How Many Shares Should I Buy?

The share version of the same formula

Stock position sizing is the same calculation as forex or crypto, minus the pip and contract-size conversions. You know the money you are willing to lose and you know how far the price has to fall before you accept being wrong. Dividing one by the other gives the number of shares, and nothing else.

Note what is not in the formula: your opinion about the company, the share price itself, or how much cash you happen to have. Those decide whether the trade is a good idea or whether you can afford the position; they do not decide the risk-limited size. A 500 USD stock and a 20 USD stock can carry identical risk if the stop distances are set so the arithmetic says so.

Worked example with whole shares

A 25,000 USD account, risking 1% per trade, buying a stock at 42.50 with a stop at 40.75, paying 2.00 USD in total commission on the round turn, with no fractional shares available.

1. Stop distance per share: 42.50 - 40.75 = 1.75 USD per share.
2. Budget after commission: 250.00 - 2.00 = 248.00 USD.
3. Shares: 248.00 / 1.75 = 141.71.
4. Round down: 141 shares.
5. Check the worst case: 141 x 1.75 = 246.75, plus 2.00 commission = 248.75 USD. Inside the 250.00 budget.
6. Position value: 141 x 42.50 = 5,992.50 USD.

Here commission is a 2 USD rounding detail. On a tighter stop or a larger budget it stops being one: the tighter the stop, the more shares you hold for the same risk, and every per-share or per-trade fee scales with the share count. This is the same mechanism the forex guide shows with pip spreads.

Stop distance moves the share count more than anything else

Fix the budget at 250.00 USD, the entry at 100.00 USD, ignore commission for clarity, and vary only where the stop goes:

Halving the stop distance doubles the shares and doubles the position value, while the money at risk stays exactly 250.00. This surprises people who read position value as a proxy for risk. A 25,000 USD position with a 1% stop and a 3,100 USD position with an 8% stop risk the same amount; the second one simply uses eight times less capital to do it.

It is also why widening a stop after the trade is open is quietly dangerous. If you bought 250 shares for a 1.00 stop and then move the stop to 4.00 without reducing size, the position that risked 250.00 now risks 1,000.00.

Fractional shares: small accounts get precision back

Whole-share rounding hurts most when the share price is high relative to the risk budget. Take a 2,000 USD account risking 1% (20.00 USD) on a stock trading at 318.40 with a stop at 311.00.

1. Stop distance per share: 318.40 - 311.00 = 7.40 USD.
2. Shares: 20.00 / 7.40 = 2.702 shares (fractional, to three decimals).
3. Check the worst case: 2.702 x 7.40 = 19.99 USD. Inside the budget.
4. Position value: 2.702 x 318.40 = 860.32 USD.

Without fractional shares you would round down to 2 shares, risking only 14.80 USD — about 26% less than planned. Being under your budget sounds safe, and mostly it is a silent performance leak: your winners are also 26% smaller, and every result you track is distorted by a rounding error rather than by your actual edge.

Availability depends on your broker and jurisdiction, and some platforms allow fractional quantities only on certain order types. Check what yours supports before assuming you can place 2.702.

What does not belong in the calculation

• Taxes and dividends. Neither is caused by the price reaching your stop. Account for them in your returns, not in your size.
• Gap risk. A stop order becomes a market order when triggered; it is not a guarantee of the fill price. If a stock gaps through your stop, the realised loss exceeds the planned one. That is a property of equities, not an error in the formula — and it is an argument for smaller size on names that gap often, such as around earnings.
• Account-level trading rules. Some regimes impose minimum equity thresholds or holding-period restrictions on certain account types, and they can override any size the arithmetic produced. Ask your broker what applies to your account.
• Your conviction. The strongest setups still lose. Sizing is the one part of the plan that is supposed to be mechanical.

And one honest limitation: the formula sizes a single position. It does not know you already hold three correlated names in the same sector, and it will happily size each one to 1% of the account. Portfolio-level exposure is a separate calculation on top of this one.

Do it once, then automate

Work the example through with your own numbers on the next trade you take. Once your result matches what the platform will accept, you have confirmed your share step and fee assumptions.

Open the position size calculator

Questions traders ask

The risk budget divided by the loss per share, with commission taken off the budget first. On this page's example — 25,000 USD account, 1% risk, entry 42.50, stop 40.75 — that is (250.00 − 2.00) / 1.75 = 141.71, rounded down to 141 shares. Change the stop and the answer changes; nothing about the company or the share price enters the division.

Only through the stop distance it produces. A 500 USD stock with a 5.00 stop and a 20 USD stock with a 5.00 stop take the same number of shares for the same risk. What actually moves the count is where the stop sits: on the table above, halving the distance doubles the shares while the money at risk stays 250.00.

Not without reducing size first. If you bought 250 shares against a 1.00 stop and move it to 4.00, the position that risked 250.00 now risks 1,000.00 — four times the budget, decided after the entry rather than before it. Widening a stop is a decision to risk more; if that is what you want, take fewer shares and say it out loud.

Yes, and materially. On a 2,000 USD account risking 20.00 on a 318.40 stock with a 311.00 stop, the arithmetic gives 2.702 shares. Rounded down to whole shares you get 2, risking 14.80 instead of 19.99 — about 26% under budget. That sounds cautious, but your winners shrink by the same 26%, so rounding quietly distorts every result you track.

Nothing in the formula says to, because a gap bypasses the stop rather than changing the arithmetic. A stop becomes a market order once triggered, so a stock that gaps through it realises more than planned. That is a property of equities, and it argues for smaller size on names that gap often — size to the loss you could actually take, not to the one the formula promises.

Related guides

• How much should you risk per trade? — where the 1% comes from.
• Position sizing on a small account — fractional shares and minimum steps.
• Lot sizes explained — the forex equivalent of share counts.
• Position sizing for crypto futures — contracts rather than lots or shares.
• Options profit calculator — the same risk budget turned into a contract count, where the most one contract can lose is the premium plus the fees.
• Forex, stock and crypto position size calculator