What does this tool do
The Compound Interest Calculator shows how savings grow with compound interest. Enter principal, annual rate, term, and compounding frequency. Optionally add regular contributions (monthly, quarterly, etc.) and choose whether they are made at the start or end of each period. View a year-by-year breakdown and the final balance. Also supports solving for the required contribution to reach a target amount.
How to use it
- Enter principal — The initial amount you invest.
- Set annual rate — Interest rate as a percentage (e.g. 6 for 6%).
- Set term — Number of periods (years, months, etc. depending on frequency).
- Choose frequency — Daily, monthly, quarterly, or annually.
- Add contributions (optional) — Regular deposits each period; choose beginning or end timing.
- View results — Final balance, total contributed, total interest, and yearly breakdown.
- Target mode (optional) — Enter a target amount to solve for the required contribution.
How it works
Compound interest: A = P × (1 + r/n)^(nt) where P is principal, r is annual rate, n is compounding frequency, t is time. For regular contributions, closed-form annuity formulas are used: ordinary annuity (end-of-period) or annuity-due (beginning-of-period). Required contribution is solved with C = (FV − P×(1+r)^n) × r / ((1+r)^n − 1) for end timing (and the annuity-due variant for beginning). Zero-rate case uses C = (FV − P) / n.
All calculations run entirely in your browser. No data is sent to any server.
Use cases & examples
- Savings — See how a lump sum grows over time.
- Retirement — Model contributions and growth.
- Comparison — Compare different rates and frequencies.
- Goals — Find how much to save monthly to reach a target.
- Education — Teach compound interest with real numbers.
Example
- $10,000 at 6% for 10 years, monthly compounding → ~$18,194
- $500/month for 20 years at 5%, end-of-period → depends on principal; use the tool for exact figures.
Limitations & known constraints
- Fixed rate — Assumes constant interest rate; no variable-rate modelling.
- No taxes — Tax treatment of interest is not included.
- Period alignment — Contributions align with compounding frequency only.
- Numerical precision — Very long terms or extreme rates may accumulate rounding error.