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Finance calculators turn the abstract math of interest, time, and money into concrete numbers you can plan around. Whether you're projecting how much a recurring deposit will be worth in twenty years, comparing what daily versus monthly compounding actually changes, or sanity-checking a savings goal, the tools here run the formula and show their assumptions.
Every calculator on this page is free and shows the formula being used — no opaque "calculate" button followed by a number. If a result depends on assumptions like "constant interest rate" or "no fees," we say so. These calculators are educational tools, not financial advice; for actual money decisions, talk to a licensed advisor.
Compound interest is interest paid on both your original principal and on previously-earned interest. After year one, you earn interest on (principal + first year's interest). After year two, on (principal + first and second year's interest), and so on. This is the mechanism behind long-term wealth-building: each year's interest joins the pool that earns next year's interest. The formula is A = P(1 + r/n)^(nt), where n is the number of compounding periods per year.
How is compound interest different from simple interest?
Simple interest pays you the same amount each period — only on the original principal. Compound interest pays on the growing balance. On $1,000 at 5% for 10 years: simple interest gives $500 total interest ($50/year × 10); compound interest gives ~$629 because each year's interest earns interest the following year. The gap widens dramatically over longer horizons.
How often should interest compound for maximum growth?
More frequent compounding always grows the balance faster, but with diminishing returns. Going from annual to monthly compounding adds noticeably to your end balance; going from daily to continuous compounding adds almost nothing. For most savings accounts, monthly compounding is standard. The compounding frequency matters less than the interest rate itself.
Is the Rule of 72 accurate?
The Rule of 72 — divide 72 by the interest rate to estimate how many years until your money doubles — is a useful mental shortcut for rates between roughly 6% and 10%. For 8%, it gives 9 years; the exact answer is 9.01 years. At rates below 6% or above 10% it gets less accurate. For exact doubling time, use t = ln(2) ÷ ln(1 + r), or use our compound interest calculator to test specific scenarios.