Position Size and the Three Limits

Your position size was probably a round number. Here is what that costs. Two people take one trade: same level, same entry, same stop, same target. One of them ends the day down one percent. The other is down six. Nothing about the setup was different. The only thing that differed was how many units each of them bought, and neither of them worked that number out. One took a round number. The other took what felt right after a good week.

By the end of this you will have three numbers, and all three come out of the same piece of arithmetic. How many units this trade is worth. How much the day is allowed to cost. How much the week is allowed to cost. The first you compute on every trade. The other two you decide once, while nothing is happening, and then obey.

Here is the sentence the rest of it hangs on. Position size is an output, not an input. You do not choose it. You calculate it, and the calculation runs from the risk you accepted — not from how much the account can buy, and not from how confident you feel about this particular chart. Everything here runs from risk. Most of the difficulty people have with that is that it sounds too simple to be the answer.

The line itself. Your risk in money, divided by the distance from your entry to your stop in money per unit, equals the number of units. If you used a buffer, that distance includes it. That is the whole formula. If you are risking two hundred and fifty dollars and your stop is fifty cents from your entry, you buy five hundred shares. If the stop is five dollars a share, you buy fifty. The dollars at risk never move. The number of units moves to keep them still.

Work it on Bitcoin. Account twenty five thousand dollars, one percent a trade, so two hundred and fifty dollars of risk. The average daily range over the last twenty days is two thousand nine hundred and five dollars. Say you're filled right at the level, with no buffer, so the stop distance is a tenth of that: two hundred and ninety one. Two hundred and fifty divided by two hundred and ninety one is zero point eight six. You buy zero point eight six of a coin.

Same account, same two hundred and fifty dollars, different instrument. Apple's average daily range over the same twenty days is six dollars and fifty one cents. A tenth is sixty five cents. Two hundred and fifty divided by sixty five cents is three hundred and eighty four shares. Zero point eight six of one thing. Three hundred and eighty four of another. Both risking exactly the same money.

Which means the size of a position tells you nothing about the risk inside it. Two people can hold three hundred and eighty four shares of the same stock and be risking amounts that differ by a factor of ten, because the only thing that sets the risk is the distance to the stop. The number of units is the answer, not the question.

Most people trade more than one instrument, and this is where the arithmetic stops being optional. If you buy the same number of units of two instruments, you are risking different amounts on each, and your results become a mixture you cannot read. A good week in the quiet one gets erased by a single trade in the loud one. That is one way an account with a perfectly respectable win rate still ends the month down.

There is a relationship inside that line worth saying out loud, because it runs the opposite way to instinct. Tighten the stop and the position gets bigger. Widen the stop and the position gets smaller. The money at risk does not change, because it is the thing being held still. A smaller stop is not a smaller trade. It is the same trade in more units.

And that is where the damage gets done, because the relationship also runs in reverse. Decide the size first, and the stop has to move to fit it. That order is backwards, and it is the same reversal this channel keeps arriving at. The invalidation price comes from the market. The distance from your entry to it is read off it. The size is the only free variable, and it is free precisely because it is computed last.

Now the part that gets left out. The arithmetic hands you a number of units. It does not ask whether your account can hold them. Zero point eight six of a coin at eighty thousand dollars is sixty nine thousand dollars of position on a twenty five thousand dollar account. Three hundred and eighty four shares of Apple at three hundred and twenty is a hundred and twenty three thousand.

And it gets stranger, because the calmer the instrument, the larger the position has to be. Over the same twenty days, the S and P five hundred fund moved four dollars and forty one cents a day on a price of seven hundred and seventy. That is a little over half a percent. A tenth of it is forty four cents. Two hundred and fifty divided by forty four cents is five hundred and sixty seven shares — four hundred and thirty seven thousand dollars of position. Seventeen times the account.

Here is what half a percent a day looks like on a chart. Same twenty days, real prices. The days are small, and that smallness is the whole reason the position has to be enormous. To lose two hundred and fifty dollars on a stop of forty four cents, you have to be holding a great deal of it.

Put the three side by side for the same two hundred and fifty dollars. Bitcoin asks for two point eight times the account. Apple, four point nine. This one, seventeen.

So there is a limit the formula cannot see, and it belongs to your account rather than to the market. If the size the arithmetic asks for is more than you can hold, the instrument is not too expensive. It is too calm for the risk you chose. Three honest responses, and you will recognise the shape of them: risk less per trade, take the stop from a longer timeframe, or accept that this instrument is not for this account yet.

That was one trade. The same arithmetic applied to a series gives you the second number. At one percent a trade, three losses in a row cost three percent. That is your day. Not because three is special, but because the number has to be chosen while nothing is happening and you can still count. Chosen during a losing session, it is not a limit. It is a negotiation.

The reason a day needs a limit at all is that the per-trade number says nothing about a run of them. A stop protects one idea. It does not protect you from taking the same idea five times because the first four annoyed you. Of everything in this video, the daily limit is the only part aimed at the trader rather than at the trade.

The third number is the week, and the usual figure is double the day. Six percent. What makes it worth having is the arithmetic of coming back. Down six percent, you need six point four to get level. Down twenty, you need twenty five. Down fifty, you need a hundred. The hole gets steeper than it looks from inside it, and the weekly limit exists to keep you out of the steep part.

One more thing about the money behind the first number, because it is the one people want to grow. The money at risk grows when the account grows. That is the only thing allowed to move it. It does not grow because the last three trades went well, and it does not grow because this particular setup looks better than usual. If the risk per trade stays at one percent, the money at risk follows the account and nothing else — which is exactly what makes the number trustworthy at the moment you have to act on it.

Two places where this stops being clean. The first you've already seen — the size your account can hold. The second is the minimum unit. If the number the formula gives you is smaller than the smallest piece you can buy, you cannot take that trade at that risk — and that is information, not an obstacle. Crypto divides finely and hides the problem. Shares divide into ones. Futures — contracts sold only in fixed, large sizes — do not divide at all, and one contract is often more risk than a small account has any business carrying.

So, the thing to go and do, and it takes twenty minutes. Take the instruments you actually trade. For each one write four things: the price, the average daily range over the last twenty days, a tenth of that as the stop, and your risk divided by that stop. If you enter with a buffer, add it to the stop first. The last column is your size, and it is the only number you need at the moment of the trade.

Then write two more where you can see them: what the day is allowed to cost, and what the week is allowed to cost. Educational content only. Nothing here is financial advice.