Compound Interest Grapher
Compound Interest Calculator — Interactive Graph
What this calculator does
This calculator shows how compound interest grows a balance over time. Use the slider to see how the timeline changes.
The math, with a worked example
Each month: balance = (balance + monthly addition) × (1 + annual rate ÷ 12 ÷ 100). The loop repeats for the chosen number of years. Starting with $1,000, adding $200 per month, and earning 7% annually, the balance reaches about $37,000 after 10 years.
Why this matters
Compound growth is boring for years and then absurd: $1,000 plus $200 a month at 7% is about $37,000 after a decade, and the curve steepens every year after. The graph exists because the intuition fails — early money is worth multiples of late money.
Methodology and transparency
Each month: balance = (balance + contribution) × (1 + annual rate ÷ 12). The rate is the assumption that owns the output: 7% reflects a long-run diversified-equity average, not a promise, and real returns arrive as lumpy years, not a smooth curve. Run it at 4% and 10% to see your range. Everything runs in your browser; no data is sent or stored.
Last reviewed: 2026-08-08
Common questions
What does the slider do?
It updates the chart in real time as you change the number of years.
Is this monthly compounding?
Yes. The calculator assumes monthly contributions and monthly compounding.
How does this apply to investing?
Use the annual return as your expected average market return. The graph shows the power of consistent contributions over time.