Bonds together if that suits the plan.
How long does an ISA transfer take?
An ISA transfer usually completes within 15 to 30 days when initiated correctly by the receiving provider. Partial transfers are possible but follow provider instructions to preserve tax status.
What are typical FX spreads from providers?
Typical spreads range from about 0.3% with specialist fintechs to 0.5%–2% with high‑street banks. A single conversion at 1% spread on £10,000 costs roughly £100 plus any fixed fees.
Are Premium Bonds safer than a bank deposit?
Capital in Premium Bonds is secure because NS&I is government backed. Bank deposits have FSCS protection up to £85,000 per institution. Safety differs by context; NS&I is not the same legal guarantee as FSCS but is state‑supported.
How do I model my own expected Premium Bonds returns?
Use Expected £/yr = Pot × prize‑fund rate. Then subtract conversion spread and any receiving fee amortised over one year. For probability of at least one win use 1−(1−q)^n with q from NS&I and n equal to number of bonds.
Tax and residency rules matter for savers with multi‑currency lives. Premium Bond prizes are tax free in the UK, and interest in a Cash ISA is shielded from UK income tax for UK residents, but that does not automatically mean those sums are tax‑free where you live. Many countries treat windfalls and interest differently: for example, US citizens remain taxable on worldwide income and must declare NS&I prizes on US returns even though they are tax free in the UK. Similarly, if you become non‑resident you can normally keep existing ISAs but you may no longer be eligible to subscribe (there are specific exceptions for Crown servants and certain occupations).
In practice that means an expatriate sterling holder should check both (a) whether their country of residence taxes lottery/windfall income or foreign interest, and (b) whether they can continue to add to an ISA. Where cross‑border tax applies, the net return comparison between a cash ISA and Premium Bonds must include the expected foreign tax bill as well as any currency conversion costs and receiving fees.
Which to choose by situation
For a sterling holder with occasional foreign receipts, a cash ISA usually gives higher net returns once FX spreads and receiving fees are accounted for. For a saver who prizes the chance of a big tax‑free prize and accepts probabilistic returns, keep a portion in Premium Bonds. For frequent conversions use a multi‑currency account to reduce spread costs before moving cash to an ISA or NS&I.
Concrete allocation rules by pot and horizon:
- £1,000, short term buffer: 100% cash ISA if funding requires conversion and fee >1% of pot.
- £10,000, medium term: 70% cash ISA, 30% Premium Bonds when prize‑fund outlook is favourable and conversion costs are low.
- £50,000 to £100,000, long term: keep a core in stocks & shares ISA for growth, a cash ISA buffer equal to 3 months expenses, and up to 10–20% in Premium Bonds if prizes appeal.
A practical rule: if (ISA rate − expected Premium Bonds rate) > (FX spread + fee/pot) then choose ISA. Replace inputs with live numbers for a definitive call.
Elige esto si: the saver wants a final rule to act on now and will replace inputs with live rates.
