Position Size Calculator

What is this?

A Position Size Calculator works out how much of an instrument to buy or sell so that, if your stop-loss is hit, you lose exactly the amount you planned to risk. It covers stocks, forex and crypto, and also reports margin, reward-to-risk and the break-even win rate.

How to Use the Position Size Calculator

A position size calculator answers one question: given what you are willing to lose and where your stop sits, how much can you buy? It works the same way for stocks, forex and crypto - only the units change.

The order matters. Decide the risk first, then let the stop distance decide the size. Sizing a position by what you can afford to buy, or by how much leverage the broker allows, is how accounts get destroyed - because it leaves the loss undefined.

1. Choose the Market

Stocks and crypto are priced per unit, so the calculator works from your entry and stop prices. Forex is sized in lots against a stop measured in pips.

2. Enter Your Account Balance

Use your tradeable capital, not your net worth. This is the base the risk percentage is taken from.

3. Set the Risk Percentage

How much of the account you accept losing if the stop is hit. Most risk management uses 1% to 2% per trade, which lets you absorb a long losing run without serious damage.

4. Enter Your Stop

For stocks and crypto, enter the entry and stop-loss prices - the stop goes below entry for a long and above for a short. For forex, enter the stop distance in pips and the pip value for one standard lot.

5. Add a Target (Optional)

Entering a take-profit price gives the reward-to-risk ratio and the potential profit, plus the win rate you would need just to break even at that ratio.

Key Formulas Used in the Calculator

Amount at Risk

$\text{Amount at Risk} = \text{Account Balance} \times \frac{\text{Risk \%}}{100}$

A £10,000 account risking 1% puts £100 on the trade. This figure is fixed before anything else is decided.

Stocks and Crypto

$\text{Position Size} = \frac{\text{Amount at Risk}} {\left|\text{Entry Price}-\text{Stop Price}\right|}$

Risking £100 with a £5 gap between entry and stop gives 20 shares. If the stop is hit you lose 20 × £5 = £100, exactly as planned.

Forex

$\text{Lots} = \frac{\text{Amount at Risk}} {\text{Stop in Pips} \times \text{Pip Value per Lot}}$

Risking £100 with a 30 pip stop and a £10 pip value gives 0.33 standard lots, or 33,333 units. A standard lot is 100,000 units, a mini lot 10,000 and a micro lot 1,000.

Reward to Risk

$\text{Reward:Risk} = \frac{\left|\text{Target}-\text{Entry}\right|} {\left|\text{Entry}-\text{Stop}\right|}$

At 3:1 you need to win only 25% of trades to break even, because 1 ÷ (1 + 3) = 0.25. At 1:1 you need 50%.

Benefits

• Covers stocks, forex and crypto in one tool

• Sizes the position so the stop loses exactly your planned amount

• Shows forex size in standard, mini and micro lots and in units

• Separates margin required from money at risk

• Gives reward-to-risk and the break-even win rate it implies

• Warns when the risk percentage is above conventional limits

• Warns when the position is larger than the account can carry

When & Where to Use

• Sizing a trade before entering it

• Checking whether a stop is too tight for your account

• Comparing the size a wider stop would allow

• Working out the margin a leveraged position needs

• Deciding whether a setup's reward-to-risk justifies the trade

• Keeping risk consistent across trades of different volatility

• Sizing crypto positions where fractional units are allowed

Who Should Use This Calculator

The Position Size Calculator is for anyone trading with a stop loss - stock, forex and crypto traders, swing and day traders, and anyone moving from discretionary sizing to a defined risk per trade.

Tips to Get the Best Deal

Set the risk first and let the stop decide the size, never the reverse

1% to 2% per trade is the conventional limit

A tighter stop allows a bigger position, not a smaller loss

Leverage changes the margin held, not the money at risk

Place the stop where the idea is proven wrong, not at a round number

Reward-to-risk below 1:1 needs a win rate above 50% just to break even

Recalculate as the balance changes so risk stays proportional

Pro Tips

• Set the risk first and let the stop decide the size, never the reverse.

• 1% to 2% of the account per trade is the conventional limit.

• A tighter stop allows a bigger position, not a smaller loss.

• Leverage changes the margin held, not the money at risk.

• Reward-to-risk below 1:1 needs a win rate above 50% to break even.

• Educational only - slippage and gapping mean a real loss can exceed the figure shown.