Questions worth answering well.

A tool that takes a starting principal, an interest rate, a time horizon, and a compounding frequency, then projects what the money grows to as each period's interest gets added back to the balance and earns interest itself. This one also handles recurring deposits and withdrawals, inflation, and tax — so the final number reflects what you'd actually keep in today's dollars. Everything runs in your browser; no sign-up, no server.

The formula is A = P(1 + r/n)^(nt) — P is the principal, r is the annual rate as a decimal, n is compounds per year, t is years. For $10,000 at 8% compounded monthly for 10 years, the keystrokes are 1 + 0.08 ÷ 12 = y^x (12 × 10) × 10000 =, which gives about $22,196. For regular contributions the formula is longer — see the formula page or just use the compound interest calculator.

Simple interest multiplies the principal by the rate and the time, once. Compound interest adds each period's interest to the balance, so the next period earns interest on the interest. Over 10 years at 8% on $10k, simple yields $18k, compound (monthly) yields about $22.2k. The gap widens the longer the money sits.

APR is the nominal annual rate; APY (also called effective annual rate) folds in the compounding. For 8% nominal compounded monthly, the APY is about 8.30%. If a bank quotes a rate without saying 'effective', assume it's APR. Enter the APR here with the matching compound frequency and the calculator shows the resulting APY.

A little. At 8%, annual compounding gives an 8.00% APY; monthly gives 8.30%; daily gives 8.33%; continuous gives 8.33% (the mathematical ceiling). Past monthly, the differences are small — the rate itself matters far more than the schedule.

A rough shortcut: at a steady annual rate r, your money doubles in roughly 72 / r years. 8% → 9 years. 6% → 12 years. 4% → 18 years. The calculator shows the exact doubling time next to the APY tile.

Start-of-period deposits earn one extra period of interest, so the end balance is slightly higher. Over 30 years at 8% monthly, start-of-period beats end by roughly 0.7% — enough to notice, not enough to lose sleep over if you can't control it (e.g. direct deposit on the 15th). This calculator adds deposits at the end of each period, so its projections lean slightly conservative.

Set the inflation rate in the Inflation & tax section — the calculator shows your future value in today's purchasing power under the main number. A $1M balance in 40 years at 3% inflation is worth about $306k in today's dollars. Historically, long-run US inflation averages 2–3%.

The tax rate is applied once, at the end of the horizon, to the interest earned only — not to your contributions. It's a simple approximation of a taxable brokerage account. For tax-advantaged accounts (401(k), Roth IRA, ISA), leave it at 0% and use the pre-tax rate.

The S&P 500's long-run nominal return is around 10% a year with substantial volatility; real (inflation-adjusted) return is around 6–7%. For planning, 7–8% nominal with 3% inflation is a common conservative pair. Short horizons (under 10 years) carry much more risk than the average implies.

Not yet — this calculator assumes a constant rate. For most planning purposes a constant long-run average is more useful than guessing a sequence; for sequence-of-returns analysis you want a Monte Carlo tool, not a formula.

The three usual causes: (1) a different compound frequency (annual vs monthly can differ by 0.3% APY); (2) start- vs end-of-period deposits — this calculator adds deposits at the end of each period; (3) one tool treating your rate as an APR and the other as an APY. Line those three up and the results should match to within rounding.

No. The calculator shows the math of compound growth under assumptions you provide. Real investments involve risk, fees, taxes, and sequence effects this tool doesn't model. Treat it as a planning aid, not a prediction.

Entirely in your browser. No input is sent to any server. To save or share a scenario, use Copy link — it creates a URL with your inputs built in. Reloading the page without that link starts again from the defaults.