Compound interest calculator with monthly contributions. Project balance over 10–40 years at your return rate. Free, private, no signup.
Compound interest grows your balance on principal plus prior gains. Example: $10,000 at 7% annually becomes about $76,123 in 30 years with no extra deposits. Adding $500/month at 7% over 30 years reaches roughly $566,764 — about $386,764 from growth alone.
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.
A = P(1 + r/n)^(nt) for lump sums; with monthly contributions the calculator sums each deposit's growth path. P = principal, r = annual rate (decimal), n = compounding periods per year, t = years.
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.
$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.