A compound interest calculator projects balance growth with monthly contributions and your chosen compounding frequency.
This compound interest calculator projects balance growth with monthly contributions and compound frequency you choose. An exponential interest calculator and compound interest calculator share the same math—time, rate, and recurring deposits drive outcomes. Free compound interest calculator runs stay private in your browser; adjust return assumptions conservatively. Compound interest calculator charts help compare 10-, 20-, and 30-year savings goals. Use the compound interest calculator with our guides on DCA and FIRE when you move from projection to contribution discipline.
Compound growth with monthly contributions
Compound interest is interest on principal plus accumulated interest. Monthly contributions dominate outcomes for most US savers over 20–30 year horizons—even small increases in contribution rate beat chasing extra return.
Use monthly compounding for brokerage and HYSA projections; match the rate to your actual APY or expected portfolio return net of fees.
Subtract inflation mentally or run a lower return scenario for goals stated in today's dollars (e.g. $1M nest egg in real terms).
Guide, examples, and methodology
Compound interest calculator
Project future value with starting balance, monthly additions, annual rate, and compounding frequency. Small rate and time changes produce large end balances—use ranges, not single-point forecasts.
Exponential interest calculator
Pair with our FIRE and retirement tools when translating today's savings into long-range wealth targets.
How to use this compound interest calculator
Enter your starting balance, optional monthly contribution, expected annual return, compounding frequency, and years invested. Results update instantly in your browser. Use monthly compounding when modeling most US brokerage or high-yield savings assumptions; use annual only if that is how your product quotes the rate.
Example (USD)
Input
Value
Result after 30 years (7% annual, monthly compound)
Starting balance
$10,000
$76,123 (no extra deposits)
Plus $500/month
Same rate
~$566,764 total
Interest earned
—
~$386,764 on top of $180,000 contributed
How we calculate
Lump-sum growth uses A = P(1 + r/n)^(nt). Each monthly contribution is compounded from its deposit date to the end of the horizon, then summed. We do not deduct taxes, fund expense ratios, or inflation unless you lower the return yourself. For purchasing-power planning, subtract an inflation assumption from your nominal return (e.g. 7% nominal minus 3% inflation ≈ 4% real).
Common mistakes
Quoting a yearly APY while the account compounds daily or monthly (understates growth).
Ignoring ongoing 401(k) or IRA fees when comparing to a headline market return.
Assuming you can keep max contributions every year without a cash-flow plan.
Comparing to a CD or Treasury without matching the same time horizon.
Monthly contributions vs lump sum (US portfolios)
Most US retirement savers compound through payroll 401(k) deferrals plus employer match. A $500/month contribution at 7% nominal over 30 years can exceed $560,000—often more impactful than optimizing a single year's return. Use this calculator to test sensitivity: raise contributions 1–2% before chasing an extra 0.5% fund return.
Real vs nominal returns
Headline market returns are nominal. If inflation averages 3% and your portfolio earns 7%, your real return is about 4%. For goals stated in today's dollars (e.g. $1M nest egg), subtract an inflation assumption from the return field or run a second scenario at a lower rate.
Who should use this calculator
Use it for HYSA projections, taxable brokerage goals, 529 planning, or back-of-envelope retirement checks. Pair with our retirement and FIRE calculators when the question is sustainable withdrawal, not just accumulation. Pair with DCA calculator when deciding whether to invest a windfall all at once or over months.
For a single deposit: A = P(1 + r/n)^(nt), where P is principal, r is the annual rate as a decimal, n is compounding periods per year, and t is years. With monthly contributions, each deposit has its own timeline; this calculator totals them.
How much does $500 a month grow in 30 years?
At about 7% average annual return with monthly compounding, $500/month for 30 years is roughly $566,764 total (about $180,000 contributed). Your starting balance and actual return change the outcome.
How does compound interest work?
You earn interest on principal and on interest already earned. Year 1: $10,000 at 7% → $10,700. Year 2: 7% on $10,700, not just the original $10,000 — that acceleration is compounding.
How much does $500/month invested over 30 years become?
At about 7% average annual return, $500/month for 30 years grows to roughly $566,764 ($180,000 contributed, ~$386,764 from compound growth). Your rate and starting balance change the outcome — run your scenario here.
Starting at 25 vs 35: how much difference does it make?
$500/month at 7% from age 25 to 65 ≈ $1.3M; starting the same contributions at 35 ≈ $567,000. Ten extra years of compounding often matter more than a slightly higher return.