Position Size Calculator
Calculate how many shares to buy based on your account size, risk percentage, entry price, and stop price. Keeps every loss within a fixed dollar limit.
Enter your account size, the share of it you’re willing to risk, your entry and your stop. You get the exact number of shares that keeps the loss at that risk if the stop is hit.填入账户规模、你愿意承担的风险比例、入场价与止损价，即可得到——若触发止损、亏损恰好等于该风险时——应买入的确切股数。
Fill in all four fields to size the trade.填入四个字段即可计算仓位。
- Dollar risk风险金额
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- Risk per share每股风险
- Position value仓位价值
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- Position as % of account占账户比例
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The number of shares to buy is your dollar risk divided by your risk per share, rounded down. Dollar risk is the slice of your account you’ll accept losing on this one trade; risk per share is the distance from your entry to your stop. Everything else — the position’s size, what fraction of the account it ties up — falls out of those two numbers.应买入的股数，等于你的风险金额除以每股风险后向下取整。风险金额是你愿意在这一笔交易上承担损失的那部分账户；每股风险是入场价到止损价的距离。其余的一切——仓位大小、占用账户的比例——都由这两个数字决定。
01The formula公式
Risk per share = | Entry − Stop |每股风险 = | 入场 − 止损 |
Shares = floor( Dollar risk ÷ Risk per share )股数 = 向下取整( 风险金额 ÷ 每股风险 )
Position value = Shares × Entry仓位价值 = 股数 × 入场价
The floor matters: you can’t buy a fraction of a share in most accounts, and rounding up would push your loss past the risk you set. Round down and your realized risk sits at or just under target — never over.向下取整很关键：多数账户无法买入零股，而向上取整会让亏损超过你设定的风险。向下取整后，你的实际风险恰好等于或略低于目标——绝不会超出。
02A worked example一个算例
Suppose you hold a $10,000 account and risk 1% per trade. You buy at $50.00 with a stop at $48.00.假设你有一个 $10,000 的账户，每笔风险 1%。你在 $50.00 入场，止损设在 $48.00。
- Dollar risk = 10,000 × 1 ÷ 100 = $100风险金额 = 10,000 × 1 ÷ 100 = $100
- Risk per share = | 50 − 48 | = $2.00每股风险 = | 50 − 48 | = $2.00
- Shares = floor(100 ÷ 2) = 50股数 = 向下取整(100 ÷ 2) = 50
- Position value = 50 × 50 = $2,500 — a quarter of the account, risking $100 to protect it.仓位价值 = 50 × 50 = $2,500——占账户四分之一，用 $100 的风险来守护它。
That $2,500 position sounds large, but the loss is capped at $100 — because the stop is close. Position size and risk are different questions; sizing off risk is what keeps a wide, low-conviction trade from quietly becoming your biggest loss.这个 $2,500 的仓位听起来很大，但亏损被限制在 $100——因为止损很近。仓位大小和风险是两个不同的问题；以风险来定仓位，能防止一笔宽止损、低把握的交易悄悄变成你最大的亏损。
03The common trap常见陷阱
The most common mistake is deciding “I’ll put $2,000 into this” and setting a stop afterward. Now your loss depends on where the stop lands — a tight stop and a loose one give wildly different risk on the same $2,000. Size from the stop instead: pick the risk, pick the stop, and let the share count follow. A wider stop simply means fewer shares, not more risk.最常见的错误是先决定「我要投 $2,000」，再设止损。此时你的亏损取决于止损设在哪里——同样的 $2,000，紧止损和松止损带来的风险天差地别。反过来从止损定仓：先定风险、再定止损，让股数随之而定。更宽的止损只意味着更少的股数，而非更多的风险。
04Where this breaks它的局限
This math sizes a single trade cleanly. It does not know about the rest of your book. A few honest limits:这套算法能干净地为单笔交易定仓，但它并不了解你账簿上的其余部分。几点诚实的局限：
- Correlated positions. Ten trades each risking 1% aren’t 1% of risk if they all move together — a sector selloff can hit them at once. Cap total open risk, not just per-trade risk.相关持仓。十笔各承担 1% 风险的交易，若齐涨齐跌，就不是 1% 的风险——一次板块抛售可能同时击中它们。要限制总的敞口风险，而不只是单笔风险。
- Gaps and slippage. A stop is a request, not a guarantee. Overnight gaps and thin liquidity can fill you well below the stop, so the real loss can exceed the planned risk.跳空与滑点。止损是一个请求，而非保证。隔夜跳空和流动性不足可能让你的成交价远低于止损，实际亏损因此可能超过计划风险。
- Position larger than the account. A very tight stop can call for a position worth more than your cash — that needs margin, with its own risks. This tool flags it rather than pretending it away.仓位大于账户。极紧的止损可能要求一个价值超过你现金的仓位——那需要保证金，并带来自身的风险。本工具会提示这一点，而非假装无事。
Decide what you’re willing to lose before you decide how much to buy. Position size is an output, not an input.在决定买多少之前，先决定你愿意亏多少。仓位是结果，不是起点。
Go deeper in the lesson on 在这节课中深入了解 position sizing仓位管理, or turn a stop into a target with the ，或用 risk-to-reward calculator盈亏比计算器.把止损换算成目标。