Compound Interest
Calculator
Project how a starting balance grows when a fixed return compounds each period, with optional recurring contributions or withdrawals. See the split between what you put in and what compounding added.
Free, no sign-up, no ads. Educational tool — not financial advice.
TL;DR
Compound interest is growth on your growth: each period's return is calculated on the new, larger balance. The future value is FV = P x (1+r)^n + contributions. Small differences in rate or time produce huge differences at the end - but the model assumes a constant return every period, which real markets never deliver. Treat the output as a projection, not a forecast.
How compounding is calculated
Each period, your balance earns the return rate r and then any contribution is added. Repeating this n times gives the future value:
FV = P x (1+r)^n + C x [((1+r)^n - 1) / r]
where P is your starting capital and C is the contribution per period. The chart separates the money you actually put in (the dashed baseline) from the growth that compounding added (the shaded area) - early on they're close, but the gap widens as returns earn returns.
Why rate and time dominate
Because returns compound, both the rate and the number of periods have an outsized, non-linear effect. Doubling the time period more than doubles the result; a couple of extra percent per period can multiply the final value over a long horizon. This is the genuine power of compounding - and also why unrealistic rate assumptions produce absurd projections.
The honest caveat
A constant return every single period is a fiction. Real returns - especially in crypto - are volatile: a few flat or negative periods reset the curve and the smooth exponential never materializes. A 10%-per-month projection looks life-changing on the chart and is almost never achievable in practice. Use this to understand the mechanics of compounding, not to predict your balance.
For a realistic, price-based view of accumulating crypto, use the DCA calculator; for staking yields, the staking calculator.
Frequently Asked Questions
What is compound interest?
It's interest earned on both your original capital and the interest already accumulated. Because each period grows a larger balance, the total grows faster over time than simple interest, which only ever pays on the original amount.
How do contributions and withdrawals work?
Enter a positive contribution per period to add money each period (like recurring deposits), or a negative number to withdraw. Contributions are applied at the end of each period and are tracked separately from compounding growth in the breakdown.
Is a constant return realistic?
No. Real markets are volatile and returns vary every period - a constant rate is a simplifying assumption. This tool is a projection of the math, not a forecast of what an asset will actually do. Treat high per-period rates with heavy skepticism.
What's the difference between this and the DCA calculator?
This tool projects growth from an assumed constant return. The DCA calculator uses real historical prices to show what regular buying would actually have produced - no assumed return. Use compounding for mechanics, DCA for a price-based reality check.
What period type should I pick?
Match the period to your return assumption. If you think in monthly returns, use 'month' with a monthly rate; for annual yields, use 'year'. The period type sets the timeline; the math is identical, just scaled to how often the return compounds.
