Compound Interest Calculator — investment growth in any currency

Compound interest is what happens when interest is added back to the balance and itself starts earning interest. Each compounding period the running balance is multiplied by one plus the periodic rate, so growth is exponential rather than linear. The longer the horizon and the more often interest is credited, the bigger the gap between compound and simple interest becomes — which is why the math behind this page is at the heart of every savings account, fixed deposit, and index-fund projection.

The closed-form expression for a fixed-rate account with periodic deposits is the sum of two standard textbook identities — the future-value of a lump sum and the future-value of an annuity:

              ⎛   r ⎞^(n·t)              ⎛(1+r/n)^(n·t) − 1⎞
FV = P × ⎜1 + ─ ⎟        +  PMT × ⎜─────────────────⎟
              ⎝   n ⎠              ⎝      r/n        ⎠

Here P is the initial deposit, r is the nominal annual rate (as a decimal), n is the number of compounding periods per year, t is the horizon in years, and PMT is the contribution per compounding period. The expression uses end-of-period deposits (ordinary annuity); for start-of-period deposits (annuity due) the contribution term is multiplied by (1 + r/n) — each contribution gets one extra period of growth.

The closely-related quantity an account actually pays over a year is the effective annual yield, or APY: APY = (1 + r/n)n − 1. This formula is the one disclosed under the U.S. Federal Reserve's Regulation DD (12 CFR §1030 Appendix A) — the same definition used by the Central Bank of Sri Lanka when comparing deposit products.

Every future value displayed on this page is cross-checked against an explicit period-by-period simulation: start with P, walk every compounding period, add the contribution, multiply by (1 + r/n). The closed form is just a folded version of the same loop, so the two must agree. If they don't, the “Simulation verified” badge in the calculator card will not light up. Zero-rate inputs short-circuit divide-by-zero in both methods and fall back to FV = P + PMT × N.

For real-money planning you usually care about purchasing power rather than nominal rupees. The inflation field, when set, divides the future value by (1 + inflation)^tto show today's-money equivalent — a useful sanity check when the horizon is long.

• U.S. SEC Investor.gov — Compound Interest Calculator & formula
• Investopedia — Compound Interest
• CFPB — Regulation DD Appendix A (APY calculation)
• Central Bank of Sri Lanka — Interest Rates of Commercial Banks

The future-value formula is a universal mathematical identity. The cited sources document the same formula, the APY definition under U.S. consumer-finance regulation, and the current Sri Lankan deposit-rate tariff used as a sanity benchmark. Last cross-checked on 2026-05-11.