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Start Learning → Browse All Articles →Fixed fractional position sizing is a method of deciding how large a position to take by risking a constant percentage of current account equity on every single trade, so the actual amount at risk grows and shrinks automatically as the account balance itself grows and shrinks. It stands in contrast to a second common approach, fixed ratio sizing, which changes position size in discrete steps tied to a defined amount of accumulated profit rather than recalculating a percentage continuously. This piece explains how each method actually works mechanically, why they behave so differently as an account compounds, and how a trader might choose between them.
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Under fixed fractional sizing, a trader first decides on a risk percentage — the share of current account equity willing to be risked on a single trade — and then works backward from that percentage, the current equity figure, and the distance to the stop-loss on the specific setup, to arrive at how many units or contracts to trade. Because the calculation uses current equity rather than a fixed starting balance, the position size recalculates automatically after every trade that changes the account balance.
The percentage itself is the constant; the actual currency amount at risk is the variable that moves with it. This is precisely what gives the method its name and its core behaviour — as the account grows through a run of winning trades, the same percentage now applies to a larger base, so the absolute amount risked on each subsequent trade grows too. The reverse happens during a losing stretch: the same percentage applied to a shrinking base means the absolute risk per trade shrinks automatically, without any manual adjustment required.
Fixed ratio sizing takes a different approach entirely. Rather than recalculating a percentage of current equity on every trade, it increases position size by one discrete unit each time accumulated profit crosses a predefined threshold, and that threshold itself is often designed to increase progressively as more units are added, making each successive step-up require more profit than the last.
This produces a scaling curve that looks quite different from the smooth, continuous recalculation of fixed fractional sizing. Position size under fixed ratio sizing moves in visible steps rather than adjusting incrementally with every single trade, and the size of the step required to reach the next increase is set in advance as part of the framework’s design rather than being derived from a percentage of the account at that moment.
The practical difference between these two approaches becomes most visible over a longer stretch of compounding, whether that compounding is favourable or unfavourable. Fixed fractional sizing scales position size aggressively during a strong run because every winning trade immediately raises the equity base the next trade’s risk percentage is calculated against, which can accelerate gains further during a favourable stretch but also accelerates losses if the strong run reverses before the trader adjusts anything.
Fixed ratio sizing, by contrast, increases size only once a defined profit threshold has actually been banked, which tends to make it more conservative during the early stages of an account’s growth, since a meaningful cushion of realised profit has to accumulate before size increases at all. This generally produces a smoother, less reactive scaling pattern than fixed fractional sizing, at the cost of being slower to capitalise on a strong run once it is underway.
Fixed fractional sizing has a genuinely useful defensive property during a drawdown: because the risk amount is recalculated as a percentage of the shrinking equity base, position size naturally decreases as losses accumulate, which slows the rate of further loss compared with holding a constant absolute size throughout the drawdown. This self-correcting behaviour is one of the main reasons the method is widely used as a starting framework.
Fixed ratio sizing generally does not reduce size symmetrically on the way down unless the framework is explicitly designed to step back down as profit gives back below a threshold. Many practical implementations only step size up on the way up and leave size unchanged during a drawdown until a separate rule intervenes, which means the defensive, size-shrinking property that comes built into fixed fractional sizing has to be added deliberately to a fixed ratio framework if it is wanted at all.
For fixed fractional sizing, the risk percentage chosen has an outsized effect on how the account behaves over time. A percentage set too high produces position sizes that swing dramatically with each trade’s outcome and can erode an account rapidly through a losing streak, since consecutive losses compound against an already-shrinking base. A percentage set conservatively produces a much smoother equity curve at the cost of slower growth during favourable stretches.
For fixed ratio sizing, the equivalent decision is the size of the profit threshold required before size steps up. A threshold set too low increases size aggressively after only a small amount of banked profit, which can behave similarly to an overly high fixed-fractional percentage in terms of the risk it introduces. A threshold set conservatively delays scaling until a genuinely meaningful profit cushion exists, trading responsiveness for durability.
Neither method is inherently superior in every circumstance; each simply encodes a different philosophy about when and how aggressively position size should respond to a changing account balance, and the better fit depends heavily on the trader’s own tolerance for the swings each approach produces.
Both methods answer the question of how much capital to risk, but neither one, on its own, answers the separate question of how far away the stop-loss should sit for a given setup — and that second question is where market volatility enters the calculation directly. A wider stop, appropriate for a more volatile instrument, means fewer units or contracts can be taken for the same percentage or the same absolute risk amount, while a tighter stop on a calmer instrument allows a larger position for that identical risk figure.
This means the practical position size produced by either fixed fractional or fixed ratio sizing is never just a function of the account balance or the accumulated profit; it is always the joint outcome of the sizing rule and the volatility of the specific instrument being traded at that moment. Applying either method without first accounting for how much room the setup genuinely needs before being invalidated produces position sizes that look consistent on paper but carry wildly different real risk from one trade to the next.
This matters more, not less, as volatility regimes shift over time. An instrument’s typical daily range is rarely constant across an extended period; it expands during turbulent stretches and contracts during calmer ones. A sizing framework that does not periodically revisit the volatility assumption it was built around can end up systematically oversized during calmer regimes it was calibrated for and dangerously undersized in terms of dollar risk once volatility expands, or the reverse, depending on when the framework was last reviewed relative to the current regime.
Some traders build hybrid frameworks that borrow properties from both methods — for example, using a fixed fractional calculation as the baseline risk model while overlaying a rule that caps how quickly the absolute risk amount is allowed to grow over a given stretch, which introduces some of fixed ratio’s step-based restraint into an otherwise continuous fixed fractional framework.
Whichever method or combination is used, the actual position size taken on every trade is worth recording alongside the reasoning that produced it — the equity figure or accumulated profit figure used, the stop distance, and the resulting size — rather than only recording the outcome of the trade itself. Without this record, it becomes very difficult to tell, after a stretch of results, whether a run of losses came from the underlying trading approach or from a sizing framework that was drifting away from its own intended parameters. Periodically reviewing this record against the framework’s original design also surfaces a common but easy-to-miss failure mode: a trader who nominally follows fixed fractional sizing but who quietly rounds position size upward after a winning trade out of confidence, or downward after a losing trade out of caution, beyond what the actual percentage calculation calls for. This kind of informal override defeats much of the purpose of adopting a systematic sizing rule in the first place, since the whole point of the rule is to remove exactly this kind of in-the-moment emotional adjustment.
Whatever the specific combination, the underlying principle worth preserving from either pure approach is that position size should never be decided arbitrarily or emotionally on a trade-by-trade basis. A documented, consistently applied sizing rule — whichever family it belongs to — removes one of the more common sources of inconsistency in how a trader manages risk across many trades over time.
Fixed fractional sizing risks a constant percentage of current account equity on every trade, recalculating continuously as the balance changes. Fixed ratio sizing increases position size in discrete steps only after accumulated profit crosses a predefined threshold.
Yes. Because the risk amount is a percentage of current equity, a shrinking account balance during a losing streak automatically produces a smaller absolute risk on the next trade, without any manual adjustment.
It tends to be more conservative in the early stages of an account, since size only increases after a meaningful amount of profit has actually been banked, whereas fixed fractional sizing scales continuously with every change in equity.
Yes. Some frameworks use a fixed fractional baseline while overlaying step-based limits on how quickly absolute risk is allowed to increase, borrowing the restraint of fixed ratio sizing without abandoning the continuous recalculation of fixed fractional sizing.
Fixed fractional sizing is generally considered more transparent for a beginner, since the calculation is the same simple percentage applied every time, whereas fixed ratio frameworks require defining and tracking profit thresholds in advance.
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