Compound Interest Calculator
Project a balance from an initial principal, fixed annual rate, term, and independently scheduled regular contributions.
Guide
What is compound interest?
Compound interest applies each interest period to the current balance, including interest previously added to that balance. This Calculator projects a balance from an initial principal, a fixed nominal annual rate, and optional regular contributions.
Formula
At each compounding event, the balance is multiplied by 1 + (annual rate ÷ 100 ÷ compounding periods per year). Contributions are scheduled independently. At a coincident event, a beginning-of-period contribution is added before compounding, while an end-of-period contribution is added after compounding.
Worked example
With an initial principal of 1,000, a 10% annual rate, annual compounding, and no regular contributions, the balance becomes 1,100 after one year and 1,210 after two years. The 210 difference from the initial principal is total interest.
Assumptions and limitations
The Calculator assumes a fixed nominal annual rate and the selected discrete compounding schedule. It does not model fees, taxes, withdrawals, changing rates, account minimums, or market volatility. A rate above negative 100% is allowed, but a negative rate can reduce the balance. Contributions cannot be negative.
Common questions
Can contribution and compounding frequencies differ? Yes. Each is scheduled on its own timeline. What happens for a fractional term? Only scheduled events at or before the requested term are applied, so no contribution or compounding event is created beyond the term. Is a contribution at the exact end included? Yes, when it is an end-of-period contribution. A beginning-of-period contribution at the exact end starts a period outside the term, so it is not included.
Methodology
The Calculator constructs a deterministic fractional-year timeline for all contribution and compounding events through the requested term. It applies beginning contributions, then interest, then end contributions whenever events coincide. Final balance equals initial principal plus total regular contributions plus total interest at internal calculation precision. Values are rounded only for display.