Compound Interest Calculator for Traders
See what a monthly return compounds to over time, and why the smooth curve never happens in a real account.
Rayner5 min read24 September 2026A compound calculator shows what happens when returns are left to build on themselves. The maths is the most quoted and least reliable thing in trading: quoted because the curve looks impressive, unreliable because it assumes every month is the same.
The tool below does the arithmetic, including regular deposits. Under it are the formula, what a realistic monthly return looks like, the effect of a single bad month, and why compounding works better as a reason to protect capital than as a plan for getting rich.
A compound calculator applies a percentage return to a balance each month and reinvests it. A 1,000 dollar account growing 3 percent a month reaches about 2,033 dollars after two years. The arithmetic is simple, but a real account never returns the same percentage every month, so the result is a shape to understand rather than a target to hit.
The calculator
The table shows the balance at intervals through the period. Try changing the monthly return by half a percent and watch what it does to the final figure over two years.
The formula
| Item | Calculation |
|---|---|
| Balance after n periods | Starting balance multiplied by one plus the rate, raised to the power of n |
| With regular deposits | The same, plus each deposit compounded for the months it was invested |
| Equivalent annual return | One plus the monthly rate, raised to the power of twelve, minus one |
A 3 percent monthly return is not 36 percent a year. Compounded, it is about 42.6 percent, which is the number the calculator shows.
What is realistic
| Monthly return | Annual equivalent | Assessment |
|---|---|---|
| 1 percent | About 12.7 percent | Good, and beats most funds |
| 2 percent | About 26.8 percent | Very good, and hard to sustain |
| 5 percent | About 79.6 percent | Exceptional, and rarely repeated across years |
| 10 percent | About 213 percent | Usually a sign of oversized positions |
Professional funds that return 15 to 20 percent a year are considered excellent. Any promise of a steady 10 percent a month should be read as a warning rather than an opportunity, as covered in forex signal scams.
What one bad month does
| Sequence over 12 months | Ending balance from 1,000 |
|---|---|
| 3 percent every month | About 1,426 dollars |
| 3 percent for 11 months, minus 20 percent once | About 1,108 dollars |
| 3 percent for 11 months, minus 40 percent once | About 831 dollars |
A single oversized loss can wipe out most of a year. That asymmetry is the real lesson of compounding, and the arithmetic behind it is in the drawdown calculator.
Why the curve never looks like this
A trading account does not return a constant percentage. It produces a run of small wins, a loss that removes several of them, a quiet month with nothing worth trading and an occasional very good week. The end point can still match the curve. The path never does.
The practical consequence is that judging progress against a smooth compounding line creates pressure to trade in conditions that do not suit the strategy, which is how a good month is usually followed by a bad one.
Withdrawals and the curve
Money taken out of an account stops compounding, which is not a problem but does change the picture. A trader withdrawing profits every month has a flat balance and a growing bank account, and the compounding curve never appears at all.
| Approach | Effect |
|---|---|
| Reinvest everything | The fastest growth on paper, and the largest position sizes |
| Withdraw a share of profits | Slower growth, and the money is safe from a later drawdown |
| Withdraw everything above the starting balance | No compounding, constant risk per trade in money terms |
Adding capital
| Approach | Balance after 24 months at 2 percent a month |
|---|---|
| 1,000 start, nothing added | About 1,608 dollars |
| 1,000 start, 100 added each month | About 4,650 dollars |
| 1,000 start, 250 added each month | About 9,210 dollars |
Figures are illustrative. For a small account, regular deposits matter far more than the return rate, and they do not require taking more risk per trade.
Using compounding sensibly
- 01Target a process, not a percentageA monthly return target forces trades in quiet markets.
- 02Compound slowlyRaise position size only after the balance has actually grown.
- 03Protect the downsideOne large loss costs more months than a good month adds.
- 04Count withdrawalsMoney taken out stops compounding, which is fine, but the curve changes.
Keep reading
- Drawdown calculator the other side of the curve
- Lot size calculator sizing as the balance grows
- Forex signal scams why steady returns are promised
- Trading tools every calculator on the site
Frequently Asked Questions
How does compounding work in trading?
Each period return is applied to the balance built so far, so gains earn gains. The effect grows with time rather than with the size of any single win.
What is a realistic monthly return?
One to two percent a month, compounded, is a strong long term result. Anything promising a steady ten percent a month is not credible.
Is 3 percent a month the same as 36 percent a year?
No. Compounded it is about 42.6 percent a year, because each month builds on the last.
Should I increase my lot size as the account grows?
Gradually, and only in proportion to the balance. Risking a fixed percentage per trade does this automatically.
What does one bad month do to compounding?
A great deal. A single 20 percent loss can remove most of a year of steady gains, because the recovery has to come from a smaller base.
Do deposits matter more than returns?
For a small account, usually yes. Regular deposits grow a balance faster than a higher return, without any extra risk per trade.
Can I use this as a forecast?
No. It shows the shape of growth under an assumption no account meets. Treat it as a way to understand the maths, not as a plan.
