Compound interest calculator
Compound interest is growth where each period's gain earns in the next period. Final capital is starting capital times (1 + rate) to the power of the number of periods, plus compounded deposits. At 2% per month, 10,000 becomes 12,682 in a year — and doubles in 35 months.
Enter the starting capital, the return per month, the number of months and the deposit made at the end of each month. The calculator returns the final capital, the total put in, the gain and how many months it takes to double on the rate alone.
To go deeper into each part of this calculation: Minimum win rate calculator · Expectancy · TWR — time-weighted return
What this calculator solves
The calculation that holds up or breaks a trading plan is this one: with the return the strategy actually delivers, where the capital will be in one, two, five years. Not with the best month's return — with the average net of costs and bad months.
It also works backwards: given a goal, what monthly rate is needed. When the answer is 15% per month, the plan has to change, not the rate.
How the calculation works
Final capital without deposits = starting capital × (1 + i)^n, with i the rate per period as a decimal and n the number of periods.
With a deposit D at the end of each period, add D × ((1 + i)^n − 1) ÷ i. If the rate is zero, that part is simply D × n.
Periods to double = ln 2 ÷ ln(1 + i). The rule of 72 (72 ÷ rate in %) is an approximation; the calculator uses the logarithm, which is exact.
Full example
Capital of 10,000, 2% per month, 12 months, 500 deposited at the end of each month. Without deposits: 10,000 × 1.02^12 = 12,682.42. Compounded deposits: 500 × (1.02^12 − 1) ÷ 0.02 = 6,706.04. Final capital: 19,388.46. Total put in: 16,000. Gain: 3,388.46.
Months to double on the rate alone: ln 2 ÷ ln 1.02 = 35.0. At 1% per month it would be 69.7 months; at 3%, 23.4. The difference between 1% and 3% per month is not 'three times more': it is three times the doubling speed.
After the calculation
The rate that goes in here should come from history, not hope. TCC computes each account's real return — TWR and MWR, net of withdrawals and deposits — and that is the number that makes sense to put in the projection.
FAQ
Is the rule of 72 reliable?
It is a good approximation for low rates. At 2% per month it gives 36 months; the exact value is 35.0. At 10%, it gives 7.2 against a real 7.27.
Does the deposit go in at the start or the end of the month?
At the end: it does not earn in the month it is made. If you deposit at the start, the real result will be slightly higher than calculated.