Position size is arithmetic, not judgement. Take the money you are willing to lose on the trade, divide it by the distance from your entry to your stop, and convert the answer into shares, lots or contracts. Conviction never enters the calculation — it is not a number, and it cannot be measured.
Most traders reverse the order of operations. They decide how much to buy first, place a stop somewhere afterwards, and end up with a risk figure they never chose. Sizing from risk runs the other way: the loss is fixed before anything else moves, and the size is whatever the stop distance permits.
The formula
| Step | What you decide |
|---|---|
| 1 · Risk amount | A fixed fraction of the account, in dollars |
| 2 · Stop distance | Entry price minus invalidation price |
| 3 · Position size | Risk amount ÷ stop distance, in trading units |
Only step one is a choice you make repeatedly, and most traders settle it once — commonly at 1% or less, for the survival reasons set out in the 1% rule in trading. Step two comes from the chart, not from you: it is the distance to the level that proves the idea wrong, which is why writing the invalidation before entry has to happen first. Step three is division.
Worked example: stocks
A $20,000 account risking 1% has $200 on the line. The setup is a reclaim of $48.20 with invalidation at $47.40, so the stop distance is $0.80 per share. Position size is $200 ÷ $0.80 = 250 shares, a $12,050 position. If the stop fills as intended, the loss is $200 — 1% of the account, exactly as decided.
Change one variable and watch the size move without the risk moving. If the invalidation sits at $46.60 instead, the stop distance doubles to $1.60 and the size halves to 125 shares. The wider stop does not cost more; it buys fewer shares. That is the whole point of the method — the risk is constant and the size absorbs the difference.
Worked example: forex
Forex substitutes pip value for per-share distance. On a standard lot of EUR/USD, one pip is worth $10; on a mini lot, $1; on a micro lot, $0.10. With $200 of risk and a 25-pip stop, the maximum loss per standard lot would be 25 × $10 = $250, which is already too much. Step down: at $1 per pip, 25 pips costs $25 per mini lot, so $200 ÷ $25 = 8 mini lots.
The mechanics of lots and the pip-value arithmetic behind them are covered in what is a lot size in trading, and you can check your own numbers against the lot size calculator before the session starts.
Worked example: futures
Futures are the least forgiving case because the contract size is fixed and you cannot buy half of one. The E-mini S&P 500 (ES) has a contract unit of $50 × the index, and a minimum price fluctuation of 0.25 index points, worth $12.50 per contract (CME Group, E-mini S&P 500 contract specifications). A 10-point stop is therefore 40 ticks, or $500 of risk per contract.
With $200 of risk, one ES contract is impossible — $500 is two and a half times the budget. The honest options are a smaller instrument (the Micro E-mini is one-tenth the size, so a 10-point stop costs $50 and four contracts fit), a setup with a tighter invalidation, or no trade. Rounding up to one ES contract because you like the chart is exactly the decision this method exists to prevent.
Why conviction sizing quietly destroys accounts
Sizing up on the trades you believe in sounds like putting money where the edge is. The problem is that conviction is not observable and does not correlate with outcome in any way you can verify. What it does correlate with is recency: confidence peaks after a run of winners and after a move is already well underway.
That produces a specific, predictable failure. Your largest positions land on the trades taken latest, chased hardest, and entered furthest from the level — the trades with the worst reward-to-risk of the week. A single one of them can undo a month, because the loss is not 1% but 4% or 6%, and the recovery arithmetic in drawdown explained is unforgiving above 10%.
There is a second, quieter cost. When size varies with feeling, your results stop being a readable signal. You cannot tell whether a losing month came from a bad edge or from three oversized trades, so nothing in your trading journal can be acted on. Fixed fractional risk makes every trade one unit of the same thing, which is what turns a sample of trades into evidence.
When the answer comes out below one unit
This happens constantly on small accounts and it is where most rules quietly die. There are exactly three legitimate responses:
- Trade a smaller instrument. Micro futures, mini and micro forex lots, and fractional shares where your broker offers them all exist for this. A Micro E-mini is one-tenth of an ES contract, which turns an impossible trade into a routine one.
- Find a tighter invalidation. Sometimes the same idea has a closer level that still genuinely disproves it — an entry on the retest rather than the break, for instance. This is legitimate only if the level is real, not if you moved the stop closer to make the size work.
- Pass. The trade does not fit the account. That is information, not failure.
What is not on the list is taking it anyway at a size your rule forbids. That converts a written system into a suggestion, and it is one of the mistakes that blow up new trading accounts.
Four things that break the arithmetic
- Slippage and gaps. The formula assumes the stop fills at the stop price. It usually does on liquid instruments in regular hours; it does not on illiquid names, on the open, or through news. Treat the calculated loss as the best case, not the guaranteed case.
- Correlated positions. Three trades at 1% each in instruments that move together is one 3% trade wearing a disguise. Size the exposure, not the ticket.
- Fees and financing. Commissions, spread and overnight financing sit outside the stop distance and eat into the planned loss. On small accounts and tight stops they are not a rounding error.
- Moving the stop after entry. Widening a stop invalidates the size you calculated for it. Once you are in, the only direction a stop should travel is toward the entry, which is what a trailing stop formalises.
Frequently Asked Questions
How do you calculate position size from risk?
Divide the money you are willing to lose on the trade by the distance from your entry to your stop, then convert that into the instrument's trading unit. If you will risk $200 and your stop sits $0.80 below entry, you buy 250 shares. The same arithmetic works for forex and futures once you substitute pip value or tick value for the per-share distance.
Should you trade bigger when you are more confident?
Conviction is not a measurable input, so sizing from it means your largest losses land on your most confident trades. Because confidence tends to peak after a run of winners and after the move has already started, conviction sizing systematically puts the most money on the trades taken latest and chased hardest. Fixed fractional risk removes the variable.
What if the position size works out to less than one share or contract?
Take the trade smaller or do not take it. The three legitimate options are a smaller instrument such as a micro futures contract, a setup with a tighter invalidation, or passing on the trade. Rounding up so the trade is possible means accepting more risk than your rule allows, which is the same as having no rule.
Does position sizing change with account size?
The percentage stays fixed; the dollar amount moves with the account. Risking 1% of a $5,000 account is $50 and 1% of a $20,000 account is $200. Because the risk scales down automatically after losses and up after gains, a fixed fraction shrinks your exposure exactly when the account can least afford it.
Bottom line
Sizing from risk is one division problem placed in the right order. Decide the dollars first, read the stop distance off the chart second, and let the size be whatever falls out — 250 shares, 8 mini lots, four Micro E-minis, or nothing at all when the trade does not fit. The discipline is not in the maths, which is trivial; it is in refusing to round up when the answer is inconvenient. Where this sits inside the wider framework is in risk management in trading, and the stop distance it depends on comes from setting a stop loss that isn't just a guess.