Share
A unit of ownership in a company.
Also called: stock · equity
Written by Javier Sánchez Ros
In plain language
Owning a share means owning a fractional claim on a company’s assets and future earnings, along with whatever voting rights the share class carries.
Share price alone says nothing about whether a company is large or small. A $8 stock can be worth more in total than a $600 one — market capitalization is what determines size.
For a trader, a share is simply a divisible unit of exposure. Because most equities trade in whole shares, position sizing usually rounds down.
Worked through
A calculation that returns 183.7 shares
- Exact size
- 183.7 shares
- Rounded down
- 183 → risk $499.06
- Rounded up
- 184 → risk $501.78
- Risk budget
- $500.00
Round down and the risk is $499.06, a fraction under budget. Round up and it is $501.78, a fraction over. On one trade the difference is $2.72 and nobody would care.
It matters because of which direction the habit points. Rounding down means every position is at or below the intended risk, so the account can only ever drift conservative. Rounding up means every position is slightly over, and across hundreds of trades a 1% rule is quietly being run at 1.02%.
The effect is small and it is free to avoid, which is the argument for making it a rule rather than a judgement. Any place where a habit can only err in one direction, choose the direction that cannot hurt you.
Fractional shares remove the rounding but not the principle: you still cannot buy a fraction of a futures contract, and on small accounts the granularity problem simply reappears somewhere else.
Why it matters
Whole-share trading means your calculated position size rarely lands on an integer. Rounding down keeps your actual risk at or below your intended risk.
Common mistakes
- Judging whether a stock is cheap by its share price rather than its valuation.
- Rounding position size up, which pushes real risk above the planned limit.
Keep exploring
These concepts are connected. Understanding one usually makes the next one easier.
The total market value of a company’s shares — share price times shares outstanding.
The amount of an asset you buy or sell in a single trade.
The number of shares actually available for public trading.
A cash payment distributed to shareholders out of company profits.
The short code that uniquely identifies a tradable instrument on an exchange.