Rule of 72 Calculator Kenya — How Fast Your Money Doubles
Divide 72 by your interest rate to estimate how many years it takes to double your money. Works for MMFs, T-Bills, Bonds, SACCOs, and Fixed Deposits in Kenya.
How Long to Double Your Money
Use the Rule of 72 to instantly see when your investment doubles.
Serrari provides educational tools — we are not financial advisors. Results are estimates based on the inputs you provide.
| Where your money is | Rate | Years to 2× |
|---|
How the Rule of 72 Works
Years to double
Magic constant
Annual rate (%)
The Rule of 72 is a mental-math shortcut derived from the compound interest formula A = P(1 + r)t. Setting A = 2P and solving gives t = ln(2)/ln(1 + r). Since ln(2) ≈ 0.693, and for small r, ln(1 + r) ≈ r, the approximation simplifies to t ≈ 0.693/r. Multiplying numerator and denominator by 100 and rounding to the nearest convenient integer gives us 72, which is more divisible than 69.3.
The rule is most accurate for rates between 2% and 20%. At 9.4% (Kenya MMF average), the Rule of 72 estimates 7.66 years; the exact formula gives 7.64 years — off by less than a week.
Where to Double Your Money Fastest in Kenya
2026 rates — the Rule of 72 shows how long each product takes to double your KES.
| Product | Typical Rate | Years to Double | Tax on Interest |
|---|---|---|---|
| Money Market Funds | 8 – 12% | 6 – 9 yrs | 15% WHT |
| Unit Trusts (Equity) | 10 – 15% | 4.8 – 7.2 yrs | 15% WHT |
| Treasury Bills | 14 – 17% | 4.2 – 5.1 yrs | Tax-exempt |
| Treasury Bonds | 12 – 14% | 5.1 – 6 yrs | 10% WHT |
| Fixed Deposits | 6 – 10% | 7.2 – 12 yrs | 15% WHT |
| SACCO Deposits | 8 – 12% | 6 – 9 yrs | 15% WHT |
| Bank Savings | 2 – 5% | 14.4 – 36 yrs | 15% WHT |
Rule of 72 vs Exact Formula — How Accurate Is It?
Side-by-side comparison for common Kenya investment rates.
| Annual Rate | Rule of 72 Estimate | Exact Formula | Difference |
|---|---|---|---|
| 3.5% (Bank savings) | 20.6 years | 20.1 years | +0.5 yrs |
| 7% (Lower MMF) | 10.3 years | 10.2 years | +0.1 yrs |
| 9.4% (MMF average) | 7.7 years | 7.6 years | +0.1 yrs |
| 11.5% (Top MMF) | 6.3 years | 6.4 years | −0.1 yrs |
| 13% (Bond YTM) | 5.5 years | 5.7 years | −0.2 yrs |
| 16% (T-Bill) | 4.5 years | 4.7 years | −0.2 yrs |
Need the full compound interest calculation? Try the Compound Interest Calculator
Rule of 72 Calculator Kenya — FAQ
What is the Rule of 72?+
How long does it take to double money in Kenya?+
Is the Rule of 72 accurate?+
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