About
Built for the long tail of compound time.
We wanted a compound interest calculator that didn't make us wait for a "Calculate" button or scroll past three ad blocks before the result. So we built one.
Promise 1
Nothing leaves your browser
No sign-up, no server call, no third-party trackers. The scenario lives in the URL — so you can bookmark or share it — and nowhere else.
Promise 2
The math is explicit
We run a month-by-month simulation so compound frequency, deposit cadence, and start-or-end timing are honored exactly — no handwaving.
Promise 3
Honest numbers
Nominal and inflation-adjusted side by side. A $1M balance in 40 years isn't the same $1M you spend today — we show both.
Methodology
How the numbers get made.
- 01
Monthly simulation
We tick 12 steps per year. Each month accrues interest atr / 12; the compound schedule folds the accrued interest into the balance on its cadence.
- 02
Deposits layered in
Deposits post at either the start or end of their period. Start-of-period lands before that period's interest accrues, so it earns one extra period.
- 03
Inflation deflation
Real value = nominal / (1 + inflation)years. We show both numbers so you can see what the balance is worth today, not just in future dollars.
- 04
Tax, applied once
The tax rate applies to interest earned, at the end of the horizon. It's a simple approximation of a taxable account; tax-advantaged accounts (401k, Roth IRA, ISA) should leave it at 0%.
What we don't model
- · Variable rates or sequence-of-returns risk (use a Monte Carlo tool).
- · Account fees or expense ratios — subtract them from the rate yourself.
- · Withdrawals during the horizon (coming soon).
- · Currency conversion — the currency selector is cosmetic, not FX-rate aware.
- · Tax-bracket progression — the tax rate is treated as flat.
Start compounding, keep compounding.
Open the calculator, change one number, and watch the long tail.