Fixed Fractional Position Size Calculator

Fixed-fractional position sizing, popularized by Ralph Vince in 'The Mathematics of Money Management' (1992), means risking a constant fraction f of your current account equity on every trade. If f = 2% and your account is $10,000, you risk $200 on the next trade. After a loss the account shrinks, so the next dollar risk shrinks too; after a gain it grows. This compounding effect makes the method geometrically optimal at the right f and prevents ruin in the way a fixed dollar amount never can.

Fixed-dollar sizing risks the same dollar amount every trade (e.g., always $200), regardless of whether your account has grown or shrunk. Fixed-fractional sizing risks the same percentage every trade (e.g., always 2% of current equity). After a series of losses, fixed-dollar sizing keeps the dollar risk constant and can wipe out a small account; fixed-fractional sizing automatically reduces the dollar risk as the account shrinks, making it mathematically impossible to lose more than 100% of equity as long as f < 100%.

Optimal f is the fraction that maximises the geometric growth rate of a trading system given its historical win rate and payoff distribution. It was derived by Ralph Vince and is related to the Kelly Criterion. In theory optimal f maximises compounding; in practice it is often aggressively high (sometimes 20–50% of equity per trade) and produces drawdowns most traders cannot stomach. Van K. Tharp and other practitioners recommend capping f at 1–2% for safety. This calculator lets you enter any f; using 1–2% is the practitioner consensus.

Step 1: Multiply your account size by the fixed fraction f to get your dollar risk — e.g., $10,000 × 2% = $200. Step 2: Find the risk per share by taking the absolute difference between your entry price and stop-loss — e.g., |$50 − $48.50| = $1.50. Step 3: Divide the dollar risk by the risk per share and floor the result — e.g., floor($200 / $1.50) = floor(133.33) = 133 shares.

Rounding up can push the actual loss above your risk budget. With 133.33 theoretical shares at $1.50 risk per share, rounding up to 134 gives a maximum loss of $201 — $1 over the $200 budget. Flooring to 133 gives $199.50, which is within budget. The floor rule guarantees the realized loss never exceeds your intended dollar risk.

Yes. The risk per share is always the absolute value |entry − stop|, which is direction-agnostic. For a short trade the stop is above the entry (e.g., short at $50, stop at $51.50 → $1.50 risk per share). The rest of the formula is identical. The direction field in the calculator is informational — it labels the trade type but does not change the arithmetic.

Most professional risk frameworks recommend no more than 1–2% of equity per trade. At 1% a ten-trade losing streak reduces the account by about 10%; at 2% the same streak reduces it by about 18% (due to compounding). At 5% a ten-trade losing streak reduces it by about 40%. Higher fractions maximise short-run growth but produce severe drawdowns. Van K. Tharp, Alexander Elder, and most trading textbooks set 2% as the upper safe limit for individual retail traders.

This occurs when the stop-loss is very tight relative to the entry price. The risk math is still valid — you are correctly sizing the number of shares to limit the loss to your dollar-risk budget — but in a cash account you cannot deploy more than your balance. Options include: widening the stop to reduce the share count, accepting a smaller position capped at floor(accountSize / entryPrice), or using a margin account where the broker extends buying power. The calculator flags when capitalRequired exceeds accountSize so you can act accordingly.

Because the dollar amount risked each trade is a percentage of the remaining account, the account can never reach exactly zero — each successive loss takes a smaller and smaller absolute bite. For example, at 2% risk per trade: after 10 straight losses the account is $10,000 × 0.98^10 ≈ $8,171 (not zero). This is the mathematical ruin-prevention property of the fixed-fractional method, and it is why it is preferred over fixed-dollar sizing for long-run account preservation.

Yes. The formula — shares = floor(riskAmount / riskPerShare) — applies to any instrument where you can express the entry and stop as a price per unit. For stocks and ETFs use price per share. For crypto express both in USD (or your base currency) per coin. For forex you would express riskPerShare as the loss per standard lot (pip value × stop pips). For futures use the contract's point value × stop points as riskPerShare and set 'shares' as number of contracts. The flooring rule may be relaxed to the instrument's minimum tradeable increment.

They are related but different. The Kelly Criterion maximises the expected log of wealth based on win probability and win/loss payoff ratio; it says nothing about stop-loss prices. Fixed-fractional sizing, as formulated by Vince, determines position size from the stop distance and a chosen fraction f. Vince showed that optimal f is mathematically equivalent to the Kelly fraction when the payoff distribution is binary (win W or lose L), but diverges for multi-outcome distributions. In practice both methods agree on the principle of risking a percentage of equity per trade, though they differ in how that percentage is computed.

'R' (risk unit or R-multiple) is the fixed dollar amount you plan to lose if the stop is triggered: R = accountSize × f / 100. Every trade is either a loss of 1R (stop hit), a break-even, or a gain expressed as a multiple of R (e.g., 2R means the profit is twice the initial risk). Using fixed-fractional sizing keeps R proportional to your account, so a string of 2R wins genuinely compounds the account, and a string of 1R losses self-corrects by shrinking each subsequent R.