Position size calculator
Enter your account, the percentage you are willing to risk, your entry, and your stop. It tells you the size that keeps a loss to exactly that amount.
What this actually does
Position sizing is the one decision that turns a losing streak from an inconvenience into a blown account. The calculation is simple, but almost nobody does it in the moment, because the market is moving and the order ticket is open.
The formula is:
size = (account × risk%) ÷ (entry − stop)
That is it. Everything else on this page is convenience: rounding down so you never round yourself into extra risk, showing what the position actually costs, and flagging when your stop is so wide that the trade is not sizeable at your risk budget.
Why size before you enter, not after
The common mistake is picking a size first — a round number of shares that feels right — and then placing a stop wherever the chart suggests. That means your risk is whatever the chart happened to give you that day. Some trades risk 0.4%, some risk 3%, and your results become noise.
Fixing the risk first and letting size follow does the opposite. Every trade costs the same when it fails, so the only thing that varies between trades is whether the idea was good.
Planned risk vs actual risk
This calculator gives you planned risk. Actual risk is what you really lost, and the two drift apart through slippage, gaps, moving a stop, or adding to a loser. That gap is where most account damage happens, and it is invisible unless you record both numbers per trade.
A worked example
- Account: $10,000
- Risk per trade: 1% = $100
- Entry: $100.00, stop: $98.00 → $2.00 of risk per share
- Size: $100 ÷ $2.00 = 50 shares
- Position cost: $5,000 — half the account, risking 1% of it
Note the last line. A 1% risk does not mean a 1% position. Tight stops produce large positions, and that surprises people the first time they size properly.
Questions
How do I calculate position size?
Divide the cash you are willing to lose by the distance between your entry and your stop. If you will risk $100 and your stop is $2 away, you buy 50 shares.
What percentage should I risk per trade?
Most professional and funded traders risk 0.5% to 1% of the account on a single trade. Prop firm rules often force this, because a 1% risk means a 10-trade losing streak costs about 10% rather than the account.
Does this work for options and futures?
Yes. Set the contract multiplier: 100 for standard US equity options, or the contract size for a futures product. The maths is the same, the multiplier just scales the per-unit risk.