Position Sizing Formula
Calculate exact position size from account risk, stop distance and leverage. No guessing, no overexposure.
Core Theory
Position sizing translates your dollar risk into a number of contracts or coins. It keeps losses fixed and prevents a single trade from damaging the account.
Position sizing is the bridge between a chart-based idea and an account-based reality. The chart tells you where invalidation sits; your risk rule tells you how many dollars that invalidation is allowed to cost; sizing is the arithmetic that connects the two. Because it is arithmetic, it should never involve judgement, conviction, or how much of a move you expect to catch.
The core formula is unchanging: Position Size = Dollar Risk ÷ Per-Unit Risk. Per-unit risk is the absolute distance between entry and stop. On a $20,000 account risking 2%, dollar risk is $400. If entry is 65,000 and the stop is 63,700, per-unit risk is 1,300, so size is 0.3077 BTC — a notional of roughly $20,000. Change the stop to 61,750 and the same $400 risk permits only 0.123 BTC. The dollar loss is identical in both cases; only the size adapts.
Leverage is the most misunderstood element of this process. Leverage does not change your risk — the stop does that. Leverage only determines how much margin you must post to hold the notional. In the example above, a $20,000 notional on a $20,000 account is 1x if unleveraged, but at 10x leverage requires only $2,000 of margin, leaving the rest free. The risk in both cases is $400, because the stop is in the same place. Traders get destroyed not by leverage itself but by using leverage to justify a notional far larger than their stop distance allows.
Liquidation price deserves separate attention. It must always sit further away than your stop, ideally by a wide margin. If your stop is 2% away and your effective leverage puts liquidation at 1.6%, the exchange closes the position before your thesis is tested — and you lose considerably more than 1R. A practical rule is to keep liquidation at least three times the stop distance away, which usually means using far less leverage than the platform permits.
Costs belong in the calculation. Taker fees on both sides, funding on positions held through funding windows, and slippage during volatile fills can collectively add 0.1-0.3R to every trade. Including a small cost allowance in your dollar risk keeps your realised R-multiples aligned with your planned ones.
Step-by-Step Execution
Work through these steps in order. Each one produces an input the next step depends on, which is what keeps the process repeatable under pressure.
- 1
Take current equity, not peak equity
Sizing from a previous account high keeps risk artificially large during a drawdown. Always recalculate from today's balance so exposure shrinks naturally when performance declines.
- 2
Compute the dollar risk
Multiply equity by your fixed risk percentage. Deduct a small allowance for fees and expected slippage so that a full stop lands at approximately −1R after costs rather than −1.2R.
- 3
Measure per-unit risk from the chart
Take the absolute difference between the planned entry and the structural stop. Use the actual fill assumption, not the ideal limit price, if you intend to enter at market.
- 4
Divide to get units and notional
Units equal dollar risk divided by per-unit risk. Multiply units by entry price to obtain notional exposure, which is the number your exchange interface cares about.
- 5
Select leverage from margin, not appetite
Choose the lowest leverage that lets you hold the required notional comfortably, then confirm the resulting liquidation price sits at least three stop-distances beyond your stop.
- 6
Check portfolio-level exposure
Sum the risk on all open positions, treating correlated assets as one. If adding this trade breaches your aggregate cap, reduce the size or skip the trade entirely.
Key rules
- Decide account risk first: 1-2% of equity is the standard.
- Measure entry price to stop price to find the per-unit risk.
- Position Size = Account Risk ÷ (Entry − Stop) for non-leveraged assets.
- With leverage, divide the raw size by leverage and confirm margin requirements.
Common Pitfalls
These are the failure modes that appear most often in real journals. Recognising them early is usually worth more than learning an additional setup.
Sizing by notional habit
Always buying '$5,000 worth' means your actual risk swings between 0.4% and 6% depending on where the stop sits. Fixed notional is not risk management; fixed risk is.
Confusing leverage with risk
20x leverage on a tiny position with a tight stop can be far safer than 2x on a huge position with a wide one. Only stop distance and size determine risk.
Ignoring liquidation distance
A liquidation price inside your stop distance means the exchange decides your exit, and the loss exceeds the plan. Always verify liquidation after selecting leverage.
Rounding size upward
Rounding 0.307 BTC up to 0.35 quietly increases risk by 14%. Round down; the difference in profit is trivial and the difference in discipline is not.
Forgetting funding costs
On a multi-day perpetual position, funding can consume a meaningful fraction of R. Include it when the intended hold spans several funding windows.
Invalidation levels
- Guessing position size instead of calculating it invalidates risk control.
- Using maximum available leverage regardless of stop distance blows up accounts.
- Ignoring exchange fees and funding costs skews true position risk.
Real-World Examples
Two stops, two sizes, one risk
A trader with $20,000 risking 2% ($400) takes two setups. The first has a tight stop 1.2% below entry, permitting a $33,300 notional. The second has a wide stop 6% below entry, permitting only a $6,660 notional. The second position looks five times smaller on the screen, yet both trades lose exactly $400 if stopped. Position size varies precisely so that risk does not.
The liquidation that beat the stop
A trader opens a 25x leveraged long with a stop 3% away, without checking that liquidation sits 3.4% away. A volatile wick reaches 3.5% before reversing, triggering liquidation and a loss of the entire posted margin instead of the planned 2%. The setup was correct — price hit the target twelve hours later — but the leverage choice removed them from the trade first.
Put this lesson into practice
Upload a chart, set your risk parameters and let the AI analyst apply this exact framework to a live setup — entry, invalidation, targets and position size.
ChartRisk AI is an educational tool only. Nothing here is financial advice.