The short version
- Less than many people think. At the three platforms I checked, monthly minimums started at £25 and lump sums at £100 to £500.
- Monthly investing lets you start small, and fractional shares mean you don't need the price of a whole ETF unit.
- On a small pot, how you're charged matters more than the minimum. A flat fee can eat a big slice of a few hundred pounds.
- An emergency fund in easy-access cash usually comes first. MoneyHelper suggests three to six months of essential outgoings.
Part of Getting started. For the whole process in order, see how to buy your first ETF, step by step.
- £25lowest monthly minimum at the three platforms checked
- £100–£500lump-sum minimums at the same three platforms
- 3–6 monthsof essential outgoings: MoneyHelper's suggested emergency fund
Minimums vary by platform
Each platform sets its own minimum, and they differ. Here are three examples, taken from each platform’s own stocks and shares ISA pages.
| Platform (example only) | Lump sum | Monthly |
|---|---|---|
| AJ Bell | £250 | £25 |
| Hargreaves Lansdown | £100 | £25 |
| Vanguard Investor | £500 | £100 |
Listed alphabetically, as checked on 13 October 2026. Minimums change, and these are examples, not recommendations. Some platforms set no minimum for moving an existing ISA across: Vanguard Investor, for one, has no minimum for ISA transfers.
Starting with a monthly amount
All three let you start with a monthly payment instead of a lump sum. At some platforms it’s also the cheapest way to deal. AJ Bell and Hargreaves Lansdown both list no dealing charge for regular monthly investing, against £5 and £6.95 for a one-off online trade.
The money is invested on a set day each month, at that day’s price. You don’t have to pick a moment, and the habit does the work.
If you already have a lump sum, whether to invest it at once or drip it in is a separate question. Lump sum or monthly? looks at the evidence.
Fractional shares
Without fractions, you buy whole ETF units. Illustration: if one unit costs £80 and you have £50, you can’t buy any. With £100, you’d buy one and have £20 left over as cash.
Some platforms let you buy part of a unit, so all your money goes in. Trading 212 says most instruments in its Invest accounts and ISAs can be traded in fractions. InvestEngine’s ISA page also lists fractional investing.
Since 5 November 2024, fractions can be held in a stocks and shares ISA, as long as your provider offers them.
Fractions don’t make a fund cheaper or better. They just remove the minimum set by the unit price.
Why flat fees hit small pots harder
Illustration only, with made-up round numbers. Imagine two platforms. One charges a flat £60 a year (£5 a month). The other charges 0.25% of your pot a year.
| Pot size | Flat £60 a year | Flat fee as % of pot | 0.25% a year |
|---|---|---|---|
| £500 | £60 | 12% | £1.25 |
| £5,000 | £60 | 1.2% | £12.50 |
| £50,000 | £60 | 0.12% | £125 |
On £500, the flat fee takes 12% a year. Your investments would have to grow 12% just to stand still. On £50,000, the same fee is less than the percentage one.
Dealing charges work the same way. A £5 charge on a £50 purchase is 10% of your money before it’s invested. On £500, it’s 1%.
Real charges vary widely. In 2019 the FCA found yearly charges on £5,000 in a stocks and shares ISA ranged from 0.2% to 2.4%. Over five years, assuming 5% growth a year, that gap could mean £650 less in returns.
So on a small pot, the way you’re charged can matter more than the minimum. The fee calculator lets you try your own numbers, and Do I need a broker? explains the charging structures.
An emergency fund first
Shares can fall sharply, and ETFs that hold them fall too. If you might need the money at short notice, you could be forced to sell after a fall. That’s why the usual order is cash first, investing second.
MoneyHelper, the free government-backed guidance service, suggests three to six months of essential outgoings in an instant-access savings account. Essentials means things like rent or mortgage payments, household bills, food and heating. So if those come to £1,000 a month, the aim would be £3,000 to £6,000.
That can sound like a lot, especially from a standing start. NS&I’s guide adds that it doesn’t need to be your first target.
If you have a mortgage, see should I pay off my mortgage or invest? for how to weigh the two.
How I'd think about it
I can’t tell you what to do, because I don’t know your circumstances. These are the questions I’d ask, roughly in this order.
- Is the emergency fund in place? If not, that’s usually the first job.
- Do you have expensive debt? Weigh the interest on a credit card or overdraft against any return you might hope for.
- Can you leave the money alone for years, through falls? Will I lose money? shows how often UK shares lost money over different periods.
- What would you pay on your amount? Check the minimum, the charging structure and whether regular investing is free.
If you’re comfortable with those answers, the starting amount matters less than many people fear.
Common questions
- Does an ETF have its own minimum?
- Only the price of one unit, if your platform doesn’t offer fractions. Unit prices differ from fund to fund.
- Is £25 a month worth investing?
- That depends on the costs. With no dealing charge and a percentage fee, costs stay in proportion. With a flat fee, they may not.
- Can I open an ISA just to transfer an old one?
- Yes, and some platforms set no minimum for that. Vanguard Investor is one example.
- Does a small pot get the same FSCS protection?
- Yes. If a firm fails and investments are missing, the FSCS can pay up to £85,000 per person, per firm, whatever the size of your pot.
- Can I increase the amount later?
- Yes. In an ISA, you can pay in up to the £20,000 yearly allowance across all your ISAs.
This page explains how starting to invest works. It isn't advice about what you should do, or a recommendation of any platform. Investments can fall as well as rise, and you may get back less than you put in. For free, impartial guidance, try MoneyHelper. Read the full disclaimer.
Sources
- Stocks and shares ISA and charges, AJ Bell (example only)
- Stocks and Shares ISA and charges, Hargreaves Lansdown (example only)
- Stocks and Shares ISA and What's the minimum investment amount?, Vanguard Investor (example only)
- Can I trade with fractional shares?, Trading 212 help centre (example only)
- Stocks and Shares ISA, InvestEngine (example only; comparison data dated 7 July 2026)
- Individual Savings Account and Child Trust Funds (Amendment No. 2) Regulations 2024, HMRC, GOV.UK
- Investment Platforms Market Study: Final Report (MS17/1.3), FCA, March 2019
- How much to save for an emergency, MoneyHelper
- Saving for the unexpected, NS&I
- Investments, Financial Services Compensation Scheme
- How ISAs work, GOV.UK
Facts checked 13 October 2026.