ToolPilot

Compound interest calculator

See how an initial balance plus monthly contributions grow with compound interest. We map monthly deposits to each compounding period (e.g. quarterly = 3× monthly per period).

Inputs

Initial investment ($1 – $10M)

$10,000

Monthly contribution ($0 – $50K)

Annual interest rate (0.1% – 30%)

Time period (1 – 50 years)

Compounding frequency

Results

Future value: $144,572.72
Total contributions: $58,000
Total interest earned: $86,573

Contributions vs interest

Share of ending balance

40.1%

59.9%

Contributions 40.1%Interest 59.9%

How it works

Future value combines growth on the lump sum and an annuity of deposits each compounding period: A = P(1 + r/n)^nt + PMT × [((1 + r/n)^nt − 1) / (r/n)], where r is the annual rate (decimal), n is compounding periods per year, t is years, and PMT is the amount added each compounding period. Your monthly contribution is converted to that period (e.g. 3× monthly when compounded quarterly). At 0% rate, growth is linear: principal plus all monthly deposits.

FAQ

Is daily compounding exactly how my bank works?

Institutions differ on day-count, posting dates, and tiers. This is a standard mathematical model for planning—not a specific product quote.

Why convert monthly deposits for quarterly compounding?

The closed-form formula needs one payment per compounding period. We use 3× your monthly amount per quarter as a simple aggregation; some accounts instead compound monthly on a running balance.

Does this include taxes or inflation?

No. Reported future value is nominal. After-tax returns and real purchasing power can be lower.

What if I change contributions over time?

This model assumes a steady monthly contribution. Lumpy or changing deposits need a month-by-month simulation for precision.