Position size calculator.

Position sizing starts from the loss you can accept, not from how much you want to buy. Decide how much of your account one trade may lose, find the distance between your entry and your stop, and the share count follows. This calculator does that fixed-fractional arithmetic, shows how big the resulting position is compared with your account, and tells you when it would need margin.

How this is calculated

Dollars at risk equal account size times the percent you choose. Risk per share is the distance from entry to stop (stop below entry for a long, above entry for a short) plus the slippage and fees per share you enter. Shares equal dollars at risk divided by risk per share, rounded down to whole shares. Position value is shares times the entry price. If you give a target, reward to risk is the target distance, less slippage, divided by risk per share. This is a standard formula, not a legal rule: the percent you risk is a personal choice.

Two regulatory facts frame the result. Regulation T (12 CFR 220.12) sets 50 percent of current market value as the required margin for a margin equity security, so a position larger than your cash needs about half of its value in your own equity. And as of 2026-10-26, the SEC's Investor.gov page on pattern day traders says FINRA adopted new intraday margin requirements, effective June 4, 2026, that replace the current day trading margin requirements including those for pattern day traders, with a transition period until October 20, 2027 for brokerage firms that need more time. Your broker may still operate under the old rules for now, so ask which applies to your account.

Worked examples (hypothetical figures, real arithmetic)

A long trade. With a $50,000 account risking 1%, you can lose $500.00. You buy at $50 with a stop at $47.50 and allow $0.05 of slippage and fees per share, so risk per share is $2.50 + $0.05 = $2.55. Shares: $500 / $2.55 rounds down to 196. The position is worth $9,800.00, 19.6% of the account, and the real risk is $499.80. With a $57.50 target the reward to risk is 2.92 to 1 and the gain at target is $1,460.20.

A short trade. Shorting at $50 with a stop at $52.50 and no slippage risks $2.50 a share, so $500 of risk allows 200 shares, worth $10,000.00.

A tight stop on a small account. With $10,000 risking 2% ($200), a $100 stock with a $1 stop gives 200 shares, a position of $20,000.00, which is 200% of the account. The calculator flags that this would need margin: Regulation T would call for about $10,000.00 of your own money.

Mistakes people make

• Sizing from the amount you want to invest. Start from the loss you accept and let the stop set the size.
• Setting a stop too tight to get a big position. A stop inside normal price noise gets hit often; the size is only as good as the stop.
• Ignoring gaps. A price can open beyond your stop, so the real loss can exceed the planned risk.
• Forgetting margin. A position larger than your cash is borrowed money with interest and margin calls.

Who this is for, and what it does not cover

It is for sizing a single stock or ETF trade with a defined stop. It does not size options or futures contracts, correlate several positions, or tell you where to put the stop. It is arithmetic for risk control, not a recommendation to trade.

Sources

Retrieved 2026-10-26. Each link was opened and checked against the claim next to it.

• 12 CFR 220.12, Regulation T margin requirements: 50 percent of current market value required for a margin equity security.
• Investor.gov: Pattern Day Trader: FINRA’s new intraday margin requirements effective June 4, 2026, replacing day trading margin requirements, with a transition period until October 20, 2027.