Compound interest: the complete hub
Every compound-interest calculator, formula, and guide on Snowballr in one place. Pick a calculator by use case, then read the matched pillar guide for the math behind it.
Quick answer: what is compound interest?
Compound interest is interest calculated on both the original principal and the accumulated interest from previous periods. The formula is A = P × (1 + r/n)n×t, where P is principal, r is the annual rate, n is compounding periods per year (12 for monthly, 365 for daily), and t is time in years. At a fixed 7% illustrative annual rate, money roughly doubles every 10 years — and the doublings compound, so 40 years produces ~16× the original, not 4×.
Calculators by use case
Pillar guides
Worked example libraries (programmatic)
For when you want to see a specific dollar/rate/year scenario without running the calculator yourself, browse our pre-computed library pages:
Research and citation resources
Reusing a number in a lesson, article or report? Use the open-data catalog for machine-readable files, the research hub for reproducible scenarios, and the citation guide for APA, MLA and BibTeX examples. Link to the specific source page and keep its assumptions visible.
The four levers that change compound interest outcomes
- Time — the dominant lever. 30 years produces ~4× more than 20 years at 7%, despite being only 50% more time.
- Rate of return — going from 5% to 8% over 30 years roughly doubles your final number. Higher-return investments can also carry more risk, so compare the actual product, fees, taxes and time horizon.
- Contribution size — direct linear effect on the final number. Useful but capped by income.
- Compounding frequency — marginal lever. Monthly captures 99.6% of the benefit vs. daily or continuous. Don't optimize for this.
The single most actionable insight: start now, even small. A 25-year-old saving $300/month for 40 years at a fixed 7% nominal rate reaches about $787,000 with month-end deposits. A 35-year-old saving the same amount for 30 years reaches about $366,000 under the same arithmetic convention. Time is the only lever that cannot be bought back.
When compound interest works against you
The same mathematics that builds wealth on the saver side destroys it on the borrower side. Credit card debt at 22% APR doubles every ~3.3 years if left unpaid. A 1% expense ratio on a 30-year portfolio costs ~25% of final wealth. Mortgage interest paid over 30 years often equals the loan principal itself. The goal of personal finance is to spend as much of your life as possible on the lender side of compound interest, and as little as possible on the borrower side.