Position Sizing: How Much Should You Put in One Stock?

Most investors decide how much to buy by feel. A round number, whatever cash is free, or how confident they are that day. A more reliable method starts from the other end: how much am I willing to lose if I am wrong? Once that is fixed, the amount to buy is arithmetic. This note is the practical how-to, and it builds on the principles in our earlier note on position sizing.

The formula

Quantity = (Capital x Risk per trade) divided by (Entry price minus Stop price)

Risk per trade is the share of your total capital you accept losing on a single idea if it goes wrong. Many careful investors keep it between 0.5 and 2 percent. The stop price is where you will exit if the idea is wrong, and you decide it before buying.

A worked example

Suppose your capital is Rs 10 lakh and you accept a risk of 1 percent, which is Rs 10,000. You plan to buy at Rs 500 and exit at Rs 460 if you are wrong. The risk per share is Rs 40. Dividing Rs 10,000 by Rs 40 gives 250 shares. The position costs Rs 1,25,000, which is 12.5 percent of your capital. If the stock falls to your stop, you lose Rs 10,000, which is 1 percent of the portfolio, as planned.

Why the stop distance changes everything

The same Rs 10,000 of risk produces very different position sizes depending on how far away the stop is.

A tight stop lets you hold a larger position for the same risk. A wide stop forces a smaller one. This is why volatile stocks, which need wider stops, usually deserve smaller positions. Many investors get this backwards, sizing every stock the same and then discovering that a volatile one hurts far more.

Add two caps

The formula is a floor for caution, not a full system. Add two limits.

• A maximum position size, such as 10 to 15 percent of capital, whatever the formula says. A very tight stop can suggest an oversized position, and gaps can jump past a stop.
• A limit on total open risk. If every position risks 1 percent and you hold twelve, a bad market day can hit many at once. Many investors cap the total at 5 or 6 percent.

Common errors

• Sizing by conviction. Strong belief feels like a reason to buy more, but conviction is not a measure of risk.
• Sizing by price. Buying 100 shares of everything ignores how much each can move.
• Sizing by leftover cash. The amount you happen to have free is not a risk decision.
• Moving the stop after buying. Widening it to avoid being stopped out silently raises your risk. After a loss, the urge to size up and win it back is the pattern in revenge trading, and sizing rules are the defence.

Recalculate as your capital changes

The formula uses your current capital, so it adjusts by itself. After a 20 percent fall in a portfolio of Rs 10 lakh, capital is Rs 8 lakh and a 1 percent risk is Rs 8,000, not Rs 10,000. Position sizes shrink after losses and grow after gains. That is intended. It slows the damage in a bad stretch and keeps you from betting more just when your judgement is most shaken.

If you add to a position later, treat the total as one idea. Combine the cost of both purchases and make sure the stop on the whole position still risks no more than your limit.

Build a one-page size sheet

List your capital, your risk per trade, your maximum position size and your total risk cap. For each new idea, write the entry, the stop, the quantity and the rupee loss if the stop hits. It takes two minutes and removes most size mistakes. It is the same idea as carrying the umbrella before the rain.

Common questions

Does this apply to long-term investing?

Yes, in a different form. Long-term investors often set a maximum weight per holding, and a review point where the thesis is tested, instead of a tight price stop.

What if the stock gaps below my stop?

The loss will be larger than planned. That is one reason for the maximum position cap and for keeping risk per trade small.

All figures are hypothetical and exclude brokerage and taxes.