Position sizing

Position sizing is the process of choosing how many shares, contracts, or lots to buy so that hitting your stop costs a fixed, planned fraction of your account. It is the one decision that ties your entry to your survival: the same setup can be prudent or reckless depending on how big you go. Get sizing right and a losing streak is an inconvenience; get it wrong and a single trade can end your account. Most traders obsess over where to enter and treat size as an afterthought — that is backwards.

How it's calculated

The workhorse method is fixed-fractional sizing. You decide what percentage of your account you're willing to lose on the trade, then let your stop distance dictate the quantity.

Work a concrete example. Your account is $50,000 and you'll risk 1%, so $500 is on the line. You're long crude oil futures (CL), where each contract moves $10 per $0.01 tick, i.e. $1,000 per full point. You buy at $78.40 with a stop at $77.90 — a $0.50 stop, worth $500 per contract.

Position size = $500 ÷ $500 = 1 contract.

Tighten the stop to $78.15 (a $0.25 stop, $250 per contract) and the same $500 of risk supports 2 contracts. Notice the size changed but the dollar risk did not. That's the whole point: your stop distance, not your conviction, sets the quantity. A position size calculator runs this for any instrument once you plug in entry, stop, and account risk.

Why it matters

Entries feel like where the money is made, but sizing is where accounts are kept or lost. You can be right on direction most of the time and still blow up if a few oversized losers land back to back. Because losses compound against a shrinking base — a 50% drawdown demands a 100% gain just to break even — the size you choose directly drives your risk of ruin and the depth of drawdowns you'll sit through.

Fixed-fractional sizing has a built-in defense: as your account shrinks, your dollar risk shrinks with it, so a bad run bleeds you slowly instead of killing you outright. Fixed-share sizing has no such brake.

The classic trap is overleveraging. Risking 5–10% per trade feels fine while you're winning, but a run of six losers — routine over a few hundred trades — carves 30–60% off your account and drops you into the mathematical hole above. Leverage makes this worse by letting you hold far more than your capital would suggest, so a normal adverse move becomes an account-ending one. Aggressive sizing models like the Kelly criterion maximize long-run growth on paper but produce swings most humans can't stomach; serious traders run a fraction of full Kelly for exactly this reason. Whatever the framework, the discipline is the same: cap the downside first, let the upside take care of itself.

Related concepts

Position sizing sits at the center of a cluster of risk ideas worth knowing. Express your results in R-multiples so every trade is measured in units of risk rather than raw dollars — it makes performance comparable across different position sizes. Understand leverage and lot size to translate your risk budget into the actual quantity your broker expects, and keep an eye on risk of ruin as your sizing gets more aggressive.

Once you're sizing consistently, the payoff shows up in your records. Logging planned risk and realized outcome for every trade in the Tradeways journal lets you check whether your real dollar risk actually matches your intended percentage — the gap between the two is where most sizing discipline quietly breaks down.