Compounding vs fixed position sizing
There are two basic ways to size every trade: risk a fixed percentage of your current equity (compounding) or risk a fixed dollar amount (fixed sizing). The choice quietly shapes your entire equity curve — how fast you grow, how deep your drawdowns feel, and how disciplined you have to be. Here is the honest trade-off, with no hype.
The two methods side by side
| Percent-of-equity | Fixed dollar | |
|---|---|---|
| Risk per trade | e.g. 1% of current equity | e.g. always $200 |
| Growth | Geometric (compounds) | Arithmetic (linear) |
| In drawdowns | Auto-reduces size — protective | Same size — slower to recover % |
| Best for | Proven edge, long horizon | Small accounts, edge still in test |
the core differencecompounding: risk = equity × 1% # grows/shrinks with the account
fixed: risk = $200 always # constant dollars
Percent-of-equity sizing captures the geometric growth the compound return calculator shows, and it shrinks your bets automatically during losing streaks, lowering your risk of ruin. The catch: as the account grows, the dollar amounts at risk grow too, and many traders flinch at sizes that are correct but unfamiliar.
How to choose
- Still validating the edge? Start fixed-dollar — it is simpler and limits damage while you learn.
- Edge proven out-of-sample? Move to percent-of-equity to compound. Quantify the edge first with the expectancy calculator.
- Want a ceiling? Use a hybrid: percent-of-equity capped at a dollar maximum, or step risk down in drawdowns.
- Whatever you choose, never exceed the Kelly fraction, and size each trade with the position sizing calculator.
This maps onto the fixed-fractional vs fixed-amount distinction in the systematic-trading literature (Ralph Vince, Van Tharp). The position sizing strategies guide covers ATR-based and Kelly variants in more depth.
Frequently asked questions
What is compounding (percent-of-equity) position sizing?
Compounding position sizing risks a fixed percentage of your current equity on every trade, so the dollar amount grows as the account grows and shrinks during drawdowns. It maximises long-run growth and lets winners compound, at the cost of deeper dollar swings and a slower psychological adjustment as size scales up.
What is fixed-dollar (fixed-fractional) position sizing?
Fixed sizing risks the same dollar amount per trade regardless of account balance — for example, always risking $200. It is simpler and produces a steadier dollar equity curve, but it does not compound: your account grows arithmetically rather than geometrically, leaving long-run gains on the table when the system is profitable.
Which is better for a trading bot?
Neither is universally better. Percent-of-equity compounding is mathematically optimal for growth and is the default for most systematic bots, but it demands discipline through deep drawdowns. Fixed sizing is gentler and easier to reason about, and is sometimes preferred for small accounts or when an edge is still being validated. Many traders start fixed and switch to percent-based once confident.
Can I combine the two?
Yes — common hybrids cap the percentage risk at a dollar ceiling, or step the risk percentage down during drawdowns (anti-Martingale in reverse). The goal is to keep most of the compounding benefit while limiting the worst-case dollar risk. Backtest any hybrid on real data before trusting it.