How to Size a Position (Risk-Based Position Sizing)
A practical guide to position sizing: how to decide how many shares or units to buy based on your account size, risk per trade and stop loss.
Educational content, not financial advice. Trading involves risk — always verify figures and consult a professional before trading.
Most new traders pick a position size by gut feel — “I’ll buy 100 shares” — and then set a stop wherever looks reasonable. That gets the process backwards. Professional sizing starts from one decision: how much money am I willing to lose if this trade is wrong? Everything else follows from that.
The one rule that matters
Decide your risk per trade as a percentage of your account before you think about share count. A common range is 0.5% to 2%. Risking 1% means a string of ten losers costs you roughly 10% — survivable. Risking 10% per trade means three bad calls can wreck the account.
The formula
Once you know your dollar risk and your stop distance, the share count is fixed:
Shares = (Account × Risk%) ÷ (Entry − Stop)
The denominator is your per-share risk — how much you lose per share if the stop is hit. Divide your total risk budget by it, and you get the largest position that keeps the loss inside your limit.
A worked example
- Account: $10,000
- Risk per trade: 1% → $100 at risk
- Entry: $100, stop-loss: $95 → $5 risk per share
Shares = $100 ÷ $5 = 20 shares. That’s a $2,000 position. If the stop hits, you lose 20 × $5 = $100 — exactly your 1%. The Position Size Calculator does this live, and also accounts for fees, fractional crypto units, and futures multipliers.
Tighter stops let you buy more (and vice versa)
Per-share risk is the lever. With the same $100 budget:
| Entry | Stop | Risk/share | Shares | Position value |
|---|---|---|---|---|
| $100 | $95 | $5 | 20 | $2,000 |
| $100 | $98 | $2 | 50 | $5,000 |
| $100 | $90 | $10 | 10 | $1,000 |
A tighter stop means a bigger position for the same dollar risk — but tighter stops also get hit more often by normal noise. That trade-off is the heart of sizing.
Why fixed-share sizing fails
If you always buy “100 shares,” your dollar risk swings wildly: a $5 stop risks $500, a $1 stop risks $100. Your account’s fate then depends on accidental stop placement rather than a deliberate plan. Risk-based sizing keeps every loss the same size, so your results reflect your edge, not your position-size luck.
Think in R, not dollars
Once you size by risk, it’s powerful to measure outcomes in R — multiples of the amount you risked — rather than raw dollars. If you risk $100 (1R) and make $250, that’s a +2.5R trade. This normalizes everything: a win on a small account and a win on a large one are the same “+2R” if both risked 1R.
Thinking in R has two benefits. It makes results comparable across position sizes, and it stops you fixating on dollar amounts (which tempts you to oversize when you feel confident). A trading log in R tells you your real edge: average R per trade is just another name for expectancy.
Why small risk per trade matters: the drawdown math
Losses compound against you, and recovering gets disproportionately harder the deeper you fall. Here’s how much gain you need just to get back to even:
| Drawdown | Gain needed to recover |
|---|---|
| −10% | +11% |
| −25% | +33% |
| −50% | +100% |
| −75% | +300% |
Risking 1% per trade, a brutal 10-loss streak costs ~10% — recoverable with an 11% gain. Risking 10% per trade, the same streak is catastrophic. Small, consistent risk isn’t timidity — it’s what keeps you in the game long enough for your edge to play out.
Scaling in and out
Real positions aren’t always all-at-once. Two common approaches:
- Scaling in: enter part of the position, add more if it confirms. Size each tranche so the combined risk (to a single stop) still respects your limit — don’t let three “small” adds quietly become a triple-size position. The Average Down / Up Calculator gives your blended entry so you can recompute risk against the real average.
- Scaling out: take partial profit at a first target, trail the rest. This locks in R while leaving runners — but it lowers your average win, so make sure the math still clears positive expectancy.
How many positions at once?
Per-trade risk is only half the picture — open positions add up. If you risk 1% on each of eight trades, you have 8% of the account exposed at once, and if those trades are correlated (all the same sector, or all long crypto), a single bad day can hit them together as if they were one oversized position. Cap your total open risk — a common ceiling is 4–6% across everything — and treat correlated trades as closer to a single risk unit. Sizing each trade with the Position Size Calculator and summing the ”$ at risk” figures gives you that running total at a glance.
Don’t forget fees and gaps
- Fees eat into the risk budget. The calculator subtracts them so your true risk stays within the percentage you chose. On small or frequent trades they matter more than traders expect.
- Gaps can blow through a stop (especially overnight in stocks, or on crypto weekends). Size as if the stop will mostly hold, but never bet the account on any single trade holding perfectly. If a name is gap-prone (earnings, low float), size down further.
Key takeaways
- Choose risk % first (0.5–2%), then derive share count — never the reverse.
- Shares = (Account × Risk%) ÷ per-share risk.
- Tighter stops allow larger positions at equal dollar risk, but get hit more.
- Fixed-share sizing makes your risk random; percent-risk sizing makes it deliberate.
Next steps
- Size your next trade with the Position Size Calculator.
- Check the trade is worth taking with the Risk/Reward Calculator.
- Set precise levels with the Stop Loss / Take Profit Calculator.