Entry № 15 · logged Oct 3, 2026Free, no sign-upCompound Interest · Investing

How long to double your money? It depends on one number.

Doubling time is the return wearing a clock: about 35 years at 2%, 10 years at 7%, under 6 at 12%. Enter your return in the slip and the doubling line posts the exact figure — then scan the year table to see what two doublings (four times the money) actually require.

Two doublings is 4×. At 7% that is roughly 20 years before deposits — the slip’s 20-year row, posted plainly.

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.

What you post Starting amount ($) Already saved or invested today. Zero is a fine start — the deposit does the early work. Monthly deposit ($/mo) Posted at the end of each month, after that month’s interest.

How it grows Annual return (%) A savings account might post 4%; a diversified portfolio has averaged near 7% after inflation-free math. Returns are never guaranteed. Years Inflation (%) Only drives the “today’s dollars” line — what the final balance buys in current terms.

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.

How Compound Interest Worksinterest on interest, posted month by month Rule of 72 Calculatorthe doubling shortcut, checked against the exact math Compound vs Simple Interestthe same deposit, two very different ledgers Monthly Investment Calculatorno lump sum? the deposit-only ledger Compound Interest Calculatorthe main slip: deposits vs growth, posted

From the ledger

Doublings, stacked

One doubling is a milestone; stacked doublings are the strategy. Money that doubles every 9.9 years multiplies by 4 in 19.9 and by 8 in 29.8 — before any deposits. That is why the return column matters more than the deposit column over long horizons, and why shaving two points of return (7% to 5%) costs a full doubling across a working life: 13.9 years per double instead of 9.9.

Fees live in this table too. A 1% annual fee is not 1% of the outcome; it stretches every doubling, and across three doublings the fee’s share of the final balance runs into double digits. The slip cannot see your fees — subtract them from the return you enter and the ledger gets honest.

Filed answers

Questions from the growth desk

How long at 7%?

9.9 years exact with monthly compounding (Rule of 72: 10.3). Two doublings — 4x — take about 19.9 years.

What about inflation?

Deflate the return first: 7% nominal at 3% inflation is roughly 4% real, doubling purchasing power in about 17.4 years.

Is this investment advice?

No — arithmetic on numbers you type.

Keep going

Also in the ledger

Savings Goal Calculatorworking backwards: what a goal needs each month Emergency Fund Calculatorfund the buffer first, then let growth run Debt Payoff Calculatorcompounding works against you here — clear it first Mortgage Calculatorthe same math, wearing a loan’s clothes Subscription Audit Calculatorfind the monthly deposit hiding in your subscriptions Hourly ↔ Salary Converterwhat the paycheck funding the deposit is worth