Ask ten blown-up trading accounts what went wrong and most will tell the same story: they were right about position sizing exactly zero times. They bought "a few thousand dollars" of something because that felt about right, took a normal-sized loss, and did it again a little bigger. Position sizing is the one habit that separates traders who survive long enough to get good from those who don't.
The good news is that it's arithmetic, not art. Once you decide how much you're willing to lose on a trade, the correct number of shares or contracts is determined — you don't get to guess.
The core idea: risk first, size second
Amateurs start with "how much do I want to buy?" Professionals start with "how much am I willing to lose?" That single reversal is the whole game. You pick a fixed slice of your account to risk per trade — say 1% — and the position size falls out of the math.
Here's the formula:
Position size = (Account × Risk %) ÷ (Entry price − Stop price)
The top of that fraction is your dollar risk — the most you'll lose if the trade hits your stop. The bottom is your per-share risk — how many dollars you lose per share between your entry and your stop. Divide one by the other and you get the number of shares that makes those two line up.
A worked stock example
Say you have a $25,000 account and you risk 1% per trade — so $250 is on the line. You want to buy a stock at $190 with a stop at $186. Your per-share risk is $190 − $186 = $4.
$250 ÷ $4 = 62.5 shares. You always round down — 62 shares — because rounding up would push your risk past your limit. Those 62 shares represent an $11,780 position (about 47% of your account), but you're still only risking $248 if the stop hits. That gap between position value and risk is the point: sizing lets you take a meaningful position while keeping the downside small and fixed.
Change any input and the size adjusts. Tighten the stop to $188 and your per-share risk halves to $2, so you can hold twice the shares for the same $250 risk. A wider stop means fewer shares. The calculator does this instantly, but it helps to feel why it moves.
Options and futures: same idea, different unit
For options, the same logic applies to the premium: your risk per contract is what you'd lose per contract if the trade goes against you (often the full debit on a long option), times 100 shares per contract. Size so the total risk across all contracts equals your account risk.
Futures add one twist. Each contract has a tick size (the smallest price increment) and a tick value (the dollars that tick is worth). To size, convert your stop distance into ticks, multiply by the tick value to get the dollar risk per contract, then divide your account risk by that number. For example, the E-mini S&P 500 (ES) moves in 0.25-point ticks worth $12.50 each; a 5-point stop is 20 ticks, or $250 of risk per contract, so a $500 risk budget sizes to 2 contracts. The futures position size calculator has the common tick values built in so you don't have to look them up.
The mistakes that actually blow up accounts
- Rounding up. "62.5, call it 65" quietly pushes every trade over your risk limit. Always floor.
- Sizing off gut feel. A fixed rule you apply every time beats a number that drifts with how confident you feel that morning.
- Ignoring account size. The math can hand you a position bigger than your cash — meaning you'd need margin (leverage). That's not automatically wrong, but you should know when it's happening rather than discover it on a bad day. Our calculator flags this and shows what you can afford with cash alone.
- Using a forex-style lot calculator for stocks. Those don't model share or contract risk correctly. Use a tool built for the instrument you're actually trading.
Put it into practice
Position sizing only works if you do it before every trade, not after a loss makes you wish you had. Decide your risk percentage once, then run each setup through the numbers. The position size calculator gives you an exact share or contract count from your account, risk, entry and stop in a couple of seconds — and it works for stocks, options, and futures.
Once you're sizing consistently, the next question is whether a given setup is even worth taking. That's where risk/reward and expectancy come in.