Compound scenario · Verified 2026-08-01
How long for $100,000 to double at 8%?
Grows to $221,964 over 10 years. You contribute $100,000; the remaining $121,964 (55%) comes from compound growth.
Final balance
$221,964
You contributed
$100,000
From compounding
$121,964
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Year-by-year breakdown
| Year | Total contributed | Interest earned | Balance |
|---|---|---|---|
| 1 | $100,000 | $8,300 | $108,300 |
| 2 | $100,000 | $17,289 | $117,289 |
| 3 | $100,000 | $27,024 | $127,024 |
| 4 | $100,000 | $37,567 | $137,567 |
| 5 | $100,000 | $48,985 | $148,985 |
| 6 | $100,000 | $61,350 | $161,350 |
| 7 | $100,000 | $74,742 | $174,742 |
| 8 | $100,000 | $89,246 | $189,246 |
| 9 | $100,000 | $104,953 | $204,953 |
| 10 | $100,000 | $121,964 | $221,964 |
Milestones in this scenario
- It takes 2 of the 10 years to reach the halfway mark ($110,982) — the second half of the balance arrives in the remaining 8 years. Compounding is back-loaded.
- Year 9 is the crossover: from then on, accumulated growth ($104,953 by that point) exceeds everything you put in.
- The final quarter of the timeline (years 8–10) produces 15% of the ending balance — the cost of quitting early is concentrated at the end.
- At 8%, money doubles roughly every 9 years (Rule of 72) — this scenario spans about 1.1 doubling periods.
How this number was calculated
Standard compound interest formula with monthly compounding (n = 12):
Balance = P × (1 + r/n)^(n × t) + PMT × [((1 + r/n)^(n × t) − 1) / (r/n)] where: P = $100,000 (initial amount) PMT = $0 (monthly contribution) r = 0.0800 (annual rate as decimal) n = 12 (compounding periods per year) t = 10 (years) Final balance = $221,964
Same closed-form math used by Investor.gov (SEC) and 7 other major calculators we tested — all produce identical results to the cent.
Related scenarios
How long does it take $10,000 to double at 7%?
→ $21,549 (11 years at 7%)
How Long for $10K to Double at 10%?
→ $22,182 (8 years at 10%)
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Open the calculator →Educational tool. Past performance does not predict future returns. Verified 2026-08-01. Math validated against Robert Shiller's S&P 500 historical dataset.