Position sizing is the least glamorous decision in trading and quietly the most important. It is not about picking winners, it is about making sure no single loss can sink you. Get this right and you can survive long enough for your edge to work.
Think of your account like a ship with watertight compartments. Size each trade so that if one floods, the ship still sails. Put everything in one compartment and a single leak sinks you.
Small slices keep you afloat
The core idea is simple: risk only a small slice of your account on any one trade. If a position can only ever cost you a couple of percent, then even a string of bad luck is survivable, an annoyance rather than a disaster. Oversize a trade and one bad day can undo months of careful gains, or end your account entirely.
Size for the bad outcome
Here is the rule that keeps sellers safe: size every position as if the worst case will happen, because sometimes it will. For a cash-secured put, your real size is the full cash to buy the shares if assigned, not the small premium you collected. Always ask, “if this goes completely wrong, can I take it without flinching?” If the answer is no, the trade is too big.
Never size to the premium
The most common beginner mistake is sizing to the reward instead of the risk. A $200 premium feels small, so it is tempting to sell ten of them, but if each one obligates you to buy $5,000 of stock, you have quietly taken on $50,000 of risk for $2,000 of premium. Count the obligation, not the payout, and your sizing stays honest.
Boring, survivable sizing is what lets the slow magic of compounding happen. The traders who last are not the ones who bet big and won, they are the ones who never bet big enough to lose everything.
- Risk only a small slice of your account per trade, so no single loss can sink you.
- Size for the worst outcome, using the full assignment obligation, not the premium collected.
- Survivable sizing is what keeps you in the game long enough for your edge to compound.
This article is educational and is not investment advice. Options involve risk, including the possible loss of principal. Examples and premiums shown are illustrative and change with the market. Practice with paper trading before committing real money.