Risk Management Arithmetic: Position Size, Risk-Reward, Drawdown

Before you read: Order Types Explained: Market, Limit, Stop and Bracket

What you will learn

• What position size is, and the generic arithmetic that connects it to dollars at risk.
• How traders frame “risk per trade”, what risk-reward ratio measures, and what it does not.
• What drawdown is, why recovering from a loss takes a larger percentage gain, and the intuition behind risk of ruin.

What does “risk” mean here?

Before a trade, can you say how much money is at stake, and how sure can you be of that number?

In these lessons, risk means the possibility of losing money, and the amount a trader plans for if a trade does not work. That planned amount is an estimate. Prices can gap and orders can fill at worse prices than expected (order types and execution), so the actual loss can be larger than the plan. In leveraged products such as futures, it can exceed the money deposited.

This lesson is arithmetic. It describes how traders frame these ideas. It does not suggest any amount, ratio or percentage for anyone.

What is position size?

Who decides how much a one-point move costs you: the market, or the size of your position?

Position size is simply the number of shares or contracts in a position. The market decides how far price moves; position size decides what each move is worth in money.

Many traders frame position size with one generic formula:

Position size = dollars at risk ÷ risk per unit

Here “dollars at risk” is the amount a trader has planned to put at risk on the trade, and “risk per unit” is the planned loss per share or per contract if the trade is exited at a chosen price, including expected costs.

Illustrative arithmetic only, not a recommendation: a fictional account of $10,000 sets a $100 risk amount for one trade. If the planned loss per share is $0.50, the formula gives $100 ÷ $0.50 = 200 shares. If the planned loss per contract on a fictional futures contract is $50, it gives $100 ÷ $50 = 2 contracts.

The formula shows the relationship; it does not make the plan come true. A gap or slippage can turn a planned $100 loss into a larger one. The fictional futures contract is a leveraged product. Leverage can cause losses larger than the amount invested.

How do traders frame risk per trade?

Why might someone describe a planned loss as a share of the account instead of in dollars?

Some traders express the dollars at risk as a fraction of the account, so the amount scales as the account grows or shrinks. Others use a fixed dollar amount. Both are ways of framing the same question: how much could this one trade cost?

Illustrative arithmetic only: $100 is 1% of $10,000. If the account falls to $9,000, a fixed-fraction framing of 1% would be $90, while a fixed-dollar framing would stay at $100. Neither number is a suggestion; the example only shows how the two framings behave differently as an account changes.

What does risk-reward ratio measure?

If a trade could make twice what it could lose, does that make it a good trade?

The risk-reward ratio compares the planned reward with the planned risk: reward ÷ risk. (Some writers state it the other way round, as risk to reward; it is worth checking which direction a source uses.)

Illustrative arithmetic only: a planned reward of $300 and a planned risk of $100 gives 300 ÷ 100 = 3, often written 3:1.

What the ratio leaves out matters more than what it includes:

• It says nothing about the odds of reaching the reward before the risk. A favorable ratio does not mean a trade is likely to work.
• Both numbers are plans. The reward may not be reached, and the loss may be larger than planned.
• It ignores costs unless they are included in both numbers.

Whether a set of trades makes or loses money depends on the combination of how often trades work, how large the wins and losses are, and costs. No single ratio settles that.

What is drawdown?

Why is a 50% loss not undone by a 50% gain?

Drawdown is the decline from an account’s highest value to a later low, usually stated as a percentage. Recovering from a drawdown requires a larger percentage gain than the loss, because the gain is applied to a smaller balance.

Illustrative arithmetic only:

The rule behind the table: gain needed = loss ÷ (1 − loss). The deeper the drawdown, the steeper the climb back.

Drawdown diagram

Illustrative diagram constructed for teaching. Not real market data.

The diagram shows the arithmetic from the drawdown table drawn as bars, showing how the gain needed to recover grows faster than the loss. It shows arithmetic only and is not real market data.

What is risk of ruin?

How many losses in a row could happen, and what would they do to the account?

Risk of ruin is the chance that losses reduce an account to a point where trading cannot continue. Formal calculations exist, but the intuition is simple arithmetic.

Illustrative arithmetic only: with a fixed $100 risk amount on a $10,000 account, ten losses in a row would cost $1,000, or 10%. With a fixed $1,000 amount, the same ten losses would cost the whole account. Losing streaks are a normal feature of any activity with uncertain outcomes; their length cannot be known in advance. The larger the amount at risk relative to the account, the fewer losses it takes to reach a level the trader cannot recover from.

What this does not tell you about risk arithmetic

• How much anyone should risk, or what ratio anyone should look for.
• How often any trade or method works.
• That a planned loss will be the actual loss; gaps and slippage can make it larger.

Check yourself

Reflect on these; there is no score.

1. Using the formula, what position size results from $200 at risk and $0.40 risk per share? Hint: 200 ÷ 0.40.
2. A trade plan has a 4:1 reward-to-risk ratio. What does that tell you about the odds of success? Hint: nothing on its own.
3. After a 25% drawdown, what percentage gain returns the account to its peak? Hint: 0.25 ÷ 0.75.
4. Why can the actual loss on a trade be larger than the dollars at risk in the plan? Hint: stops trigger; fills can differ.

Last reviewed November 8, 2026. Spotted a mistake? Report an error.