What getting the lot size wrong costs
Sizing up by mistake multiplies everything at once: the planned risk, the spread cost and the size of the slippage. Doubling the lot does not just double the loss — it doubles the costs you never counted, and it is the sum of the three that blows the daily limit.
The cost of sizing wrong is not the lot difference: it is risk, spread and slippage multiplied together, against a limit the firm checks on the executed result.
The maths almost everyone does halfway
The correct sizing is: cash risk ÷ (stop distance in pips × pip value). With 100k of capital, 1% risk and a 20-pip stop on EURUSD, risk is 1,000 dollars, the pip is worth 10 per lot, and the size is 1,000 ÷ (20 × 10) = 5 lots.
What that leaves out is the cost of executing. At 1 pip of spread, those 5 lots are born 50 dollars down. With 2 pips of slippage on the stop, the worst case is not 1,000 — it is 1,150. That is 15% more than planned, on a trade that followed the plan.
Sizing against the full limit means, in practice, trading above it. The right maths leaves room for the fill to be worse than the request, because it often is.
Sizing up: what each mistake costs in cash
A decimal-place slip is the most common and the most expensive. Typing 5 where 0.5 belonged is ten times the size — and what was 1% risk becomes 10%, above any firm's daily limit.
A smaller error counts too. In the same example, 6 lots instead of 5 is 20% more risk: 1,200 of planned loss instead of 1,000, plus 60 of spread and 120 of slippage in the worst case, against a 5% daily limit that allows 5,000. It looks roomy — until the third trade of the day.
The difference is not linear where it matters. The loss grows with the lot, but the chance of recovering the same day falls, because the remaining limit shrinks. Two 20% errors eat almost half of a day that should have fitted four trades.
Sizing down has a cost too, and a different one
Trading smaller than planned does not blow the account, but it breaks the statistics. A strategy measured at 1% risk and executed sometimes at 0.3%, sometimes at 1%, produces a record you cannot draw an expectancy from — and without expectancy there is no way to know whether it works.
On a funded account there is a specific cost: several firms require a minimum number of valid days and some a minimum daily profit for the day to count. Irregular size stretches the challenge without reducing the risk of failing it.
Sizing down beats sizing up, and it is still a mistake. The fix is the same in both cases: use the number the maths gave, not the one that felt reasonable at the time.
Why this closes funded accounts rather than just hurting
The firm checks the limit against the result that showed up on the account, not against what you meant to risk. The sentence matters: between your intention and their number sit the spread and the slippage, and neither appears in your plan.
On a trailing-drawdown firm the effect is larger. The floor rises with the equity high, so every new balance shrinks the total room — and a position that is too big on a day with little room breaks the total limit even while respecting the daily one.
There is one way out and it is arithmetic: size against the limit minus a margin. Holding back 20 to 30% of the room for the fill to be worse than the request turns a bad day into a loss instead of a closed account.
- Calculate the size, do not estimate it: cash risk ÷ (stop in pips × pip value).
- Add spread and slippage to the worst case before accepting the number.
- Size against the room minus 20 to 30%, never the full room.
- Check the decimal place before sending — the costliest error and the easiest to avoid.
FAQ
What risk percentage per trade should I use?
That is your decision and depends on your strategy and your firm's rule. What this guide shows is the cost of each choice: the higher the percentage, the fewer trades fit inside the day's limit and the less room there is for the fill to come out worse than planned.
Is pip value always 10 dollars per lot?
Only when the dollar is the quote currency, as in EURUSD and GBPUSD. On USDJPY and on crosses the value depends on the current quote, and using 10 as if it were fixed gets the position size wrong.
How much slippage should I set aside?
It cannot be predicted, and anyone promising a number is inventing one — it depends on the order book at the instant of the order. What you can do is measure your own: compare requested and executed prices on your trades and use the worst case you have seen as the margin.
If I only use limit orders, does that solve it?
It solves the entry, in exchange for sometimes not entering. It does not solve the stop: a stop is a market order by definition, and the stop is exactly where slippage hurts the firm's limit.