Position size calculator for stocks and forex

Written by the Tactfolio research teamPublished 4 min read

A position size is how many shares, or how many lots, you buy in one trade. This position size calculator picks that number from the most you are willing to lose. It divides the money you risk by how much you lose per share, or per lot, if your stop-loss is hit.

  • Position size = money you risk ÷ loss per share (or per lot) at your stop.
  • Risking 1% of a $10,000 account means a planned loss of $100.
  • With a $2 stop on a $50 stock, that is 50 shares.
  • With a 20 pip stop on EUR/USD, that is 0.5 lot.
  • A stop-loss order is not a promise. Prices can jump past it.

Result

Shares to buy
50 shares
Money at risk if the stop is hit
$100.00
Position value
$2,500.00 (25% of the account)

Result

Trade size
0.5 lots (50,000 units)
Money at risk if the stop is hit
$100.00
Value of one pip at this size
$5.00

How to use the position size calculator#

There are two calculators on this page: one for stocks and ETFs, and one for forex. For stocks, type the money in your account, the share of it you will risk, your buy price and your stop-loss price. For forex, type your stop distance in pips and the price of the pair instead. Each shows the size to trade and the money at risk.

The prices start at round example numbers. They are not live prices, so type the prices your broker shows you.

How do you calculate position size?#

Position size is the money you risk, divided by what you lose on each share or lot when the stop is hit.

money at risk = account × risk per trade %
shares = money at risk ÷ (buy price - stop-loss price)
lots = money at risk ÷ (stop distance in pips × value of one pip on 1 lot)

The calculator rounds down, to whole shares or to 0.01 lot. Rounding down keeps your loss at or under the amount you chose. The value of one pip comes from the pip calculator formula.

A worked example for stocks#

Say you have $10,000 and will risk 1% on one trade. You want to buy a stock at $50 and sell if it falls to $48.

  1. Money at risk: $10,000 × 1% = $100.
  2. Loss per share at the stop: $50 - $48 = $2.
  3. Shares: $100 ÷ $2 = 50 shares.

50 shares at $50 cost $2,500. That is 25% of your account, but only $100 of it is at risk if the stop works.

A worked example for forex#

Say the same $10,000 account trades EUR/USD at 1.1000 with a stop 20 pips away. The account is in US dollars.

  1. Money at risk: $10,000 × 1% = $100.
  2. One pip on 1 lot of EUR/USD is worth $10, so 20 pips on 1 lot is $200.
  3. Lots: $100 ÷ $200 = 0.5 lot, which is 50,000 units.

At 0.5 lot, each pip is worth $5. A 20 pip loss is $100.

Why size from the stop?#

Sizing from the stop keeps each loss the same size, whatever you trade. A wide stop gets a small position. A tight stop gets a bigger one. Either way, a loss costs you about the same share of your account.

The size of your risk matters more than any single trade. If you lose 10% of your account, you need about 11% to get back to where you were. If you lose 50%, you need 100%.

Common mistakes with position size#

  • Picking the size first and the stop later. Set the stop where the trade idea is wrong. Then size the trade to fit it.
  • Forgetting gaps. A stock can open far below your stop after bad news. Then you lose more than you planned.
  • Ignoring leverage. In forex, a small deposit can control a big trade. Check the full value with the margin calculator.
  • Raising risk after losses. Bigger bets to win money back can turn a bad week into a big drop. If you add to a losing stock, check your new average with the stock average calculator.

Leverage makes losses bigger as well as gains. You can lose more than you put in.

Try it on Tactfolio#

Tactfolio’s stock and ETF strategies don’t use stop-loss orders. They hold US stocks, ETFs or cash and rebalance on a schedule you choose, which is another way to control risk. In the Tactfolio strategy builder you can backtest a rule on past prices and see its worst drop before you risk money. The classic strategies show how well-known rules sized their risk. To see how a price move hits a leveraged account, try the leverage calculator. Every tool is on the free trading calculators page.

Common questions#

What is a position size calculator?#

A position size calculator tells you how much to buy so that hitting your stop-loss loses a set amount. You give it your account size, the share you will risk, your entry and your stop.

How do I calculate lot size in forex?#

Find the money you will risk, like 1% of your account. Divide it by the stop distance in pips times the value of one pip on 1 lot. For a $100 risk and a 20 pip stop on EUR/USD, that is $100 ÷ ($10 × 20) = 0.5 lot.

What is the 1% rule in trading?#

The 1% rule is a habit some traders follow: never lose more than 1% of the account on one trade. On a $10,000 account, that means a planned loss of $100 or less. It is a rule of thumb, not a law.

Does a stop-loss order guarantee my loss?#

No. A stop-loss order becomes a market order when the price is hit. If the price jumps past your stop, the order fills at the next price, which can be much worse.

Should I size every trade the same?#

Sizing from the stop gives every trade the same risk, not the same size. That keeps one bad trade from doing much more damage than another.

Sources

  • Stop Order, Investor.gov, U.S. Securities and Exchange Commission, accessed October 16, 2026
  • Types of Orders, Investor.gov, U.S. Securities and Exchange Commission, accessed October 16, 2026

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