Imagine a delivery driver telling his boss:
“Don’t worry. I always carry ten boxes. Very consistent.”
Today, the boxes contain pillows.
Tomorrow, they contain bricks.
Same number of boxes.
Very different afternoon for the suspension.
The driver is measuring quantity.
He should also be measuring weight.
Traders make a similar mistake.
“I always trade one lot.”
“I always buy 100 shares.”
“I never change my contract size.”
That sounds disciplined.
But a fixed position size does not mean a fixed amount of financial risk.
The instrument can change. Your stop distance can change. The money gained or lost for each price movement can change.
If you want consistent risk, the quantity often needs to change.
That is the principle behind position sizing.
What Is Position Size in Trading?
Position size is the quantity you trade: shares, contracts, units or lots.
It answers:
“How much am I buying or selling?”
Your planned trade risk answers a different question:
“How much money would I lose if I exited at my planned stop?”
Connecting those two questions is how you calculate an appropriate position.
CME’s position-sizing guidance begins with your stop location and the cash or percentage amount you are willing to risk. You then calculate the quantity that fits those conditions. www.cmegroup.com
The sequence is simple:
Find the invalidation point → set the cash-risk budget → calculate the quantity.
The quantity comes last.
Why Trading One Lot Can Mean Different Risks
A lot is a unit of trading quantity.
It is not a universal amount of money at risk.
Its meaning depends on the instrument and the broker’s contract specifications.
Even on the same instrument, one lot can create different planned losses when your stop changes.
Consider a hypothetical contract worth $10 per point per lot.
| Position | Stop distance | Planned loss before costs |
|---|---|---|
| One lot | 5 points | $50 |
| One lot | 20 points | $200 |
| One lot | 50 points | $500 |
The trader did not increase the lot size.
The planned loss still increased tenfold.
That is the problem with treating “one lot” as a risk rule.
It controls the number of boxes.
It ignores their weight.
Consistency means controlling what you can lose—not always trading the same quantity.
The Position-Sizing Formula
For a straightforward position whose value changes proportionally with price:
Position size = cash-risk budget ÷ planned loss per share, unit or contract
For shares:
Number of shares = cash-risk budget ÷ distance between entry and stop
For a product quoted in points:
Number of lots = cash-risk budget ÷ (stop distance × monetary value per point per lot)
If your platform uses ticks or pips, use those units consistently.
Account for relevant costs and currency conversion. Round the result down to a permitted trading increment.
These formulas estimate the loss at your intended exit price. They do not guarantee the eventual loss: gaps and execution slippage can produce a worse fill.
Example: Calculating a Stock Position
Suppose your planned cash-risk budget is $120.
Your entry is $52.
Your setup becomes invalid at $48.
That gives:
$52 − $48 = $4 of planned risk per share
Then:
$120 ÷ $4 = 30 shares
At those prices, 30 shares would create a planned loss of $120 before costs.
Now consider another setup with the same entry, but a valid stop at $50.
The distance is $2.
$120 ÷ $2 = 60 shares
You can take twice as many shares while keeping the same planned cash loss.
| Setup | Entry | Stop | Risk per share | Shares | Planned loss |
|---|---|---|---|---|---|
| Wider stop | $52 | $48 | $4 | 30 | $120 |
| Narrower stop | $52 | $50 | $2 | 60 | $120 |
The second position is larger.
Its planned stop-loss amount is the same.
That distinction is the whole point.
However, the two positions are not identical in every respect. The larger position has greater market exposure and may behave differently during a gap.
Equal planned stop risk does not mean equal exposure to every possible event.
Example: Calculating Lots for an Index Trade
Imagine a hypothetical index product where:
- One lot gains or loses $10 per point.
- Your stop is 25 points from entry.
- Your cash-risk budget is $100.
- The platform permits increments of 0.1 lot.
First calculate the planned loss for one lot:
25 points × $10 = $250
Then calculate the quantity:
$100 ÷ $250 = 0.4 lot
At your intended stop price, 0.4 lot would lose $100 before costs.
Now suppose another valid setup needs a 40-point stop.
40 × $10 = $400 per lot
$100 ÷ $400 = 0.25 lot
But the platform only allows increments of 0.1.
Round down to 0.2 lot.
The planned loss becomes:
0.2 × $400 = $80
Do not round up to 0.3 simply because it is closer.
That would create $120 of planned loss and exceed your $100 budget.
Your risk budget is a ceiling. You do not have to spend all of it.
These contract values are illustrative. Check your own symbol’s specifications before using the calculation.
Margin Is Not Your Stop-Loss Risk
This is another expensive misunderstanding.
Your platform says a position requires $200 of margin.
You conclude:
“I’m risking $200.”
Not necessarily.
Margin is the amount required to support the position. Your profit or loss depends on the exposure and price movement.
Return to our delivery truck.
The deposit required to rent the truck does not tell you how heavy its cargo is.
Likewise, a low margin requirement does not mean the position has a small potential loss.
Before entering, distinguish three numbers:
| Number | What it tells you |
|---|---|
| Position exposure | How much underlying market value you control |
| Required margin | What the platform requires to support the position |
| Planned stop-loss risk | What you expect to lose if your intended exit executes as planned |
They answer different questions.
“The broker lets me open it” is not evidence that the position fits your risk plan.
Don’t Move the Stop to Fit the Size You Want
Suppose your setup needs a stop 30 points away.
The position you want would risk too much.
You have a straightforward solution:
Reduce the quantity.
Instead, some traders move the stop to ten points away.
Now the numbers fit.
But does the trade?
If the tighter stop sits inside movement your setup normally needs to tolerate, you have changed the strategy to accommodate a preferred lot size.
CME specifically advises placing stops at logical levels rather than arbitrary prices or locations easily reached by normal market movement. www.cmegroup.com
The stop should reflect the trade.
The quantity should reflect the budget.
Do not force the chart to justify the quantity.
If even the smallest permitted position exceeds your budget, pass or use a suitable smaller contract.
Some trades are valid opportunities that your account cannot appropriately accommodate.
That is a sizing decision—not a missed obligation.
Include Costs Before Pressing Buy
The basic formula gives you a starting point.
Execution makes it less tidy.
Commissions and spreads affect results. Depending on the product and holding period, financing or other charges may also matter.
Your calculation must also use the correct account currency.
For a simple illustration, if your total planned budget is $100 and estimated relevant charges are $5, you have $95 available for price-based stop risk.
But avoid double-counting costs already incorporated into your assumed executable entry and exit prices.
And do not treat an allowance for slippage as a guarantee.
The aim is a realistic estimate—not mathematical certainty where none exists.
How Much Should You Risk Per Trade?
There is no universally correct percentage.
A percentage rule must fit your strategy’s losing periods, your financial circumstances and your account restrictions.
CME notes that the familiar 2% threshold is arbitrary; it is not a rule proving that every trader should risk that amount. www.cmegroup.com
For a prop account, you also need to consider the remaining distance to its loss limits.
For any account, consider existing positions.
Three trades with the same cash-risk budget may still fail together if they depend on the same market move.
Position sizing begins with one trade.
Risk management continues across the whole account.
A Position-Sizing Check You Can Repeat
Before submitting an order, write down:
- Entry: What executable price am I planning around?
- Invalidation: Where does the setup fail?
- Cash budget: What planned loss is acceptable?
- Unit value: What does one share, lot or contract lose over that distance?
- Quantity: What permitted size fits the budget after costs?
Then verify the platform’s estimated profit or loss at the intended stop, where that feature is available.
If the estimate differs from your calculation, investigate before entering.
Check the contract value, units, currency conversion and order quantity.
A misplaced decimal point does not care how good your chart analysis was.
Consistent Risk Requires Flexible Quantity
Trading the same lot size can feel orderly.
But orderliness is not the same as risk control.
Ten boxes of pillows.
Ten boxes of bricks.
The number remains consistent while the burden changes.
Your job is to measure the burden first.
Let the setup determine the stop.
Let the budget determine the size.
Then accept that some trades require smaller quantities—and some should be left alone.
Don’t decide how much to trade and hope the risk fits. Decide what you can risk, then calculate how much to trade.