Entry № 15 · logged Oct 3, 2026Free, no sign-upCompound Interest · Investing
What does the deposit become? The growth ledger posts it.
Compounding is quiet for a decade and loud afterwards. Post a starting amount and a monthly deposit and this slip compounds interest month by month — separating what you put in from what growth added, deflating the result into today’s dollars, and dating the year your money doubles.
Prefilled: $10,000 plus $300 a month at 7% for 20 years. Run it below — the interest line is closer to the deposit line than most people expect.
The growth ledger, Form CI-15. Post a starting balance and a monthly deposit; the slip compounds interest monthly, posts the year-by-year balance, and separates what you deposited from what growth added.
| Year | You posted | Interest posted | Balance |
|---|
Math: each month the balance earns one-twelfth of the annual return, then your deposit posts. Steady returns are a modelling convenience — real markets wander year to year, and taxes and fees are not in this ledger.
Around this tool
From the ledger
How the ledger compounds
Each month does two things in order: the balance earns one-twelfth of the annual return, then your deposit posts on top. That ordering is the entire mechanism — next month’s interest is calculated on a balance that already includes last month’s interest, so growth feeds on itself. Early on the deposit dominates and the interest line looks like a rounding error; given enough years the interest line overtakes the deposits and then laps them, which is why the start date matters more than the deposit size in almost every honest comparison.
The slip posts three separations most calculators blur. First, deposits versus interest: on the prefilled ledger you post $82,000 over 20 years ($10,000 to start, then $300 a month) and growth adds the rest — the exact split is stamped above the year table. Second, nominal versus today’s dollars: at 3% inflation a future dollar buys less, so the slip deflates the final balance into current purchasing power. Third, time itself: the doubling line gives the exact doubling time from the monthly math and the Rule-of-72 estimate beside it, so you can watch the shortcut earn its keep.
Two honest limits. A steady return is a modelling convenience: real portfolios string together good and bad years, and the order of those years matters in ways a single rate cannot show. And this ledger ignores taxes and fees, both of which skim the interest line in real accounts. Treat the output as the shape of the truth — compounding’s curve, timing, and crossover — not as a promise about any particular account.
Disclaimer: arithmetic on numbers you type, not investment advice. Past average returns do not guarantee future ones; a fiduciary who knows your ledger has the final word.
Filed answers
Questions from the growth desk
How is compound interest calculated here?
Monthly. Each month the balance earns annual-rate ÷ 12 in interest, then the monthly deposit posts. Over 20 years that is 240 postings. The year table shows the balance, cumulative deposits, and cumulative interest at each year-end so you can check any row by hand.
What return should I enter?
Match the account. A high-yield savings account posts its APY (around 4% in 2026). A diversified stock portfolio has historically averaged roughly 7–10% a year before inflation, with violent year-to-year swings the steady-rate model smooths away. When unsure, run 5% and 8% and plan around the lower ledger.
When does interest overtake my deposits?
On the prefilled ledger ($10,000 start, $300/mo, 7%) the interest total passes the deposit total well inside the second decade — the year table shows the exact crossing. Shorten the horizon or cut the return and the crossing slides later; that sensitivity is the real lesson of the tool.
What is the Rule of 72?
Divide 72 by the annual return to estimate doubling time: at 7%, about 10.3 years. The slip posts the exact monthly-compounded doubling time (9.9 years at 7%) beside the shortcut so you can see the error is a rounding error, not a flaw.
Does inflation really change the answer?
It changes what the answer buys. The prefilled 20-year balance deflated at 3% inflation is worth notably less in today’s dollars — the slip posts both numbers so the gap is explicit rather than a footnote.
Is this investment advice?
No — it is arithmetic. It does not know your debts, taxes, or risk tolerance. Clear expensive debt first (that compounding runs against you), fund an emergency buffer, then let this ledger run.
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