Compound interest calculator
Growth of a lump sum plus regular contributions, with the interest shown separately.
How it works
Projects a starting balance plus optional regular contributions forward at a given rate, and separates what you paid in from what the interest added.
Compounding frequency matters less than people expect. At 7%, moving from annual to monthly compounding adds about 0.23 percentage points to the effective rate — real, but small next to the effect of the rate itself or the time invested. What dominates is time: the same monthly contribution started ten years earlier typically ends up worth roughly twice as much, because the early contributions have the longest to compound.
These are nominal figures. Inflation erodes them, and the honest way to read a thirty-year projection is to subtract expected inflation from the rate first — a 7% return with 3% inflation is 4% in purchasing power, which turns a striking number into a realistic one.
This is arithmetic, not advice. It assumes a constant rate, which no real investment has.
This tool runs entirely in your browser. Nothing you enter is sent to a server, logged or stored, and the page keeps working with the network disconnected.
Common questions
- What is the compound interest formula?
- A = P(1 + r/n)^(nt) for a lump sum, where P is the principal, r the annual rate, n the compounds per year and t the years. Regular contributions add a separate annuity term.
- Does compounding frequency make much difference?
- Less than most people assume. At 7%, annual to monthly compounding adds about 0.23 percentage points to the effective rate. Time and the rate itself matter far more.
- Should I use a real or nominal return?
- For long projections, subtract expected inflation from the rate. A nominal 7% with 3% inflation is 4% in purchasing power.
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Sources
- Calculated on this page — At One Place