A position size calculator answers the single most important question you face before any trade: how many shares can I buy without putting more of my account at risk than I have decided to lose? It does this by converting a fixed-percentage risk budget into a concrete share count, using the distance between your entry price and your stop-loss as the per-share risk. The output is not a guess or a round lot — it is the largest position that still keeps your loss capped at your chosen amount if the stop is hit.
The concept is often called fixed-fractional position sizing, and it is the backbone of nearly every durable risk framework. Instead of betting an arbitrary dollar amount or buying “as many as I can afford,” you decide in advance that no single trade may cost you more than, say, one percent of equity. The calculator then scales the size up when your stop is tight and down when your stop is wide, so that a wrong-side move always costs the same fraction of capital regardless of the instrument or the setup.
Why position sizing decides whether you survive
Most accounts are not destroyed by bad analysis — they are destroyed by good analysis sized recklessly. A trader can be right on direction more than half the time and still go broke if a handful of oversized losers wipe out a long string of correctly sized winners. Fixed-fractional sizing breaks that link by making the dollar consequence of being wrong a constant you choose, not a number that happens to you.
Constant fractional risk also makes your results comparable and your edge measurable. When every trade risks the same 1R, your wins and losses can be expressed in R-multiples, your expectancy becomes a clean number, and a drawdown of “ten losers in a row” means a predictable, survivable dent rather than an account-ending event. You cannot honestly evaluate a strategy until each trade carries the same weight.
Finally, sizing by stop distance imposes discipline at the exact moment emotion is highest. Because a wider stop automatically produces a smaller position, you are no longer tempted to take a sloppy, far-away stop on a full-size position — the math punishes it for you. The calculator turns risk control into an input you set when calm, not a decision you improvise when the trade is moving.
The formula
Shares = (Account × Risk%) ÷ |Entry − Stop|
SharesThe number of shares (or contracts) to trade — the output. Round down to a whole number to stay within your risk budget.
AccountYour total account equity, or the capital base you size against. Use current equity, not the high-water mark.
Risk%The fraction of the account you are willing to lose on this trade, expressed as a decimal (e.g. 1% = 0.01).
EntryThe price at which you plan to enter the position.
StopThe price at which you will exit for a loss — your predetermined stop-loss.
|Entry − Stop|The absolute per-share risk: the dollar distance between entry and stop. The absolute value keeps the result positive for both long and short trades.
Account × Risk% is your dollar risk per trade (your 1R). Dividing by per-share risk gives the share count. The formula ignores commissions, slippage, and gap risk — a stop can fill worse than its price, so treat the output as a ceiling, not a guarantee.
Worked example: a long swing trade
- · Account equity: $50,000
- · Risk per trade: 1% of equity
- · Planned entry: $42.50
- · Stop-loss: $40.75
Dollar risk (Account × Risk%)$50,000 × 0.01 = $500
Per-share risk (|Entry − Stop|)|42.50 − 40.75| = $1.75
Raw shares ($500 ÷ $1.75)285.71 shares
Rounded down285 shares
Actual dollar risk (285 × $1.75)$498.75
Position value (285 × $42.50)$12,112.50
Buying 285 shares caps your loss at $498.75 if the $40.75 stop is hit — right at your 1% budget — even though the position is worth over $12,000.
How to use it
- 01Enter your current account equity in the Account field.
- 02Set the percentage of that account you are willing to risk on the trade (1% is a common starting point).
- 03Enter your planned entry price.
- 04Enter your stop-loss price — the level where the trade idea is wrong.
- 05Read the share count, then round down to a whole number before placing the order.
- 06Sanity-check the position value against your buying power and any concentration limits before you commit.
Common mistakes
Sizing off buying power instead of risk
Buying “as many shares as the account can hold” ignores the stop entirely. One ordinary adverse move on a max-size position can cost a double-digit percentage of equity.
Setting the stop to fit the size
Deciding the share count first and then moving the stop closer to justify it inverts the process. The stop belongs at the price that invalidates the trade; the size adjusts to it, never the reverse.
Forgetting slippage and gaps
The formula assumes you exit exactly at your stop price. Earnings gaps, halts, and fast markets can fill far worse, so a 1% planned risk can become a multiple of that. Size with that tail in mind.
Not resizing as equity changes
Risking a percentage of a stale, larger account number after a drawdown quietly increases your real risk. Recalculate against current equity so the fraction stays constant.
FAQ
What percentage of my account should I risk per trade?
Most professional risk frameworks land between 0.5% and 2% per trade, with 1% the common default. Lower is more conservative and survives longer losing streaks; higher grows the account faster but raises the odds of a deep drawdown. Pick a number you can hold to through a losing run.
Does this work for short positions?
Yes. The absolute value in |Entry − Stop| makes the per-share risk positive whether your stop is above the entry (a short) or below it (a long). The share count is identical for the same dollar distance.
Should I size against the whole account or a subset?
Use the capital base you genuinely trade against. If part of your equity is reserved or held elsewhere, size against the working balance. The key is consistency — use the same definition every time so your risk fraction stays meaningful.
Why round the share count down rather than up?
Rounding down keeps your realized risk at or just under your budget. Rounding up pushes you over the line on every trade, and those small overshoots compound into a meaningfully larger drawdown over hundreds of trades.
How does this relate to leverage and margin?
Position sizing controls risk; margin controls how much of that position your cash funds. A risk-based size can still demand more buying power than you have — especially with a tight stop on a high-priced stock — so always check the resulting position value against your margin limits.
What if my stop is so tight the calculator returns a huge share count?
A very tight stop produces a large share count and a large position value, which can exceed your buying power or breach concentration limits. When that happens, the binding constraint is capital or sanity, not risk — cap the position there and accept that you cannot take full risk on that idea.