You can have the best entry in the world and still lose your account if your position size is wrong. Position size, meaning how many lots you take, determines what a losing trade costs you. For a prop firm trader it is the most important calculation of the day, because one oversized trade can eat your daily limit in one go.
This article is educational and not investment advice. The examples are rounded; check pip values and contract sizes with your own broker.
What is position size?
Position size is the size of your trade, expressed in lots. The right number of lots depends on three things: how much you want to risk, how far your stop loss is and what a pip is worth for your instrument. Change one of the three and the right lot size changes with it.
Why 'just 1 lot' or 'always 0.5 lots' does not work
Many traders use a fixed lot size. It feels simple, but your risk then swings widely with your stop loss:
- With 0.5 lots on EUR/USD, a 10-pip stop costs about $50
- The same 0.5 lots with a 40-pip stop costs about $200
- That is four times the risk, without you consciously choosing it
Sizing by risk instead of by lots keeps your loss per trade the same, no matter how far your stop is.
The formula in three steps
- Dollar risk = account size × risk percentage
- Lot size = dollar risk ÷ (stop loss in pips × pip value per lot)
- Check: does this risk fit within your daily and max drawdown limit?
Example: on a $10,000 account you risk 1%, so $100. With a 20-pip stop on EUR/USD (about $10 per pip per lot) that is 100 ÷ (20 × 10) = 0.5 lots. Don't want to do the maths yourself? Use our free lot size calculator.
Why this matters even more at a prop firm
At a prop firm you have not only your own risk rules but also a daily and a maximum drawdown limit. Those limits differ per firm and can change, so always check them on your firm's website. Work out how much of that limit a single trade uses:
- Risk 1% against a 5% daily limit and one loss costs a fifth of your day's room
- Risk 2% and two losses in a row are enough to use up almost your whole day
- The less room to the limit, the smaller your position should be
Our calculator shows this instantly: enter your own limits and see what percentage of your daily and max limit the trade uses.
Common mistakes
- Increasing lot size after a loss to win it back; that is the start of revenge trading
- Moving your stop loss without adjusting lot size
- Ignoring pip value: gold, indices and JPY pairs work differently from EUR/USD
- Setting risk by feel instead of a fixed percentage
More on those behavioural mistakes in trading psychology and revenge trading and on placing your stop in how to set a stop loss.
From calculating to sticking with it
A formula only helps if you use it every trade. That makes position size a discipline question too: do you keep to your rule, even after two losses? By tracking per trade what risk you took and whether you followed your rule, you can see after a few dozen trades whether you are truly consistent.
Frequently Asked Questions
Read also: Free lot size calculator · How to set a stop loss · Revenge trading