Compound interest
Definition
Compound interest is capital growth where profit stays in the account and starts earning on its own. The trade size is computed from the current balance, so every win makes it larger, and the equity line turns from a straight line into an accelerating curve.
How compounding works
Compound interest is neither a technique nor a strategy but a way of handling profit: it stays in the account instead of being withdrawn. From there the mechanics are simple. If the trade size is set as a percent of the deposit, then after a win the deposit is larger — so the next trade is larger, and so is its profit. Profit starts producing profit.
The other side is just as automatic. After a loss the deposit is smaller, and the next trade shrinks by itself. That is exactly why a fixed percent of the balance is considered the money-management standard: it speeds you up while you grow and holds you back during a drawdown, without any discipline on your part.
What it gives over the distance
A $1,000 deposit, a 2% return per trading session, profit left in the account:
| Sessions | With compounding | Flat $20 per trade |
|---|---|---|
| 10 | $1,218.99 | $1,200.00 |
| 36 | $2,039.89 | $1,720.00 |
| 50 | $2,691.59 | $2,000.00 |
| 100 | $7,244.65 | $3,000.00 |
By the tenth session the gap is nineteen dollars — easy to miss. By the hundredth, compounding delivers 2.4 times more at exactly the same return. Nothing changed except that the profit was never withdrawn.
The same effect in different numbers: 1% a day over 250 trading days is a twelvefold increase in capital. This is why, in money management, boring percentages get discussed more seriously than a week-long sprint.
Putting it to work
Running the options beats guessing at them: the compound interest calculator shows the balance by sessions, days or years and draws the equity curve, with a contributions mode alongside.
Keep in mind that the calculation shows a ceiling, not a forecast. It assumes an even return in every session, which trading never delivers: a real curve runs through drawdowns, and compounding cuts both ways. The point of the calculation is different — to see that the distance matters more than the size of the percent.
How it differs from a flat amount
A flat amount means the trade is always the same: $20 at a $1,000 deposit, at $500 and at $2,000. It is easier to count, but it does not react to the state of the account: during a drawdown the share at risk grows ($20 out of $500 is already 4%), while during growth it holds you back instead.
Compound interest ties the size to the balance. The share at risk stays constant while the amount moves on its own — both up and down. The difference is not in the growth maths but in the fact that the second approach protects the account through a bad stretch without your involvement.
Frequently asked questions
How is compound interest different from simple interest?
Simple interest is always measured against the starting sum: 2% of $1,000 is $20 on the first trade and on the hundredth alike. Compound interest is measured against the current balance, so the trade size grows with the account. Over a short run the difference is barely visible; over a long one it decides everything.
How many sessions does it take to double the deposit at 2% per session?
Thirty-six. The exact figure is 35.003 sessions, but whole winning sessions do not come in fractions, so it rounds up. That is the answer to "why do modest percentages work": doubling comes not from the size of the percent but from the fact that it is applied to an account that has already grown.
Why does the real curve come out worse than the calculation?
Because the calculation assumes the same return every session, while real trading comes with drawdowns. Compounding works downwards too: after a loss the next trade size shrinks, and climbing out of a drawdown is slower than the growth was. The calculation shows a ceiling, not a promise.