Let’s talk about the one calculation that matters more than your entry, more than your indicator setup, more than whatever chart pattern you think you spotted on the 15-minute timeframe. Position sizing. It’s boring. Nobody gets excited about it. And it’s the single biggest reason retail accounts blow up — not because the trade idea was bad, but because the size was wrong for the stop that was set.
Here’s the thing nobody tells you when you start trading: your stop-loss placement and your position size are not two separate decisions. They’re the same decision, done backwards. Most beginners pick a number of shares first — “I’ll buy 100 shares” — and then figure out where to put the stop. That’s exactly backwards. You should decide how much money you’re willing to lose on the trade, find where the chart actually invalidates your idea, and let those two numbers tell you how many shares to buy. This calculator does that math for you.
Position Size Calculator
This calculator assumes fills at your exact entry and stop prices and does not account for slippage, gaps, commissions, or partial fills. For educational purposes only — not investment advice.
How to Use It
Account size — your total trading capital. Not your net worth, not including the house — the actual capital in the account you’re trading with.
Risk per trade (%) — this is the percentage of your account you’re willing to lose if the trade goes wrong and hits your stop. The old-school rule of thumb floating around every trading forum is 1-2% per trade, and it’s a cliché for a reason — it’s math that actually works. Risk 2% per trade and you can be wrong ten times in a row before you’re down 20%. Risk 10% per trade and three bad trades in a row puts you in a hole that takes a 50%+ gain just to climb out of. Asymmetry is not your friend on the downside.
Entry price — where you’re planning to get in.
Stop-loss price — where you’re planning to get out if the trade doesn’t work. This needs to come from your chart, your support/resistance level, your ATR-based stop, whatever your actual strategy uses — not from “how much am I okay losing.” That’s what the risk percentage field is for. Mixing these two up is the single most common position-sizing mistake I see.
Take-profit price (optional) — if you’ve got a target, enter it. This gives you your risk-to-reward ratio, which tells you how much you stand to make relative to how much you’re risking. A trade with a 1:3 risk-reward ratio can be wrong more often than it’s right and still be profitable over time — that’s the math that lets professional traders sleep at night with a sub-50% win rate.
Hit calculate and you’ll get your share count, your dollar risk (should match your target risk % almost exactly), your total position value, and — if you filled in a target — your risk-reward ratio.
Where People Actually Screw This Up
Confusing position size with risk. These are not the same number, and conflating them is how accounts get wrecked. Buying $10,000 worth of a stock is your position size. How much you actually lose if it drops to your stop is your risk. A tight stop on a big position and a wide stop on a small position can carry identical dollar risk while looking completely different on your brokerage screen. The calculator’s whole job is translating “how much am I willing to lose” into “how many shares does that actually mean,” and that translation depends entirely on the distance between entry and stop.
Using round numbers instead of real risk. “I’ll just buy $5,000 worth” is not a risk calculation, it’s a guess wearing a risk calculation’s clothes. Your stop distance determines everything. A stock with a stop 2% below entry lets you buy a much bigger position for the same dollar risk than a stock with a stop 10% below entry. Volatile stocks demand smaller positions for the same risk budget — that’s not caution, that’s arithmetic.
Ignoring that risk percentage compounds against you nonlinearly. Losing 10% requires an 11% gain to recover. Losing 50% requires a 100% gain to recover. This isn’t linear, and it’s exactly why the 1-2% guideline exists — it keeps the math survivable even through a genuinely bad stretch.
Forgetting slippage and gaps. This calculator assumes you get filled at your exact stop price. Real markets gap, especially around earnings, especially in smaller-cap names. Your actual loss on a gapping stock can blow straight through your calculated stop with no fill in between. If you’re trading names that gap hard, either size down from what the calculator suggests or treat the output as a floor on your risk, not a ceiling.
Averaging down and forgetting to re-run the math. Adding to a losing position without recalculating your total risk is how a properly-sized single trade quietly turns into an oversized one. Every time your position changes, your risk changed with it — run it again.
Quick Reference
R-multiple — a way of expressing gains and losses in units of initial risk rather than dollars. If you risked $200 and made $600, that’s a 3R winner. Traders who think in R-multiples tend to size more consistently because it forces every trade to be measured against the same yardstick, regardless of account size or ticker price.
Risk-reward ratio — the ratio between what you stand to lose (entry to stop) and what you stand to gain (entry to target). A 1:2 ratio means your target is twice as far from entry as your stop.
Win rate vs. expectancy — a high win rate with poor risk-reward can still lose money over time, and a low win rate with strong risk-reward can still be highly profitable. Position sizing discipline is what lets a strategy with a sub-50% win rate actually work in practice instead of just on paper.
The math here is simple on purpose. It’s not going to tell you where to place your stop — that’s your chart-reading and your strategy’s job. What it will do is stop you from finding out the hard way that “I’ll just buy a few hundred shares, seems reasonable” was never actually a risk decision to begin with.
