Position sizing

In short

You have probably stared at a trade you were about to make and felt a quiet, uneasy question rise up: how much of my money should I put into this one thing? That question has a name, and the name is position sizing. It means deciding, before you place a trade, how many shares or contracts to buy. You base that choice on the total money in your account and the loss you can stand. Get this right and a losing trade stings but does not break you. Get it wrong and one bad day can erase weeks of careful work. These pages walk through the idea plainly and show the math.

The whole of it

What it is

A friend of mine used to say that courage without a plan is just recklessness wearing a brave face. Position sizing is the plan. It is a rule you set for yourself ahead of time, and it answers one question. What is the most of my account I am willing to lose on a single trade? Notice that this is not about how much you hope to gain. It is about how much you can lose and still come back to trade another day. That puts survival first and profit second.

You have probably sensed that this is not about picking the right stock or predicting the market. It is a money management choice. You make it after you have chosen what to trade and where you will exit if the trade goes against you. It sits between your idea and your order button. Think of it as a seat belt you buckle before the drive, not after the bump.

How it works

I once watched a man at a county fair spend half his money on the very first ring toss. He did not win. He had nothing left for the rest of the afternoon. Trading without position sizing works the same way, and the fix is simple enough to do on a scrap of paper.

If you are gathering what you need, there are three inputs, and each flows into the next. Your account size is the total money set aside for trading. Your risk per trade is the percent of that account you will lose if the trade goes fully against you. Your stop loss distance is the gap, in dollars per share, between your entry price and the price where you will exit to cut the loss. Those three numbers are all you need.

Now you do two small steps of math. First, the risk amount in dollars equals account size times risk percent. Second, position size in shares equals that risk amount divided by the stop loss distance per share. That is the whole engine. Everything else is trim.

One thing trips people up. The size of a position and the risk of a position are not the same thing. A large position with a tight stop can risk very little. A small position in a jumpy stock with a wide stop can risk a great deal. You control the risk amount, and the share count follows from it.

The numbers, and where to find yours

You have probably wondered whether anyone besides you gets a say in how you size a position. Some do. If you hold a margin account, which lets you borrow from your broker to trade, the borrowing rules come from your broker and from the Financial Industry Regulatory Authority, known as FINRA. FINRA posts its margin rules on its website at finra.org. Your broker must also show you its own margin terms. Read both.

A friend of mine learned the hard way that futures numbers shift. Each futures contract carries a margin requirement set by the exchange where it trades. The CME Group publishes its margin figures at cmegroup.com. They change with market conditions, so check them on the day you trade. If you are looking at options, you pay or collect a premium, and your risk depends on how the trade is built. The Options Clearing Corporation offers educational material at theocc.com, and it is a good first stop. Whatever the product, the dollar limits and percents that rule your account are set by your broker and the exchange, so look them up on their pages rather than trusting a number you heard secondhand.

I think the most freeing thing a person can learn is that one number here belongs wholly to you. No regulator sets your personal risk percent per trade. Many trading books name common starting points, but those are opinions, not rules. What matters is a number you can lose again and again without it changing how you think. Only you know it.

A worked example

You have probably met someone like Maria. She has saved $20,000 in her trading account. After some honest thought, she decides she will not lose more than 1 percent of it on any one trade. So her risk amount is $20,000 times 0.01, which is $200.

She finds a stock at $40 a share. She decides that if it falls to $37, she was wrong, and she will exit. Her stop loss distance is $40 minus $37, which is $3 a share.

Now the division. Maria takes her $200 risk amount and divides it by $3. That gives 66.67, and she rounds down to 66 shares. Rounding down keeps her under her limit. The full value of the position is 66 times $40, which is $2,640. She puts $2,640 to work but risks only a small slice of it.

Say the stock does fall to $37 and she exits. Her loss is 66 times $3, which is $198. That is under her $200 limit, and well under 1 percent of her $20,000. She is still in the game. She can take the next trade. Keep in mind she only gets that result if she truly exits at her stop.

Where it goes wrong

I once told myself I would move my stop just a little if a trade crept close to it. That is where this whole system most often falls apart. Position sizing only protects you if you honor the stop you planned. Move it once and the math no longer holds. Your loss can grow past the amount you sized for.

A second weak spot is slippage. That means your real exit price is worse than your planned stop price. It happens in fast markets, when the price jumps past your stop before your order fills. It is a real cost. Learn how your broker handles stop orders before you lean on them.

A friend of mine sized each trade with care and still got hurt in one rough week. He had ten open trades, each risking 1 percent. A broad drop hit all ten the same morning. That is $200 lost ten times over on a $20,000 account, or $2,000, which is 10 percent. Some traders set a cap on total risk across all open trades for this reason. It is a personal rule, not a legal one, but it is worth thinking through.

Keeping the rule steady in good times is harder than it sounds. You feel sharp after a winning streak. You nudge the risk percent up. Then one bad trade costs more than a dozen small wins gave you. Watch for that pull. It is strong.

Questions to answer before you leave this page

Before you close this page, it is worth sitting with a few honest questions, because only your true answers will help you here. What is the total dollar amount in your trading account right now, and is it kept apart from the money you need for living costs this month or next? Have you written down, not just thought about but truly written down, the percent of that account you will lose on one trade, and does it feel bearable even if you lose it five times in a row? For the trade you are eyeing today, have you picked a clear price where you will admit you were wrong, and does it come from the chart or just from a round number that felt comfortable? Do you know whether your broker fills a stop order as a market order or a limit order, and have you read its own notes on that point? And if you ran Maria's math with your own account size, your own risk percent, and your own stop distance, does the share count feel right in your gut, or does it feel so small that you already want to override it, since that urge to override is the thing most worth your attention?