Balance over time
The curve bends upward because interest compounds on interest. The table below is the same data year by year, for anyone who came looking for a specific one.
Total balance, end of each year
| Year | Paid in | Interest | Balance |
|---|
How it works
Interest is calculated on the balance at the end of every compounding period and added to it, so the following period earns interest on a larger amount. Contributions are added monthly and then compound for whatever time remains in the schedule, exactly like the original deposit.
A = P × (1 + r ÷ n)^(n × t) — lump sum, no contributions
r = annual rate ÷ 100, n = compounding periods per year, t = years
With monthly contributions there is no closed form that stays readable, so the schedule is simulated month by month: interest accrues on the running balance at an effective monthly rate derived from your chosen compounding frequency, then the month's contribution is added. One loop, auditable line by line — and the headline figure is simply the last row of the table printed above.
Worked example
$10,000 to start, $200 added every month, 6% annual interest compounded monthly, over 20 years.
- Effective monthly rate: 6% ÷ 12 = 0.5%.
- Compounding periods: 20 × 12 = 240 months.
- The deposit alone: $10,000 × 1.005240 = $33,102.
- The top-ups: $200 × (1.005240 − 1) ÷ 0.005 = $92,408.
The balance grows to $125,510. Of that, $58,000 was paid in and $67,510 is interest — more than everything contributed on top of the initial deposit.
Common questions
What is compound interest?
Interest calculated on both the original principal and the interest already earned. Each period's interest joins the balance, so the next period earns interest on a larger figure — growth accelerates the longer it runs.
How does compounding frequency change the result?
More frequent compounding earns slightly more, since interest is added to the balance sooner and starts earning its own interest sooner. The gap between annual and daily compounding is usually small at everyday savings rates, but widens at higher rates and over longer periods.
How are monthly contributions handled?
Each contribution is added at the end of its month and compounds for whatever time is left in the schedule, exactly like the initial deposit. That's why regular contributions compound too, not only the starting amount.
Is this the same as an investment return?
This calculator assumes a fixed, known rate applied consistently. Real investments fluctuate year to year, so treat the result as a projection under a constant-rate assumption rather than a guarantee.