Lot Size: The Stop Decides It
Lot size is the quantity of an instrument that one unit of your order represents. In foreign exchange the conventions are a standard lot of 100,000 units of the base currency, a mini of 10,000 and a micro of 1,000. It decides what each pip is worth.
The lot is the quantity, and the quantity is the risk. Everything else follows from that.
How it works
A lot is the quantity of the instrument that one unit of your order represents. Buy one lot and you have bought whatever that lot is defined as.
In forex the sizes are fixed by convention. A standard lot is 100,000 units of the base currency of the currency pair, a mini lot 10,000, a micro lot 1,000 and a nano lot 100. Brokers differ in which they accept.
And the lot decides what each pip is worth. That is the whole function: a larger lot does not improve the analysis, it only makes every pip more expensive.
Size is an output, not an input
So size comes from the stop, never from the balance. Set the loss you will accept from your risk management rules, place the stop loss, then divide.
Pick the lot first and you have picked your risk blind. Fixing the money-per-pip before the stop leaves the loss to whatever level the stop lands on. That is the market deciding, not you.
Rounding to a whole lot on a small account distorts it. If the correct answer sits between the sizes your broker offers, granularity decides the outcome.
And leverage only decides what you are allowed to do. It sets the maximum a margin account permits — a ceiling, not a recommendation.
In practice
A large lot in a thin market is a different fill. Volume decides how much size the book absorbs quietly, and what fills cleanly at midday may not at the open.
The same lot is a different risk on a slower chart. A daily bar travels further than a five-minute one, so a size copied across timeframes changes what it can lose.
And a gap multiplies the loss by the lot size. An opening gap can jump straight past a stop; the size is the defence, not the stop.
Stop distance and lot size are one decision, not two. Stop loss placement sets the distance and the distance sets the size.
And costs scale with the lot, at 2% of a bar. Every execution error scales with it too, in proportion to the size behind it.
The contract size is fixed; what you risk is not. In futures the exchange defines the contract, so the number of contracts is your only lever.
Working the number in words
Start with the money, because it is the only figure you control outright. Take your trading capital, apply the fraction you have set as your risk per trade, and you have the amount a losing trade may cost.
Then measure the stop distance in pips, either from structure or from a volatility measure such as the 14-bar average true range (ATR). That distance belongs to the chart rather than to your preference.
Then divide twice. Permitted loss divided by stop distance gives the money you can afford per pip. That figure divided by the value of one pip for the lot type you are trading gives the number of lots to order.
The pip value is the part this page cannot fill in for you, because it depends on the pair, the quote currency and the prevailing rate. Read it off your platform rather than assuming it.
What lot size is not
It is not leverage. Leverage is the permitted maximum; the lot is what you chose.
It is not a measure of conviction. A bigger lot on a better idea only makes the loss bigger.
It is not a fixed personal setting. Conditions move, and the correct lot moves with them.
It is not margin. Margin is what the position ties up; the lot is what it risks.
When it fails
The market was quiet
In a quiet market an oversized lot feels fine. Inside a trading range the bars are small and the size is never tested.
The bars got bigger
Bar ranges on this site’s shared 576-bar history run from 0.17 at the tenth percentile to 1.101 at the ninetieth, a ratio of 6.5. A lot suited to one end of that is wrong at the other.
The granularity was too coarse
A small account often cannot express the correct size in the lots offered. Rounding then decides your risk, and rounding up is the expensive direction.
The stop moved, the lot did not
A trailing stop changes the distance while the position is open. Trailed by a multiple of the average true range, the median position here survived 3 bars at one range, 10 at two, 22 at three and 32 at four.
The cost was counted once
A round trip here costs 0.0098 price units, 2% of a median bar’s range and 45% of the smallest. That is per lot, every trade, win or lose.
A gap opened past the stop
A stop is an instruction, not a floor. Price can open beyond it, and the loss is then the gap multiplied by the lot.
The original data
In research/broker-coverage.json, a scan of the 31,760 videos in
research/search-study-corpus.jsonl, “lot size” appears in 17 titles at a median of 74,124 views
across 13 channels, with a maximum of 1,351,533. “Position sizing” appears in 163 titles at a
median of 1,722 across 135 channels. Ten times the supply and a fortieth of the median audience,
for the same idea under two names: the beginner’s phrasing is what people search, the professional
phrasing is what gets published. Neither “risk of ruin” nor “expectancy” appears in a single title.
And research/series-measurements.json, built by site/measure_series.py, sizes the part that
moves. On the same history the 14-bar average true range has a median of 0.5994, a tenth
percentile of 0.2823 and a ninetieth of 0.7954 — a ratio of 2.82 between quiet and active
conditions. A lot sized in a quiet week is therefore about three times too large once conditions
turn active. So recompute the size from current volatility before
every trade: carrying yesterday’s lot into today’s market changes your risk without deciding to.
Related
Risk per trade is the fraction that feeds this calculation, and the only number in it you choose outright.
Leverage is the ceiling the broker sets, which is why it is so often mistaken for the size to take.
And forex is where the standard, mini and micro conventions come from, along with the pip value this page deliberately leaves blank.
For a long time I traded the same size out of habit, and I would have told you it was discipline. It was not. It was that I had picked a number early on, it had not hurt me yet, and re-deriving it every morning felt like busywork next to finding the next setup. What changed it was noticing that on the days the market was moving fastest, my ordinary size was quietly the largest bet I had ever placed.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.