Position Size Calculator: How Many Shares or Contracts Should You Trade?

By Nummius · October 18, 2026

Position sizing answers one question: how many shares or contracts can I trade so that, if my stop is hit, I lose no more than I decided in advance? It is one of the simplest calculations in trading and one of the most important, because it is what keeps a bad trade, or a bad week, from becoming a bad year. Use the calculator below, then read on for the formula, worked examples and the traps the formula does not cover.

The position size formula

Every fixed-risk position sizing method comes down to the same three steps:

1. Decide your risk in dollars: account balance × risk percentage. With $25,000 and 1% risk, that is $250.
2. Measure the risk per unit: the distance between entry and stop × the point value. For stocks the point value is 1, so it is simply the price distance.
3. Divide and round down: dollar risk ÷ risk per unit = position size.

The order matters. The stop comes first, from the chart: below support, beyond a swing point, or outside normal volatility. The size is then derived from the stop. Never the other way around.

Example 1: a long stock trade

You have a $25,000 account and risk 1% per trade, so $250. You want to buy at $50.00 with a stop at $48.50.

• Risk per share: $50.00 − $48.50 = $1.50
• Position size: $250 ÷ $1.50 = 166.67, rounded down to 166 shares
• Loss if the stop is hit: 166 × $1.50 = $249
• Position value: 166 × $50 = $8,300, about 33% of the account

Notice that the position is worth a third of the account, yet the planned loss is only 1%. Position size and risk are different things.

Example 2: a short stock trade

Shorts work the same way, with the stop above the entry. With a $10,000 account and 0.5% risk, the dollar risk is $50. Short at $120 with a stop at $123 and the risk per share is $3, so the size is $50 ÷ $3 = 16.67, or 16 shares.

Example 3: futures, ES versus MES

Futures add one step: the point value, also called the multiplier. One point on the E-mini S&P 500 (ES) is worth $50; on the Micro E-mini (MES) it is $5. The E-mini Nasdaq-100 (NQ) is $20 per point and the Micro (MNQ) is $2.

With a $50,000 account and 1% risk, the dollar risk is $500. Using illustrative prices, you plan to buy at 5,000.00 with a stop at 4,992.00, an 8-point stop.

• ES: 8 points × $50 = $400 per contract. $500 ÷ $400 = 1.25, so 1 contract, risking $400.
• MES: 8 points × $5 = $40 per contract. $500 ÷ $40 = 12.5, so 12 contracts, risking $480.

Micro contracts let smaller accounts size precisely instead of being forced into all or nothing. Forgetting the multiplier is the most expensive futures sizing mistake: treat ES like a stock and you would compute 62 contracts instead of 1.

How much should you risk per trade?

There is no universal number, but many traders work with somewhere between 0.5% and 2% of their account per trade, and 1% is a common convention. The reason is losing streaks. Every strategy has them, and the risk percentage decides how deep the hole gets:

The table compounds each loss on the shrinking balance. The asymmetry in the last column is the point: deep drawdowns need much larger gains to recover, so small fixed risk buys you time to be right.

What the formula does not cover

• Slippage and gaps. A stop is an instruction, not a guarantee. Fast markets fill stops at worse prices, and overnight gaps, especially around earnings, can jump straight past them. Size down when gap risk is high.
• Buying power and leverage. A very tight stop can produce a position larger than your account can hold. Cap the size by buying power or margin, not only by risk.
• Correlation. Three long positions in semiconductor stocks at 1% each behave much like one 3% bet on semiconductors.
• Volatility. Volatility-based stops, for example a multiple of the Average True Range (ATR), place the stop outside normal noise. Because the size is derived from the stop, a wider stop automatically means fewer shares, which is exactly what you want in a volatile market.
• Commissions. On small accounts and frequent trading, include them in the risk per unit.

Example 4: sizing with an ATR-based stop

Volatility-based stops plug straight into the same formula. Say a stock trades at $80 and its 14-day Average True Range is $2.00. You place the stop two ATRs below entry, at $76, a distance of $4.00. With $250 of risk, the size is $250 ÷ $4.00 = 62.5, so 62 shares. If the same stock were twice as volatile, with an ATR of $4.00, the stop would be $8.00 away and the size would halve to 31 shares. The dollar risk stays at $250 either way; the position adapts to the market.

Fixed fractional versus fixed dollar risk

Fixed fractional sizing recalculates the dollar risk from the current balance before each trade: risk shrinks after losses and grows after wins. Fixed dollar sizing keeps the same dollar risk until you deliberately change it, which makes results easier to compare. Both are valid; what matters is choosing one and applying it consistently. Record it in your trading journal so you can tell sizing effects apart from strategy effects.

Common position sizing mistakes

• Choosing the size first and the stop second, then placing the stop wherever the dollar loss feels acceptable rather than where the trade idea is proven wrong.
• Ignoring the futures multiplier.
• Rounding up. Always round down; the leftover is a small safety margin.
• Widening the stop after entry. It silently increases risk beyond the plan.
• Adding to a losing position without recalculating the total risk.
• Sizing up after a loss to win it back. That is the core of revenge trading.

Position sizing in Nummius

In the Nummius trading journal, the Open Trade dialog suggests a size from your account balance, your risk-per-trade setting and the entry and stop prices, using the contract’s point value for futures, so an ES trade is sized as an ES trade. The same formula is used on the backtesting charts. The pre-trade plan records entry, stop and target before you click, which also gives you the planned risk-reward ratio of every trade. See the pricing page for what is included in the free and Core plans.

Key takeaways

• Set the stop from the chart, then derive the size from the stop.
• Position size = dollar risk ÷ (stop distance × point value), rounded down.
• Keep risk per trade small enough to survive a long losing streak.
• Gaps, slippage, correlation and leverage can all push real losses above the plan.

This article and calculator are for educational purposes only and are not financial advice. Position sizing limits risk but cannot remove it: slippage and price gaps can cause losses larger than planned. Check contract specifications and margin requirements with your broker.