For a linear instrument, start with raw quantity = usable loss budget ÷ estimated loss per unit at the exit. Estimated loss per unit must include the distance from entry to the planned exit, the instrument multiplier and account-currency conversion, plus a defensible allowance for spread, commissions, slippage, and funding where they apply. Round down to the venue’s valid quantity increment, then recompute the worst planned loss. A stop trigger is not a guaranteed fill.
The Position Sizing Formula (Plain English)
The calculation is not “How large a trade can my buying power open?” It is “What quantity keeps the loss inside a budget if the trade exits under the scenario I have defined?” Place the technical or thesis invalidation first. Moving the stop closer merely to obtain a larger quantity changes the trade rather than solving the sizing problem. For a deeper invalidation workflow, see the stop-loss placement methods.
| Variable | What to enter | Failure to avoid |
|---|---|---|
| Loss budget | The maximum planned account-currency loss for this trade after considering portfolio and rule limits | Treating available margin or the firm’s entire loss limit as a per-trade budget |
| Stop distance | Absolute distance between the expected entry basis and the planned exit trigger or conservative exit scenario | Choosing the quantity first and moving the stop to make it fit |
| Value per price unit | Account-currency P&L for one unit, share, coin, lot, or contract per pip, point, tick, or price unit | Using a multiplier for a different contract, quote currency, or settlement type |
| Cost reserve | A documented allowance for applicable spread, commissions, fees, slippage, funding, borrow, and currency conversion | Adding spread twice or pretending the stop must fill at its trigger |
If the budget itself comes from a percentage, write down the denominator: current cash balance, current equity/NAV, or a smaller rule-aware capital base. Then calculate loss budget = declared denominator × chosen risk fraction. There is no evidence-based percentage that is automatically correct for every strategy, account, market, holding period, or trader. The risk-management framework shows how drawdown tolerance, loss clustering, open exposure, and strategy evidence constrain that choice.
Round down, then verify twice
- Round down to a tradable increment. Shares may be whole or fractional, spot FX may use units or lots, and futures contracts are integers. The venue decides the valid increment.
- Recalculate the planned stop loss. Multiply the rounded quantity by loss per unit and add the fixed reserve.
- Check margin and liquidation separately. A quantity may fit the stop-loss budget but still fail an initial-margin, maintenance-margin, concentration, or program contract limit.
- Check portfolio risk. Add open positions and pending orders that can lose together; do not grant each correlated trade the full standalone budget.
Calculate, round, and verify before sending the order
Enter the account basis, chosen loss budget, entry, stop, instrument value, and costs. Save the inputs so the planned risk can be compared with the actual fills later.
Position Size vs. Order Size vs. Lot Size
Order size is the quantity submitted in one order. Position size is the net quantity actually open after fills, partial fills, reductions, and reversals. Lot size is a venue or product convention used to express quantity. They can match at entry, but they are not interchangeable.
| Term | Question it answers | What changes it |
|---|---|---|
| Order size | How much am I asking the venue to buy or sell now? | Amendments, rejects, cancels, and partial execution |
| Open position size | What net exposure is actually open? | Every fill, reduction, add, assignment, and reversal |
| Lot or contract size | What quantity convention and multiplier does this product use? | Instrument specification and broker/venue implementation |
| Average entry | What is the weighted entry basis of the filled position? | Fill prices and quantities; it is not an average of position-size numbers |
When an order fills in pieces, recompute the remaining risk from the actual weighted entry, current open quantity, exit order state, and fees. A calculator based only on the requested order can understate risk after a partial fill or overstate it after a reduction.
Example 1: Forex Position Size in Units and Lots
This is constructed arithmetic for a USD account and a linear EUR/USD trade, not a quote, recommendation, or broker promise. Suppose the declared account basis is $10,000, the chosen risk fraction is 0.75%, the planned budget is therefore $75, and $3 is reserved for all-in costs and adverse execution. A long entry basis of 1.0800 and stop trigger of 1.0770 are 30 pips apart.
| Step | Calculation | Result |
|---|---|---|
| Spendable stop budget | $75 planned loss − $3 reserve | $72 |
| Loss per EUR unit | 30 pips × $0.0001 per pip per unit | $0.003 |
| Raw quantity | $72 ÷ $0.003 | 24,000 EUR units |
| Lot expression | 24,000 ÷ 100,000 units per standard lot | 0.24 standard lot |
| Verification | 24,000 × $0.003 + $3 | $75 planned loss |
The $0.0001 pip value per EUR unit works here because USD is both the quote currency and the account currency. It is not a universal pip-value table. With a different account currency or cross-pair, convert the quote-currency P&L into the account currency using a declared rate and side. Broker rounding, tiered margin, financing, and execution policies still need a platform check.
OANDA documents both unit-based order sizing and margin calculations in the account’s home currency. That supports the method, but it does not prove that every broker accepts one-unit increments or applies the same conversion side. Use the forex lot-size calculator, then reconcile its result with the exact ticket preview.
Example 2: Linear Crypto Position Size and Leverage
This deliberately uses a fictional linear USD/USDT-settled contract called XYZ so no number can be mistaken for a live token price. Assume $5,000 of account equity, a chosen 0.8% planned-loss fraction, a $40 loss budget, and a $4 cost/slippage reserve. The constructed entry is $100 and the stop trigger is $96, so the price distance is $4 per coin.
| Step | Calculation | Result |
|---|---|---|
| Spendable stop budget | $40 − $4 reserve | $36 |
| Raw quantity | $36 ÷ $4 per coin | 9 XYZ |
| Constructed notional | 9 × $100 | $900 |
| Stop verification | 9 × $4 + $4 | $40 planned loss |
| Illustrative initial margin at 5× | $900 ÷ 5, before venue-specific additions | about $180 |
For the same nine-unit position and the same two execution prices, the linear price-difference P&L is unchanged by the leverage setting. That narrow statement does not mean leverage leaves total risk unchanged. Leverage permits more notional exposure, reduces margin headroom, and introduces venue-specific maintenance-margin and liquidation behavior. Fees, funding, mark-price rules, and a liquidation before the stop can change the realized outcome. Inverse or coin-margined contracts also use a different P&L formula; use the exact contract specification instead of this linear shortcut.
The CFTC warns that leverage amplifies gains and losses in virtual-currency derivatives. Treat the exchange’s liquidation estimate as a separate constraint, never as proof that the stop will execute first. This guide does not substitute for the venue’s contract specification or risk disclosure.
Example 3: Stock Position Size With a Cost Reserve
This is a fictional XYZ stock example, not a live security quote. Assume $25,000 of current equity, a chosen 0.6% planned-loss fraction, a $150 budget, and a $6 reserve for costs and execution variance. The constructed long entry basis is $50.00 and the stop trigger is $47.60, a $2.40 distance per share.
| Step | Calculation | Result |
|---|---|---|
| Spendable stop budget | $150 − $6 reserve | $144 |
| Raw shares | $144 ÷ $2.40 | 60 shares |
| Constructed position value | 60 × $50.00 | $3,000 |
| Stop verification | 60 × $2.40 + $6 | $150 planned loss |
The $3,000 position value is not the same as the $150 planned loss. Nor is $150 a guaranteed maximum. Investor.gov explains that a stop order becomes a market order when triggered and may execute materially away from the stop price. A stop-limit controls the acceptable price but may not execute at all. Overnight gaps, halts, thin liquidity, and partial fills belong in scenario analysis rather than being hidden by the formula.
Futures Need the Exact Contract Multiplier and Whole-Contract Rounding
The formula stays recognizable, but the unit is one contract and the multiplier is product-specific. CME lists the E-mini S&P 500 multiplier at fifty dollars per index point and the Micro E-mini S&P 500 multiplier at five dollars per point; E-mini Nasdaq-100 and Micro E-mini Nasdaq-100 use different multipliers again. Never infer a multiplier from a similar ticker.
| Constructed input | ES result | MES result |
|---|---|---|
| Loss budget before costs | $120 | $120 |
| Stop distance | 6 index points | 6 index points |
| Loss per contract at stop | 6 × $50 = $300 | 6 × $5 = $30 |
| Raw contract count | 0.4; cannot round up inside the budget | 4 before costs |
| Executable decision | Skip or use a smaller product | Reduce below four if commissions/slippage are not reserved elsewhere |
Whole-contract rounding is why “risk percentage” alone cannot determine an executable order. The smallest valid unit may exceed the budget. Skipping the trade is a valid output; rounding up is a new risk decision.
How the Same Lot Size Turns 1% Into 3%
| Constructed EUR/USD trade | Quantity | Stop | Price-distance loss before costs |
|---|---|---|---|
| A | 1 standard lot | 10 pips | $100, or 1% of a $10,000 account |
| B | 1 standard lot | 30 pips | $300, or 3% of the same account |
The arithmetic is exact only for the stated USD-account, USD-quote, one-standard-lot scenario and excludes costs and slippage. The lesson is narrower than the old page claimed: a fixed quantity produces different planned loss when stop distance or unit value changes. It is not evidence that fixed lot size is the universal leading cause of drawdowns.
Spread, Fees, Slippage, Gaps, and Partial Fills
Do not apply a universal “stop plus spread” rule. For a long trade, the planned entry may execute at the ask while the sell stop ultimately executes against available bids; for a short, the sides reverse. Depending on how the platform reports entry and stop prices, adding the quoted spread again can double-count it. Model the actual order sides and use the broker or venue’s fee schedule.
| Adjustment | Sizing treatment | Evidence to retain |
|---|---|---|
| Spread | Use expected entry and exit sides; do not add it twice | Bid/ask snapshot, order side, and platform convention |
| Commission or fee | Subtract fixed cost first or include a per-unit rate in the denominator | Exact tier, venue, currency, maker/taker or round-turn rule |
| Slippage | Use a declared adverse scenario informed by comparable executions, not a promise | Trigger, actual fill, depth/liquidity state, and event context |
| Gap or halt | Run a larger discontinuous-loss scenario; a normal stop-distance formula is insufficient | Session boundary, news/event flag, halt state, and next eligible fill |
| Partial fill | Recompute open quantity, average entry, remaining order state, and exit coverage | Every order and fill ID with timestamp and quantity |
How Do I Size Variable Stops Inside Prop-Firm Drawdown Limits?
The FTMO 2-Step and Topstep Trading Combine rows above matter because their rules change the calculation. They share a global $100K first evaluation stage, but not a market or loss-floor model. Use the current values and official links before applying either worked method.
- Translate each rule into a current breach floor. Use the program’s exact balance/equity definition, included costs, reset clock, and trailing/static method.
- Measure capacity now. Compare current rule equity with each applicable floor. Include realized session P&L and open P&L exactly as the program defines them.
- Reserve simultaneous exposure. Subtract loss scenarios for open positions and pending entries that could be active together.
- Choose an internal buffer. Slippage, commissions, data latency, liquidation behavior, and rejected exits mean the breach limit itself is not a sensible target.
- Apply the instrument formula. Use the smallest remaining budget, the actual variable stop, and the correct multiplier; round down to a valid size.
FTMO’s published 2-Step loss logic includes open positions, commissions, and swaps and uses a daily reset boundary, while its maximum-loss floor is static. Topstep’s Maximum Loss Limit is monitored against unrealized and realized P&L and trails from end-of-day balance before locking; its Daily Loss Limit is a separate optional checkout control for the current Trading Combine offer. Dividing a headline daily limit by a guessed number of trades ignores those differences and can overstate safe size.
The detailed prop-firm position-sizing guide covers the program workflow. For a current scenario, use the tracked calculator below and reconcile its inputs with the exact program card instead of copying an old rule amount from prose.
Model the rule floor before sizing the order
Use current program inputs, open risk, planned trades, and a declared safety buffer. A calculator is a scenario tool; the firm’s official rule page remains controlling.
One Correct Position Size Can Still Create the Wrong Portfolio Risk
Standalone sizing assumes only one loss scenario matters. In practice, several trades may share an index, currency, sector, event, liquidity window, or stop level. Five individually acceptable positions can behave like one concentrated position when the common driver moves.
Define a joint scenario before entry: which open and pending positions can lose together, how much of each stop is realistically executable, and which exposures offset only under normal conditions. The trade-correlation risk guide explains why pairwise historical correlation alone is not a guarantee during stress.
A Reproducible Position-Size Worksheet
| Field | Store before entry | Verify after execution |
|---|---|---|
| Account basis | Balance/equity/NAV or exact rule-aware base, timestamp, currency | No silent denominator change |
| Loss budget | Chosen fraction or amount plus portfolio/program cap | Planned budget preserved as the frozen R denominator |
| Instrument | Exact symbol, venue, contract, multiplier, settlement and quantity increment | Filled instrument matches the specification used |
| Entry and exit plan | Expected entry side/basis, stop trigger, order type, gap scenario | Trigger, activation, fills, rejects and partials retained separately |
| Costs | Commission/fee tier, spread handling, slippage and financing reserve | Actual costs reconcile to source statement |
| Quantity | Raw result, rounding rule, final executable size | Open/closed quantities and average basis reconcile |
| Constraints | Margin, liquidation, concentration, contract and prop-rule headroom | No limit was assumed from stale text |
Test the worksheet on historical or simulated trades before making it a live rule. The backtest workflow shows how to freeze a rule and evaluate it on untouched data rather than tuning size after seeing outcomes.
Trader’s Second Brain is our product. The useful role here is not to choose a risk percentage for you. It is to preserve planned size, frozen risk, source fills, fees, exit evidence, and realized R so the calculation can be audited after execution. TSB cannot guarantee a stop fill or infer every venue’s contract rules; verify the exact source and program first. If that workflow fits, open TSB and record the next sizing decision.
Sources and Verification Method
The formula examples are constructed arithmetic, not performance evidence or individualized advice. We rechecked the regulatory order and leverage warnings, contract multipliers and linked prop rules against current official sources on September 22, 2026. A published specification does not guarantee that an order will fill at the planned price.
- Investor.gov: stop, stop-limit, and trailing-stop order behavior
- CFTC: retail forex margin and leverage risks
- CFTC: virtual-currency trading and leveraged derivatives risk
- CME Group: E-mini and Micro E-mini multipliers and ticks
- OANDA: unit sizing, home-currency margin conversion, and stop limitations
- FTMO: current Trading Objectives and loss-floor definitions
- Topstep: current Maximum Loss Limit method
- Topstep: current optional Daily Loss Limit scope
