Compounding vs Fixed Lots: Which Sizing Model Fits Your EA?

The direct answer: fixed lots keep every trade the same size regardless of account balance, while compounding (percent-risk) sizing scales positions with equity - growing them as the account grows and, crucially, shrinking them in drawdown. Neither is universally better. Fixed lots suit evaluation and comparison; compounding suits long-run accounts that can tolerate equity-dependent swings. The right choice depends on what you're using the EA for.

How each model works

• Fixed lots: the EA always trades, say, 0.10 lots. Monetary risk per trade is constant if the stop distance is constant, but as a percentage of the account it drifts: it shrinks as you grow and - dangerously - grows as you lose.
• Percent-risk compounding: the EA risks a fixed fraction of current equity per trade, say 1%, and derives the lot size from the stop distance - the calculation covered in how to calculate position size. Monetary size floats; percentage risk stays constant.

An illustrative example

Illustrative numbers only. Two accounts start at £10,000 and each takes the same sequence: five losses, then five wins, all with equal stop and target distances.

• Fixed lots, risking £100 per trade: five losses take the account to £9,500; five wins return it to exactly £10,000. Symmetric, linear, easy to reason about.
• Compounding at 1%: five losses compound to about £9,510 (0.99⁵). The five wins then compound from the lower base to roughly £9,995. Slightly less lost in the streak - each losing trade was smaller than the last - but a touch slower to recover, because wins are earned on a smaller base.

Over short sequences the difference is pennies. Over hundreds of trades it becomes structural: compounding cushions drawdowns and accelerates growth phases, at the price of path-dependence - your outcome depends on the order of wins and losses, not just their count. The cushioning matters because of the asymmetry in drawdown recovery maths: sizing down in a hole keeps the hole shallower.

When fixed lots make sense

1. Evaluating a new system. A fixed-lot equity curve shows the raw edge without sizing effects distorting it - useful while you accumulate a fair sample of trades.
2. Small accounts near minimum lot sizes. If 1% risk computes to less than the broker's minimum lot, percent-risk sizing silently becomes oversizing. Fixed minimum lots at least make the risk explicit.
3. Comparing systems. Identical sizing isolates strategy quality from money management.

When compounding makes sense

1. Long-run deployment. Percent-risk keeps risk proportionate through both growth and drawdown, which is the practical implementation of the 1% rule.
2. Ruin control. Because size shrinks with equity, percent-risk sizing can't hit zero in any finite number of losses - a mathematically friendlier profile, as the Kelly criterion framework makes precise. (Small print: real accounts still face minimum lots, margin floors and gaps.)

Common mistakes

• Confusing compounding with martingale. Percent-risk sizing reduces size after losses. Anything that increases size after losses is a different - and far more dangerous - animal.
• Backtesting compounded, judging fixed. A compounded backtest's headline return is dominated by late-period size. Check the fixed-lot curve to see whether the edge itself was stable.
• Compounding too aggressively. Percentage risk in the exponent cuts both ways - see Kelly and volatility scaling for why practitioners size well below the theoretical optimum.

Systems built with Nebula enforce hard per-trade risk caps whichever sizing mode you deploy, so the choice between fixed and compounding becomes a question of preference and account context - not a loophole through which risk quietly escalates.

Trading carries a high level of risk. Nothing here is financial advice; past performance does not predict future results.