How to use this calculator
A single deposit drives this tool: set the one-time amount, the annual return you want to assume, and the holding period. Two options refine the result:
- Compounding: monthly suits funds and many accounts; yearly suits products quoted as annual effective rates.
- Expected inflation: optional. Set it and the fourth metric switches from exact doubling time to the maturity value in today's purchasing power.
The chart deliberately draws three lines: your rate, plus dashed lines two points lower and higher. A single smooth curve looks like a promise; the band is a more truthful picture of a return assumption. The table tracks value, yearly growth and the running multiple; watch how the "growth that year" column accelerates even though the rate never changes. The Investment Calculator can also model a one-time amount (with contributions set to zero), but use this page when the money goes in exactly once and you want the doubling time and the ±2% assumption band it draws around your rate.
Deploying a lump sum
A lump sum might come from a bonus, inheritance, maturing deposit, property sale or vested stock. This calculator does not decide whether it should be invested; it shows what one deposit becomes under a return and horizon you choose. At an assumed 10% compounded monthly, $100,000 becomes about $445,392 in 15 years, a 4.45× multiple ($417,725, or 4.18×, with yearly compounding).
The multiple's intuition is doubling. Money at 10% compounded monthly doubles roughly every 7 years, so a 15-year horizon fits a little over two doublings, and 2 × 2 × a bit ≈ 4.45 falls right out. This is also why the year-by-year table looks back-loaded: the default investment gains about $64,500 over its first five years but $174,700 over its last five, at the same unchanging rate, because each doubling operates on a bigger base. Cutting a 15-year plan to 10 doesn't cost a third of the outcome; it costs $174,700 of the ending value.
A market investment made all at once is fully exposed to the next move in prices. A staged purchase exposes the money gradually but leaves part of it waiting, so its result depends on both the market path and the return earned before each purchase. The SIP Calculator models regular contributions, and the lump sum vs monthly investing guide explains the trade-off without assuming one path will occur.
Finally, mind inflation on long holds. At 3% inflation, the default's $445,392 buys what $285,880 buys today — still a strong real outcome, but a third smaller than the nominal headline. For a windfall meant to fund something specific years from now, the real figure is the one to plan with.
Formula and methodology
One deposit and no additions make this the purest compounding case:
Pthe one-time investment;tyears heldrannual return as a decimal;mcompounding periods per year
Doubling time follows directly by solving FV = 2P:
- with yearly compounding this reduces to ln 2 ⁄ ln(1 + r)
- the Rule of 72 approximates it as 72 ⁄ rate
How good is the Rule of 72? Against exact annual-compounding doubling times computed by this library: at 6% it says 12.0 years (exact 11.90), at 8% both say 9.0 (exact 9.01), at 10% it says 7.2 (exact 7.27), at 12% it says 6.0 (exact 6.12). Tight through the single digits, drifting slightly optimistic above 10%.
Worked example
Monthly rate = 0.10 ⁄ 12 = 0.008333; periods = 180. Growth factor: (1.008333)180 = 4.45392.
FV = 100,000 × 4.45392 = $445,392, with $345,392 of growth on money touched exactly once.
Milestones from the table: $164,531 at year 5, $270,704 at year 10, $445,392 at year 15; each five-year block adds more than the one before. And the assumption band matters: the same windfall at 8% ends at $330,692, at 12% it ends at $599,580. The two dashed lines on the chart are that sentence, drawn.
What changes the result
- Horizon sets the multiple. At 10% monthly the default doubles about every 7 years: 4.45× at 15 years came from the time, not from cleverness. The biggest risk to a lump sum plan is usually withdrawing it early.
- The rate assumption is a band. ±2 points around 10% spans $330,692 to $599,580 at 15 years, a $268,888 range on identical money. Compare several return assumptions and check whether the outcome would still be acceptable under the lower one.
- Entry timing. All-at-once and staged investing have different exposure paths. Compare them over a defined schedule and avoid treating the constant-return output as evidence about next month's market.
- Inflation and fees both compound against you: 3% inflation converts the default's 4.45× nominal into 2.86× real, and every 1% of annual fees costs multiples of itself over 15 years.
Assumptions and limitations
- A constant return is assumed; real markets deliver the average through wild detours, and a lump sum feels every one of them from day one. The ±2% band understates how wide real outcomes can range.
- Results are pre-tax and pre-fee; net your expected costs out of the return before entering it.
- No withdrawals or top-ups are modeled — for a lump sum plus ongoing contributions, use the Compound Interest Calculator.
- Doubling times assume the entered rate holds indefinitely, which no rate does. Treat them as intuition-builders, not schedule commitments.
Frequently asked questions
Should I invest a windfall all at once or spread it out?
This calculator cannot answer that from a constant return. Investing at once creates full market exposure immediately; staging reduces early exposure and keeps some money uninvested for longer. Compare a defined staged schedule, your need for the cash, transaction costs and how you would respond to an early loss. The companion guide shows both modeled paths.
How accurate is the Rule of 72?
Very, in the range that matters. Library-computed exact doubling times with annual compounding: 11.90 years at 6% (rule says 12.0), 9.01 at 8% (rule: 9.0), 7.27 at 10% (rule: 7.2), 6.12 at 12% (rule: 6.0). With monthly compounding money doubles slightly faster — 6.96 years at 10%. For mental math on any investment pitch, 72 ÷ rate is reliable to within a few months.
What does the growth multiple actually tell me?
It's the ending value per unit invested — a horizon-and-rate summary that's easier to compare than either input alone. 4.45× means every $1,000 of the windfall became $4,454. Multiples also expose long-horizon stakes clearly: the difference between 8% and 10% for 15 years reads as "just two points" but is 3.31× versus 4.45×, about a third more money from the same deposit.
Why show the result at rate ±2% instead of just my rate?
Because a single projected curve manufactures false confidence. Expected returns are estimates with wide error bars, and 15-year outcomes at 8%, 10% and 12% span $330,692 to $599,580 on the default input. Seeing the band tells you what your plan risks and what it might deliver — if the goal only works on the top line, the plan needs more money or more time, not more optimism.
Does this calculator work for fixed deposits and bonds too?
Yes, with the right settings: use the deposit's rate with its actual compounding frequency (yearly or quarterly for many FDs; use yearly if you're given an effective annual yield). For bonds where coupons are paid out rather than reinvested, growth is linear, not compound — the Simple Interest Calculator models that case and shows the reinvestment gap explicitly.