To work out how small you're going to trade, you need to see what the company charges to buy and sell. If they charge a flat rate of £12 to buy and £12 to sell, then you need to make £24 to break even, so if you bought £100 worth of shares then you'd need to make 24% just to break even. Increase the amount of shares purchased to £300, then you'd only need an 8% increase to break even, which is still far too much unless you get really lucky. Now trade with £1000 pounds (at the aforementioned £12 quid fee) and you'd only need to top 2.4% to start making money which is far more realistic. If you're in it for the long haul then the fees start looking less important, but if it takes 3 years to break the fee structure then you'd have been better off shoving it in a high interest account.