You know your biggest size. Do you know what you took last Tuesday?

You can say your biggest position size without looking it up. One lot. Two lots. It comes out fast. Now the harder question. What did you actually take last Tuesday afternoon, after the morning had already gone against you? There are two sizes in your trading. Only one of them is in your head.

The first is the size you believe you take. Steady, round, the one you would say out loud to a friend over tea. The second is the size you took, order by order, on the quiet days and on the other ones. That second number is just as much yours as the first. It is only written somewhere you rarely open.

The distance between the two is not a verdict on you, and it is not something you have to tell anybody. It is a quantity. You can count it this evening, and a counted number stops being an argument you keep having with yourself.

Counting it from your own order history

This is only counting. It changes nothing you have already done and it needs no new app. Many brokers let you download your order history as a file you can open in a spreadsheet. Check what yours offers, and if it does not, the same count can be done off the screen by hand. Half an hour, once, is enough to have the number for the first time.

• Write down the size you believe you take, before you open anything else. One line, with today's date on it. This is the number you are about to test, so it has to be written before you look.
• Pull your last ten closed trades. Not your best ten, not the ones you remember, not this month if this month has been thin. The last ten in order.
• Against each one, write the quantity that actually went through. Where you added to a position, write the largest quantity you held at any one time, not the first order and not what was left at the end.
• Mark each line either at or under your own number, or above it. Two boxes only. Nothing in between.
• Count the marks. Six above out of ten is six out of ten. There is nothing to work out after that.
• Then count it a second way, by the day instead of the trade. On each of your last five trading days, add together every position you took that day, whether or not it was still open at the close. Then, separately, the most you had open at any one moment.

Two things come out of a first count. One is the number itself. If it sits further from your remembered figure than you expected, that is worth knowing and nothing more. You have never been asked to check it before, so there was no reason for it to be accurate.

The other is where the lines above your number sit in the ten. Spread across all of them, or bunched into the same kind of day. Only your own count can say which, and the answer is only about your account.

The number you remember was set on a quiet evening

Think about when you decided your maximum size. Almost certainly on a weekend, or late on an evening with nothing open and nothing moving. That is the sensible time to decide it. It is also the easiest time. When you set it, nothing was open. Nothing was moving. The morning had not happened yet.

Now look at when size is actually chosen. In the few seconds between deciding to get in and the order going through. With the quantity box in front of you and the price moving while you look at it. With the morning still attached to you: a day that has gone well, a day that has gone against you, a position you closed twenty minutes ago. One number is recalled by someone with time. The other is typed by someone without any.

Nothing dramatic has to happen for the two to separate

There is no single order where you sit back and decide to leave your own number behind. There is one order that is slightly larger because the setup looked obvious. Another slightly larger because the previous one worked. Another larger because the previous one did not, and this one is meant to settle the day. Each one made sense at the time.

Added up across a month, they describe a size nobody ever agreed to. That is why the remembered figure survives. It is never contradicted by any single order, only by the total, and the total is the one thing you never look at.

There is a second reason it does not survive a real day. Your number covers one trade. It does not cover three trades open at the same time. It does not cover the whole day added up. And the same quantity puts more money at stake when your exit sits further away.

If the number you wrote down is a per-position number, it says nothing about three positions open at the same time. Whether you want those added together is yours to decide, and the count works either way as long as you decide before you start. Two sides of the same position can also offset each other, so added together they may come to less than either one alone.

Suppose the number you wrote down works out to ₹4,000 a position. Six positions across one afternoon, each at your own number, add up to ₹24,000: ₹4,000 × 6 = ₹24,000. Put your own figure in and the sum takes a minute.

That total is what the afternoon could have cost if every one of the six had gone against you in full. It is not the amount at stake at any single moment, and if the six were taken one after another it was never all in play at once. Work out what share of your own account it is.

That is the only arithmetic on this page. Note what it does not include. We are not telling you that six is too many, and we are not telling you what your number should be. We do not know your number. You do, and counting against your own number is the only comparison that means anything.

What the count gives you and what it does not

If the lines above your number do sit together, that is the part worth keeping. A size you took once is just an order. A size that turns up after the same kind of morning is something you can see arriving next time. You are not looking for an explanation, and you do not need a name for it. You are looking for when.

The honest limit of all this is that counting does not reach into next Tuesday and hold your hand still. What some people find afterwards is that the number in the quantity box is harder not to see while they are typing it. That is all this does, and it may not do that for you.

So keep the two numbers apart, and keep them both. The size you believe you take is a statement about yourself. The size you took is a record of what happened. When the record and the statement disagree, you do not have to pick a side tonight, and you do not have to promise anybody anything. You only have to know the gap, in your own figures, written in your own hand.

Next Tuesday afternoon the question will not be what your maximum is. You already know that. The question will be whether you know what you did the last time the afternoon looked like this one.