Position-size calculator
Loss first, stake second. Decide what this trade is allowed to cost you, set the stop where the idea is wrong, and let those two numbers choose the stake — not confidence.
Decide what you're willing to lose on one idea — most professionals use 1–2% of the account — and where the trade is wrong (your stop). The stake follows: risk ÷ stop distance = £/pt. Beginners do it backwards: pick a stake that feels exciting, then discover the risk. At 2% you can be wrong twenty-five times in a row and still have three-quarters of your account.
The margin figure shows what the provider will hold; if it's a large share of the account, the position is big even when the planned risk looks small. The margin & P/L calculator shows the same trade from the exposure side, and risk management (chapter 11) covers where stops belong: at the level that proves you wrong, never at a round number of pounds.