Options position sizing is the discipline of deciding how much capital to risk on each trade so no single loss can end the account. Desks size by fixed fractional risk, not by conviction: cap the downside, keep the upside convex, and survive the variance long enough for an edge to compound.
Sizing starts with the loss, not the target
Most retail position sizing runs backward. The trader picks a strike, imagines the payoff, and buys however many contracts the account can afford that day. The size becomes a function of excitement. A desk inverts the whole process. You decide what you are willing to lose on the position first, and the number of contracts falls out of that constraint. The target is a hope; the loss is a decision. Sizing from the loss is what separates a book that survives a bad month from one that does not.
This matters more in options than in shares because a long option can go to zero. The premium you pay is the entire position at risk, which is a feature when the size is set correctly and a trap when it is not. Treat each premium as money you have already written off, and the question becomes simple: how much of the book can disappear on this one idea without changing how you trade tomorrow?
Fixed fractional risk: the discipline that keeps you in the game
The core tool is fixed fractional risk. You expose the same small, constant slice of the book to each position, sized so the worst case is bounded before you ever click. When you win, the book grows and the next position grows with it. When you lose, the book shrinks and the next position shrinks automatically, which is exactly the behavior you want during a cold streak. The method is unglamorous, and it is the reason professional books compound instead of detonate.
Compare it directly to how conviction-led sizing behaves under stress:
| Dimension | Conviction-led sizing | Fixed fractional sizing |
|---|---|---|
| What sets the size | How sure the trade feels | A predefined risk budget per position |
| Worst-case loss | Open-ended, varies with emotion | Capped and known before entry |
| Effect of a losing streak | Compounds into a deep drawdown | Shrinks position size automatically |
| Upside | Occasionally large, often given back | Convex, left to run on the survivors |
| Survivability | Fragile to variance | Built to outlast variance |
The right-hand column is not more conservative for its own sake. It is the only column where a normal run of losing trades, which is guaranteed to arrive, cannot end the account. Retail alert feeds rarely talk about this, because capped downside does not sell as well as a screenshot of a triple. But the capped downside is precisely what lets the screenshots keep coming.
Asymmetry is the point of options
Done right, sizing turns options into a machine for buying convexity. Each position has a floor you defined on entry and a ceiling you left open. Most trades resolve near the floor; a minority run, and because you never let a single idea exceed the fixed risk budget, the runners more than pay for the duds. The job is not to be right often. It is to be sized so that being right occasionally, with the losses kept small and bounded, is enough.
This is why a real desk obsesses over the shape of the payoff before it obsesses over the direction. A position with a small, fixed, known cost and an open-ended upside is structurally survivable. A position sized by feeling, where one bad week can take a chunk of the book that takes months to earn back, is structurally fragile no matter how good the analysis behind it was.
Most blowups I have watched were never a forecasting problem. The trader was right often enough; they were just sized so that one ordinary losing streak ended the account before the edge could ever pay off. Justin Katz, @Bluedeerc
Kelly, and why professionals run a fraction of it
The Kelly criterion is the formal version of this instinct: a formula for the bet size that maximizes long-run growth given your edge and your odds. It is the mathematical answer to how much to risk. The catch is that full Kelly assumes you know your edge precisely, which no trader does, and it produces a wildly volatile equity curve that most people cannot stomach. So professionals run a fraction of it, deliberately sizing under the theoretical optimum to trade raw growth for survivability. Under-betting a real edge still compounds; over-betting it is how accounts that pick well still go to zero.
The practical takeaway is that good sizing is conservative on purpose. The goal is never to maximize this trade. It is to still be solvent and unemotional for the next long run of trades, because that is the horizon over which an edge actually expresses itself.
How Equidamus Markets applies it
The Equidamus record has been built in public on X as @Bluedeerc since 2019, by a desk veteran with roughly ten years across two funds and a prop firm. Sizing discipline is the part that does not photograph well, so retail feeds skip it, but it is the spine of everything in that public timeline, tracked trade by trade since 2023. Every entry and exit is posted in real time before the outcome, with per-contract math anyone can replicate, which means the downside on each idea is visible at the moment of entry rather than reconstructed afterward. The point of posting the math is not to flex the wins. It is to show that the losses were bounded by design.
If you take one thing from a desk's approach to risk, take this: protect the downside relentlessly and let the upside take care of itself. A survivable book with capped losses and convex winners beats a brilliant one that cannot outlast a normal stretch of variance. Sizing is where that survival is won or lost, long before the thesis is ever right.
By the numbers
Frequently asked questions
- What is position sizing in options trading?
- Position sizing is deciding how much capital to put at risk on a single trade before you enter. The professional approach is fixed fractional risk: you cap the dollars exposed per position to a small, constant slice of the book, so no individual outcome, win or loss, can dominate the account.
- How do desks size an options position?
- They start from the loss, not the target. You define the maximum you are willing to lose on the trade, size the number of contracts so that loss equals your fixed risk budget, and let the asymmetric payoff of a long option do the rest. Conviction adjusts which trades you take, not the size cap.
- What is the Kelly criterion and should traders use full Kelly?
- The Kelly criterion is a formula for the bet size that maximizes long-run growth given your edge and your odds. Most professionals run a fraction of it, often half or less, because full Kelly is brutally volatile and assumes you know your edge precisely (https://en.wikipedia.org/wiki/Kelly_criterion). Sizing too aggressively is how accounts that pick well still blow up.
- Why does position sizing matter more than being right?
- Because variance is unavoidable and ruin is permanent. A study of retail day traders found that about 97% lost money over time, often less from bad picks than from oversized positions that could not survive a normal losing streak (https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3423101). Survive the variance and the edge compounds; size too big and one cluster of losses ends the game.