Compound Interest Calculator
Project savings growth from principal, contributions, rate, and time.
Year-by-year breakdown
| Year | Contributions YTD | Interest YTD | Balance EOY |
|---|
What is compound interest calculator?
Compound interest is interest that earns interest. Each period the account balance grows by balance × rate, and that new larger balance is what next period’s interest is calculated on. Over long time horizons the effect dominates the original principal: $10,000 left alone at 7% for 40 years becomes roughly $150,000 — fifteen times the starting amount, with no further contributions.
The standard formula for a lump-sum investment is A = P × (1 + r/n)^(n × t), where P is the principal, r is the annual rate as a decimal, n is the compounding frequency per year, and t is years. Add periodic contributions and the formula gets messier; this calculator simulates the account period by period rather than apply a closed-form formula, so the math stays correct even for irregular compounding/contribution combinations.
The biggest lever for most savers isn’t rate of return — it’s time. A 25-year-old contributing $200/month at 7% until age 65 has $526,000 at retirement. The same 35-year-old has only $245,000 — half — because they lost the first decade of compounding. The year-by-year breakdown makes this concrete: in early years contributions dominate, but in the last decade interest earned each year exceeds total contributions for the year.
When to use a compound interest calculator
- Projecting a 30-year investment plan — A 35-year-old with $10,000 saved and $500/month going into an index fund wants to see what they'll have at 65 assuming a 7% average return.
- Comparing compounding frequencies — Same principal, contributions, and rate — see how much extra growth you get from monthly vs annual compounding over a long time horizon.
- Setting a savings target — Work backward from a goal: how many years of $200/month at 6% will it take to hit $50,000? Adjust years until the final balance matches.
How to use the Compound Interest Calculator
- Enter starting principal and contributions — Type the amount you start with and how much you'll add each month or year. Use the segmented toggle to pick monthly or annual contributions.
- Set rate and time horizon — Enter the expected annual rate as a percentage (7 = 7%) and the number of years. Pick the compounding frequency that matches the account (most savings/investment accounts compound daily or monthly).
- Review the year-by-year breakdown — Expand the schedule to see contributions, interest earned, and balance for each year. Download as CSV to model further in a spreadsheet.
Worked examples
Long-horizon investment
Input: $10,000 + $500/mo, 30 years, 7%, monthly compounding
Output: Final balance ≈ $689,000 (contributions $190,000, interest $499,000)Compounding bump
Input: $10,000, $0 contribution, 10 years, 7%, annual vs monthly
Output: Annual: $19,672. Monthly: $20,097.More frequent compounding gives slightly more growth at the same nominal rate.
No-interest baseline
Input: $0 + $200/mo, 5 years, 0%
Output: Final balance $12,000 (all contributions, no interest)