How To Calculate Position Size From Your Stop Loss

You found the setup, picked the entry and know exactly where the stop goes. Then you type in a share count that feels about right, 100 shares, maybe 200, because that's what you usually buy. That one guess decides how much you'll lose if you're wrong, and you never actually decided it.

Position sizing fixes that. You choose the dollar loss first, and the stop tells you how many shares fit inside it. Three steps, on every trade.

Step 1: Set Your Dollar Risk

Start with the account, not the stock. Decide what share of the account you're willing to lose on one trade if the stop is hit, and keep that percentage fixed, for example 1%. The risk-first lesson explains why a small, fixed number keeps you trading through a losing streak.

Dollar risk = account size × risk percent.

On a $40,000 account at 1%, that's $400. This is your R for the trade, the unit our guide to R-multiples uses to grade every result. It's the most the stop should cost you.

Step 2: Measure Your Risk Per Share

Risk per share is the distance from your entry to your stop.

Risk per share = entry price - stop price, for a long trade.

Say you plan to buy at $80.00 with a stop at $76.00. Your risk per share is $4.00. For a short, flip it: stop price minus entry price.

The stop has to come from the chart, not from the size you'd like to trade. Put it where the trade idea is clearly wrong, such as just past the level that defines the setup. If you nudge the stop closer just so you can buy more shares, you're sizing a trade you don't actually believe in.

Step 3: Divide And Round Down

Shares = dollar risk ÷ risk per share.

$400 ÷ $4.00 = 100 shares. If the stop is hit at $76.00, you lose 100 × $4.00 = $400, exactly 1R.

Now change only the stop. Same stock, same $80.00 entry, but this setup calls for a stop at $78.50. Risk per share is $1.50, so the size is $400 ÷ $1.50 = 266.67 shares. Always round down, so 266 shares. The worst case is 266 × $1.50 = $399.

Two trades, two very different share counts, the same planned loss. That's the whole point. A tight stop earns a bigger position, a wide stop forces a smaller one, and your risk stays the same.

Check What The Position Ties Up

One more check before you send the order. Multiply shares by the entry price to see how much capital the trade uses.

266 shares × $80.00 = $21,280. That's more than half of a $40,000 account in one stock, even though the risk is only 1%.

If that's more than you want in a single position, or more than your cash covers, cap the size. Set a maximum position value as part of your plan, for example 25% of the account. On $40,000, that's $10,000, or 125 shares at $80.00. The risk drops to 125 × $1.50 = $187.50. Ending up with less risk than planned is fine. Ending up with more never is.

What you shouldn't do is borrow to make the size fit. Lesson 9 on leverage and margin shows how borrowed buying power magnifies losses just as fast as gains.

Sizing An Options Trade

The same three steps work for options, with one extra number: the contract multiplier. FINRA's investor guide to options notes that a standard-size options contract is equal to 100 shares of the underlying security, and that for the buyer of an option, the premium paid is the maximum loss.

Say you want to buy a call priced at $2.50. One contract costs $2.50 × 100 = $250, and that's your risk per contract if you're willing to let it expire worthless. With the same $400 of dollar risk, $400 ÷ $250 = 1.6 contracts. Round down to 1.

If your plan is to sell the option once it falls to $1.50, your risk per contract is $1.00 × 100 = $100, and the size is $400 ÷ $100 = 4 contracts. That exit is a plan, not a promise, since an option can open well below your exit price on a gap. Keep that in mind before you choose a planned exit over the full premium.

Join The Pod and size every trade in seconds instead of guessing. Lesson 4 walks through sizing shares and contracts step by step, the Position Size Calculator turns your account size, risk percent, entry, stop and contract multiplier into an exact share or contract count, and the private trade journal tracks the R on every trade you log. Join The Pod today and let your stop set your size.