Fixed-Dollar vs. Fixed-Fractional Sizing — A Fixed Amount or a Fixed Percentage per Trade?

There are two broad ways to size positions: fixed-dollar sizing risks the same amount each time, while fixed-fractional sizing risks the same percentage of the balance each time. Both follow rules, but behave very differently when the account grows or shrinks. The choice is less about which is universally better and more about understanding where each can fail.

Defining the Two Methods

Fixed-dollar sizing specifies a monetary loss per trade. The rule is “lose $40 if the stop is hit,” and that $40 stays unchanged whether the account holds $2,000 or $1,200. Fixed-fractional sizing specifies a percentage: “lose 2% of the current balance if the stop is hit.” The amount changes with the balance. The basic formula for working backward from stop distance to trade size is in position sizing.

Both methods produce the same number at the starting point. They diverge after the balance moves, so they must be compared across a sequence of trades.

Losing Streaks — Fixed Fractional Automatically Shrinks

Start with the losing side. Keep stop distance and the win/loss sequence identical, then apply ten consecutive losses to reveal the difference.

The guide describes a ten-loss streak as a normal possible occurrence across dozens of trades near a 50% win rate, rather than a special accident. Probabilities by streak length are discussed in losing-streak probability. Extending the streak widens the gap: after 20 losses, fixed-dollar sizing leaves $1,200 (−40.0%), while fixed fractional leaves $1,335.23 (−33.2%).

More important than the remaining balance is the remaining risk percentage. Fixing the dollar amount means it automatically consumes a larger share as the account shrinks.

This is the structural weakness of fixed-dollar sizing: it becomes more aggressive as the account deteriorates, exactly when performance is worst. Risk of ruin explains how per-trade risk translates into account failure probability. Drawdown recovery covers the increasing return needed to recover deeper losses.

Winning Streaks — Fixed Fractional Leads Through Compounding

The opposite side works symmetrically. During wins, fixed-fractional sizing calculates from the growing balance, so the amount risked grows too.

Fixed fractional therefore includes automatic adjustment that earns more through wins and loses less through losses in these streak examples. This is why percentage-based sizing is often the starting point in calculations seeking maximum long-term growth. The Kelly criterion derives an optimal fraction from probability and reward-to-risk.

Why Only Fixed Fractional Loses on an Otherwise Break-Even Sequence

Fixed fractional has a cost too. With equal counts of wins and losses and equal gain/loss magnitudes, such as ten wins and ten losses, fixed-dollar sizing returns exactly to break-even while fixed fractional falls short.

This loss arises because percentage gains and losses multiply. Larger swings increase it: ±2% costs 0.04% per win/loss pair, while ±10% gives 0.90 × 1.10 = 0.99, losing 1% per pair. Increasing the fraction can therefore enter a region that reduces expected growth instead of increasing it. Fees raise the actual break-even requirement further; see Profit Factor and expectancy and R multiples for the calculation framework.

Fixed Fractional May Become Unexecutable in a Small Account

Practical constraints can block fixed-fractional sizing regardless of theory. Exchanges have minimum order quantities and notional amounts. An order calculated below those minimums cannot be submitted successfully.

Smaller accounts encounter this constraint more often. Even when fixed fractional is the intended principle, actual small-account operation often approaches fixed-dollar sizing. The essential point is to keep checking what percentage of the balance the fixed amount represents. Otherwise it quietly rises to 5% or 10%, as in the earlier table.

A Compromise: Stepped Sizing with Less Frequent Recalculation

A common practical approach is to calculate from a percentage but not change the amount every trade. Recalculate weekly or after a specified balance change, keeping the dollar amount fixed between recalculations.

Adding an upper and lower bound addresses most weaknesses. The upper rule might be “recalculate immediately if the fixed amount exceeds 3% of the current balance.” The lower rule might be “skip trading if the calculated size falls below the minimum order value.” Combining this with a daily loss cutoff provides multiple safeguards; see daily loss limits.

Whichever method you use, separately count how many positions are open simultaneously. Even with 2% risk per trade, five highly correlated positions can create effective risk closer to 10%. See portfolio heat for the combined calculation.

Which Method Fits Which Situation?

The choice depends on account conditions and execution constraints rather than personal taste.

Without records, even which method you are using becomes unclear. Thinking you use fixed-dollar sizing while increasing size only after wins can combine the disadvantages of both. Recording the risk amount and its percentage of the balance for every trade is covered in the trading journal; broader allocation is covered in capital management.

Summary

Fixed-dollar sizing is easier to calculate but increases relative risk as the account deteriorates; fixed-fractional sizing fixes the risk percentage but introduces multiplicative drag and minimum-order constraints. The guide suggests that many accounts find a balance by calculating fractionally, recalculating less frequently, and adding upper and lower bounds.

Caution

The capital amounts, risk fractions, stop distances, winning and losing streak lengths, and minimum order values are hypothetical examples illustrating the calculation structure of the two sizing methods, not actual measurements from an account or strategy. The consecutive PnL calculations use simplified assumptions of independent trade outcomes and constant gain/loss magnitudes. Actual markets differ because of slippage, fees, funding, and correlation between trades. Minimum order quantities and values vary by exchange and instrument; check actual values with your exchange. No sizing method guarantees future profits or avoidance of losses. Leveraged trading can lose all principal, and investment decisions and their consequences are your responsibility.