Compound Interest Calculator
Calculate how your investments grow over time using the power of compound interest. Plan your retirement with scientific precision.
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Compound Interest
Method and basis
How the figure above is derived, and when not to act on it.
The 8th Wonder of the World
Einstein reportedly called compound interest the "eighth wonder of the world." The mathematics behind it are deceptively simple, yet profoundly powerful.
The formula: A = P(1 + r/n)^(nt) + PMT × [(1 + r/n)^(nt) − 1] / (r/n)
Where P = principal, r = annual rate, n = compounding frequency, t = time in years, PMT = monthly payment.
📊 The Rule of 72
The Rule of 72 is a mental shortcut: divide 72 by your annual interest rate to find approximately how many years it takes to double your money.
| Rate | Years to Double |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
💡 Why Monthly Contributions Matter More Than the Principal
A £500/month contribution at 7% for 30 years generates £567,000 — from only £180,000 contributed. The remaining £387,000 is pure compound growth.
Starting 10 years earlier (40 years total) would produce £1,197,000 — more than double — from only £60,000 extra contributed. Time is the most powerful variable.
🌍 Real-World Return Benchmarks (Long-Term Historical Averages)
| Asset Class | Nominal Return | Real (Inflation-Adjusted) |
|---|---|---|
| Global Equities (MSCI World) | ~9–10% | ~6–7% |
| US S&P 500 | ~10.5% | ~7.5% |
| Bonds (10yr Govt) | ~3–5% | ~1–2% |
| Cash/Savings | ~2–4% | ~0–1% |
| Property (UK) | ~6–8% | ~3–5% |
The Cost of Waiting
Every year you delay costs you compounding years. If you invest £10,000 at age 25 at 7%, it becomes £149,745** by 65. The same investment at 35 becomes only **£76,123**. A 10-year delay costs you **£73,000 — despite investing the same amount.
Frequently Asked Questions
Q: How often should I compound? A: More frequent compounding (monthly vs annually) makes a small but real difference. A £10,000 investment at 7% for 20 years: annually = £38,696; monthly = £40,088. Favour funds/accounts with monthly compounding.
Q: What's a realistic return assumption? A: For a diversified global equity portfolio (e.g., a low-cost index fund), 6–8% nominal (4–6% real) is a reasonable long-term planning assumption, based on 100+ years of data.
WinSport Editorial & Research Standards
All formulas and reference values used in this tool are sourced from peer-reviewed scientific literature and validated clinical guidelines. Our research team continuously audits each calculator for accuracy and updates methodology as new evidence emerges.
Read the Science
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