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Project Your Compound Growth

Calculate how your money grows over time with the compounding interest formula.

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Growth, inflation and tax assumptions
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How this calculator works

Watch your investments grow over time with our free compound interest calculator. Compound interest is the secret to building long-term wealth, earning you interest on your principal and on your accumulated earnings. Input your starting deposit, monthly contribution, annual rate, and investment period to see a detailed visual chart of your future financial growth.

Method
Effective-monthly compound growth with beginning-of-month recurring contributions (annuity due), optional simplified annual tax drag, and optional constant-rate inflation adjustment.
Source
Investor.gov compound-interest guidance; OpenStax mathematics of finance; CalcOS Compound Interest Projection Contract v1.1.0.
Last reviewed
2026-08-24
Method limitations
  • The entered return, inflation, contribution, and tax rates are held constant; actual investment returns and tax treatment vary.
  • Monthly contributions are assumed to occur at the beginning of each month.
  • Taxable mode applies a simplified annual tax drag to positive annual growth and does not model lots, deductions, losses, fees, withdrawals, or jurisdiction-specific rules.
  • Tax-deferred and tax-free modes do not model withdrawal taxes, eligibility, account limits, or penalties.
  • Inflation-adjusted value is an estimate of purchasing power, not a guarantee or financial advice.

Engine-backed example

Compound growth with monthly contributions: worked example

How much could $10,000 grow to with $500 added at the beginning of each month for 20 years at an 8% annual return compounded monthly?

Initial amount
$10,000
Monthly contribution
$500 at the beginning of each month
Return and time
8% annually, compounded monthly, for 20 years
Tax treatment
Tax-free projection; no inflation adjustment
Future Portfolio Value$345,741.64
Total Principal Invested
$130,000.00
Total Interest Earned
$215,741.64
Net Post-Tax Value
$345,741.64

The projection adds each monthly contribution before applying that month’s effective growth. It assumes a constant return and does not model fees or market volatility.

Trust and review

Who created and reviewed this calculator?

Author
CalcOS Editorial TeamResponsible for calculator explanations, examples, assumptions, and search-intent clarity.
Reviewed by
CalcOS Scientific ReviewTechnical review of compound-growth equations, contribution timing, compounding conversion, tax and inflation estimates, boundaries, engine parity, and narrative examples; not licensed financial advice.
Substantive review date
2026-08-24Basic future-value mode, beginning-of-month contributions, effective monthly growth, compounding frequency, simplified annual tax drag, inflation adjustment, rounding, worked-example parity, and estimate limitations.

Sources

Assumptions used by this calculator
  • The entered annual return and inflation rates remain constant for the full projection.
  • Monthly contributions are added at the beginning of each month before that month’s effective growth is applied.
  • The selected nominal compounding frequency is converted to an effective monthly growth rate.
  • For taxable accounts, the calculator applies the entered capital-gains rate to positive annual growth as a simplified annual tax-drag estimate.
  • Tax-deferred and tax-free selections do not model withdrawal taxes, eligibility rules, fees, market volatility, or jurisdiction-specific treatment.
  • Inflation-adjusted value is a purchasing-power estimate, not a guaranteed future outcome.

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