Position Sizing: A Practical Guide to Risk Budgeting

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Position sizing decides how much capital a trade is allowed to expose before the order is placed. It cannot make a weak setup profitable, but it can stop one ordinary loss from becoming a portfolio-level problem.

This guide uses a 1% risk budget as a worked example. One per cent is not a universal rule or a recommendation. Every trader must choose a limit that fits their capital, instrument, experience and ability to absorb loss.

Learning objective

By the end of this lesson, you should be able to convert a chosen cash risk into a theoretical position size using the distance between entry and stop-loss.

The three numbers you need

1. Account value: the capital used to calculate the risk budget.
2. Chosen risk percentage: the maximum planned loss if the stop works as expected.
3. Risk per unit: the distance from entry to stop, adjusted for the instrument's point, tick or contract value.

The basic framework is:

Risk budget = account value × chosen risk percentage

Theoretical position size = risk budget ÷ risk per unit

The answer is theoretical because fees, spread, slippage, gaps, currency conversion and contract specifications can increase the real loss.

Worked share example

Assume a hypothetical trading account of £10,000. The trader chooses to risk 1% on one idea, so the cash risk budget is £100.

The planned entry is £50 per share and the stop is £48. The price risk is therefore £2 per share.

£100 ÷ £2 = 50 shares

Fifty shares is the upper theoretical size before costs. If estimated fees and slippage add £10 of risk, the usable price-risk budget becomes £90 and the size falls to 45 shares. Rounding down keeps the planned exposure within the budget.

Put the stop before the size

A stop should sit where the trade idea is invalidated, based on market structure or a tested method. Moving the stop closer merely to buy more units reverses the process and can place it inside normal market noise.

For a long trade, risk per unit is normally entry minus stop. For a short trade, it is stop minus entry. Futures, CFDs, options and leveraged products require the correct point value, contract multiplier, margin rules and potential gap risk; a simple share calculation is not enough.

Portfolio risk matters too

Five positions risking 1% each do not always represent five independent risks. UK bank shares may fall together; gold miners can respond to the same gold and currency move; technology shares and Bitcoin can become correlated during a broad risk-off event.

Track total open risk and concentration by theme, sector and catalyst. A portfolio can be oversized even when every individual position follows its own limit.

Common mistakes

• Choosing the position first and inventing the stop afterwards.
• Treating required margin as the maximum possible loss.
• Ignoring spread, commission, slippage and overnight gaps.
• Using the same unit size for instruments with different volatility.
• Increasing size after a losing streak to recover money quickly.
• Adding correlated trades without checking combined exposure.

Pre-trade checklist

• Is the entry condition defined?
• Does the stop mark a real invalidation point?
• What is the cash risk after costs and likely slippage?
• Is the contract or point value correct?
• Does the rounded-down size stay within the chosen budget?
• What is the total risk across open positions?
• Could an earnings release, central-bank decision or weekend gap bypass the stop?

The Money Man lesson

Position sizing is a control process, not a prediction tool. Define the invalidation point, calculate the cash loss, reduce for costs and round down. If the resulting size feels too small, the answer is not automatically a tighter stop; the setup may simply offer an unattractive relationship between risk and potential reward.

Educational information only. This example is hypothetical and does not account for every product rule, tax, fee or market condition. Stop orders can fill at a worse price than requested, and losses can exceed the planned amount, especially with leverage.