Type the money in your account, the percentage of it you are willing to lose on one trade, the price you intend to buy at and the price you would get out at. The page works out how many shares fit inside that risk — as a four-decimal figure and as whole shares — what those shares cost, what share of the account they take up, and, if you already hold the stock, what your average cost per share becomes after the buy. Nothing is looked up: every number below is one you typed. The stop is the input that carries the arithmetic: the distance from your entry price down to your stop price is what turns a percentage of the account into a share count, so these are the same four boxes a stop loss calculator works from.
The account and the trade
Account size — the money you are trading withThe whole account, not the amount you feel like using today. The share of it below is measured against this figure.
Risk per trade — percent of the accountThe part of the account you are prepared to lose if this trade goes against you. There is no house rule here and no default filled in for you — it is your number.
Entry price — the price you intend to buy atThe price you plan to pay per share. This page has no quote feed, so it cannot and will not fill this in for you.
Stop price — the price at which you are outWhere you would close the position if it goes against you. It has to be below the entry price; if it is not, the page says so and stops rather than turning the two around.
If you already hold some — optional
Shares you hold now — optional
Your average cost per share — optionalFill in both boxes and the page pools this buy into your position and shows the new share count and the new average cost. Leave both empty and it stops at this trade on its own.
Shares to buy — whole shares—
Shares to buy — with fractions—
Money at risk — account × risk %—
Risk per share — entry − stop—
Cost of the position—
Share of the account—
Shares after this buy—
Average cost after this buy—
How the position size is worked out
Four numbers come from you and everything else is derived from them. The stop price is the only thing that turns a percentage of your account into a number of shares: the wider the distance between your entry and your stop, the fewer shares fit inside the same money.
Money at risk = account size × risk % ÷ 100
Risk per share = entry price − stop price
Shares to buy = money at risk ÷ risk per share
Cost of the position = shares to buy × entry price
Share of the account = cost of the position ÷ account size × 100
Average cost after the buy = (money already in + cost of this buy) ÷ (shares already held + shares bought)
Worked example: an account of 10,000.00, a risk of 1%, an entry price of 50.00 and a stop price of 48.00. The money at risk is 10,000.00 × 1 ÷ 100 = 100.00. The risk per share is 50 − 48 = 2.0000. So the size is 100.00 ÷ 2.0000 = 50.0000 shares — 50 whole shares if your broker does not take fractions. Those shares cost 50.0000 × 50 = 2,500.00, which is 2,500.00 ÷ 10,000.00 = 25.00% of the account. Note the two sides of that trade: a quarter of the account is committed, but only 100.00 of it — one percent — is at risk, because the stop sits 4% below the entry price.
Worked example with an existing position: the account holds 25,000.00, the risk is 2%, the entry is 60.00 and the stop is 54.00. The money at risk is 500.00 and the risk per share is 6.0000, so the size is 83.3333 shares, and those shares cost 4,999.99 — 19.99% of the account. You already hold 100.0000 shares at an average cost of 40.0000, which is 4,000.00 of your money in the stock. After this buy you hold 100.0000 + 83.3333 = 183.3333 shares for 4,000.00 + 4,999.99 = 8,999.99, so your average cost becomes 8,999.99 ÷ 183.3333 = 49.0908. That is the number a position size calculator normally leaves out: not only how many shares this trade is, but what it does to the average cost you were carrying.
Rounding: every figure on this page is rounded down, never up — the same rule as the rest of the site. Share counts and per-share figures are shown to four decimals, amounts and percentages to two, so the number you read is never larger than the one that was computed. Each line of the arithmetic is printed under the results using the figures shown above it, so you can check the whole chain by hand.
The same trade, three ways of measuring it: 10,000.00 on the account, 2,500.00 committed to the shares, and 100.00 actually at risk if the stop is hit. The gap between the second bar and the third is the whole point of a stop, and it is why a quarter of the account can be committed while one percent of it is at risk.
The size the risk asks for, and the money you have to buy it with
Nothing in the formula above asks whether you can pay for the shares it returns. It takes your money at risk, divides it by the risk per share and prints a size. The cost that size implies is printed beside it, but nothing in the arithmetic stops that cost from being larger than the account.
Take one account and change one number. Same 10,000.00, same 1% risk, same 50.00 entry. Only the stop moves:
Stop price
Below the entry
Shares to buy
Cost of the position
Share of the account
48.00
4.00%
50.0000
2,500.00
25.00%
49.00
2.00%
100.0000
5,000.00
50.00%
49.50
1.00%
200.0000
10,000.00
100.00%
49.75
0.50%
400.0000
20,000.00
200.00%
49.90
0.20%
1,000.0000
50,000.00
500.00%
The last column is the one worth reading twice, because it has nothing to do with how big the account is. Cancel the account out of the arithmetic and what is left is a ratio between two percentages:
Share of the account committed = risk % ÷ stop distance in % of the entry price
Check it against the table: 1 ÷ 4.00 = 25.00%, 1 ÷ 2.00 = 50.00%, 1 ÷ 1.00 = 100.00%, 1 ÷ 0.50 = 200.00%, 1 ÷ 0.20 = 500.00%. Run the same entry and stop on an account ten times the size and that column does not move a hundredth, which is what it means for a commitment to be a ratio: the account decides how many dollars, not what share of itself goes out.
Try it from the other side. A 2% risk on a 60.00 entry with a stop at 54.00 is a stop sitting 10.00% below, so the commitment is 2 ÷ 10.00 = 20.00% of the account, whether the account is 25,000.00 (83.3333 shares costing 4,999.99) or 100,000.00 (333.3333 shares costing 19,999.99). Both print 19.99% rather than the clean 20.00%, and that gap is the rounding rule working in the safe direction: every step is cut downwards, so the figure on the page lands a hundredth under the exact one instead of a hundredth over.
That ratio also gives the line where the money runs out. The whole account is committed exactly when the stop sits your risk percentage below the entry, which in dollars is the entry price times the risk percentage divided by 100: 50.00 × 1 ÷ 100 = 0.5000, so a stop at 49.50. Any stop tighter than that asks for more than the account holds. Ask for 2% on the same entry and the line moves to 1.0000, a stop at 49.00.
This runs against the way it is usually put. A wider stop means fewer shares, everyone learns that first, and by itself it reads like the cautious direction. It is also the direction that costs less money: 2,500.00 committed at the 48.00 stop against 20,000.00 at the 49.75 stop, on the same hundred dollars of risk. Tightening the stop is what pushes the arithmetic past the account, which is why the remedy offered on one of the ten pages read before writing this one, to lower your risk or find a closer stop, only half works. Lowering the risk does help. Moving the stop closer raises the cost.
When the size turns out to be too big to fund, what follows is not a bent risk limit. You simply take less risk than you asked for. The most this account can buy at 50.00 is 10,000.00 ÷ 50.00 = 200.0000 shares, whatever the stop says. At the 49.75 stop the formula wants 400.0000 shares, and buying 200 of them risks 200 × 0.2500 = 50.00, which is 0.50% of the account rather than the 1% that was typed in. At the 49.90 stop the same 200 shares risk 200 × 0.1000 = 20.00, which is 0.20%.
Look at what those two figures are. They are the stop distances, 0.50% and 0.20%. Once the money sets the size instead of the risk doing it, the percentage actually put at risk stops being the one in the box and becomes the distance from your entry down to your stop, no matter what risk percentage was typed. The closed forms above have not changed, they have stopped being the ones that decide.
The two lines in the results box, the cost of the position and its share of the account, are where this shows itself on every run, including runs nobody expected to be near the line. There is no separate cash field here, so the comparison this page can make is against the account size you typed. Cash you are holding back, cash already committed to something else, and borrowed buying power all sit outside these six boxes.
Tightening the stop is what pushes the cost up, not down: the same hundred dollars of risk asks for 2,500.00 at a 2.0000 stop and 20,000.00 at a 0.2500 stop. Once the size is capped by the money rather than by the risk, the percentage actually at risk becomes the stop distance, whatever went into the risk box.
Why the page gives two share counts
Because not every account can hold a part of a share. If your broker takes fractions, the four-decimal figure is your size. If it only takes whole shares, the whole-share figure is the largest order that still keeps your loss inside the limit you set — the fractional part is what you leave on the table, and the cost line spells out what that order would come to instead.
And because a position is usually added to, not opened from nothing, the optional boxes pool this buy into what you already hold. The stock average calculator on this site does the same pooling from a list of buys you type, and the stock split calculator carries a share count and an average cost through a split in either direction. Here it is the other way round: the new buy is worked out first, then the average cost it produces.
What is counted, and what is not
This page uses only the six numbers you type. Nothing else goes into the arithmetic.
There is no live quote on this page. The entry price and the stop price are both typed by you. This page never fetches a price, has no connection to a market data source and cannot show you a quote that is not real — if you want the current price of a stock, this is not the place to find it.
Your stop is not a guarantee. The arithmetic assumes you get out exactly at the stop price. A gap, a trading halt or a fast market can fill you well below it, so the money you actually lose can be larger than the figure on this page. The number here is what the trade is set up to lose, not a cap on what it can lose.
No word on where the stop should go. This page does not suggest a stop, does not size one from volatility, and takes no view on whether your stop is sensible. The distance from entry to stop is the input that does all the work, and it is yours.
One trade per run. The page sizes one buy against one stop. It does not check whether this position added to an existing one breaks a limit across your whole account, and it does not track other trades.
No advice. Nothing here ranks the trade, says whether to take it, or compares it with holding something else. It is division and multiplication on the numbers you supplied.
Fractional shares: one assumption. The cost of the position and the average cost after the buy are worked on the four-decimal size, which is the size that exactly matches your risk. If you cannot buy fractions, the line under the cost result shows the whole-share order instead, and that order costs a little less than the figure in the box because it buys a little less.
Not counted: commissions, fees, taxes, spreads, slippage, dividends, currency conversion, margin interest and borrowing costs. None of them fits into a box here. If you want a commission inside the arithmetic, add it to your entry price before typing it, and remember that it makes the real risk a little bigger, not smaller.
Not built: several tickers at once, importing a statement, saving your history, or signing in. There is no account here and nothing is stored.
The result describes the trade you entered — this account, this risk, this entry, this stop. It is not a view on the stock.
Frequently asked questions
What does a position size calculator work out?
How many shares to buy so that a move from your entry price to your stop price costs a chosen percentage of your account. From your four numbers it gives the money at risk, the risk per share, the share count as a four-decimal figure and as whole shares, the cost of the position, what share of the account that uses, and — if you fill in the optional boxes — your average cost per share after the buy.
Is a position sizing calculator the same as a position size calculator?
Yes. Both names mean this tool and the page answers to both, along with "calculate position size" and "stock position size". The arithmetic does not change with the wording: the money at risk divided by the distance from your entry to your stop gives the number of shares.
How do I calculate position size for a stock?
Three steps. Work out the money you are willing to lose: account size times risk percentage, divided by 100. Work out the risk per share: entry price minus stop price. Divide the first by the second. The result is your size in shares. Type those four numbers above and the page does the three steps for you, and prints each one.
Which number do I use as the risk percentage?
Yours, and it is not on this page anywhere. This page has no rule of thumb, no preset and no default, because the right number depends on your account, your plan and how the rest of your money is invested. Type the figure you have already decided on; the page only does the division.
What if my stop is above my entry price?
The page says so and stops. For a long position the stop has to sit below the entry, otherwise the risk per share is negative or zero and there is no size to work out. Rather than quietly using the difference and pretending the two prices were the other way round, the page names the two boxes and asks you to check them.
Should I use the whole-share figure or the four-decimal one?
Whichever your account can hold. If your broker fills fractional shares, the four-decimal figure is exactly the size that matches your risk. If it only takes whole shares, take the whole-share figure — it is the largest order that still keeps the loss inside your limit, and the cost line tells you what it comes to.
Why does a wider stop mean fewer shares?
Because the money at risk is fixed by your account and your percentage. If you allow the price more room to move before you are out, each share can lose more, so fewer shares fit inside the same money. Halve the distance from entry to stop and you double the size; double it and you halve the size.
How does the existing-holding part work?
Fill in the shares you hold and your average cost per share, and the page treats this buy as an addition to that position. It adds the money already in the stock to the cost of this buy, adds the shares you hold to the shares you are buying, and divides, which gives your average cost per share afterwards. If you fill in only one of the two boxes, the page asks for the other one rather than assuming an average cost.
Does the page tell me where to put my stop, or whether to take the trade?
No. The stop distance is yours, and the page takes no position on whether it is a good one or whether the trade is worth taking. It sizes the trade you described, which is arithmetic, and stops there.
Can I use this page as a risk reward ratio calculator?
It works out one side of the ratio and not the other. The risk side is here in two lines under the results, the money at risk and the risk per share, and both of them come straight from your entry and your stop. The reward side has to be measured against a target price, and there is no target box on this page, so no ratio is printed and none is implied by anything shown.
Does it account for fees, taxes or slippage?
No, and none of them are hidden inside the figures either. Commissions, fees, taxes, spreads and slippage are all left out, so the real cost of the trade is a little higher than the cost shown here, and the real loss if the stop is hit can be a little bigger than the money at risk shown here. There is no field for any of them, and the page will not pretend to know what your broker charges.
What happens if I change the risk percentage?
The size moves with it, in a straight line. Double the risk percentage and the money at risk doubles, so the share count doubles. Halve it and the share count halves. The risk per share does not move at all, because it only depends on the two prices.
How is this different from the other calculators on this site?
They work on a position you already have; this one works out the buy before you make it. The stock average calculator pools buys into an average cost, the average down calculator shows what one more purchase does to that average, the DCA calculator takes a fixed amount of money per period instead of a fixed risk, the stock profit calculator adds the fees on both sides and the sale, the stock split calculator carries a position through a split, and the dividend calculator and dividend reinvestment calculator work out what your shares pay you. This is the one that answers how many shares to buy in the first place — and then feeds that buy back into your average cost.
How do I calculate how many shares I can buy?
Money you can spend divided by the price of one share, cut down to the whole share unless your broker takes fractions. On this example, 10,000.00 at 50.00 is 200.0000 shares, or 200 whole ones. That is a different number from the one the risk works out, and this page leads with the risk answer, because it is the stop that decides the size here: 50.0000 shares on the figures above, not 200. The size you can act on is whichever of the two is smaller, and when it is the affordability one, the risk you end up taking is smaller than the percentage you typed as well.
How many shares is a good amount to buy?
This page has no good number to hand you and will not invent one, because nothing you have typed says anything about whether the trade is a good one. What the arithmetic does give is two hard bounds, and they are worth knowing before anyone offers you a rule of thumb. Above the affordability count the order cannot be funded. Above the risk count a move from your entry to your stop costs more of the account than the percentage you chose. Anything at or below both is a size these numbers can describe, and everything inside that range is preference, position limits and judgement this page does not have.
Does it work on my phone, and do I need an account?
Yes to the first, and no to the second. The fields stack to a single column on a narrow screen and the results stay inside the width of the page. There is no sign-up, no pop-up and nothing to install: the whole calculation runs in your browser, and your numbers are not sent anywhere.
This page is a calculator, not investment advice — it is arithmetic, not a recommendation to buy or to sell anything. A stop price is not a guarantee: a gap, a halt or a fast market can fill you below your stop, so the money you actually lose can be more than the figure shown here. Taxes, commissions and slippage are not counted anywhere on this page. Contact: contact@stockavg.com