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.
| Where your money is | Rate | Years to 2× |
|---|---|---|
| Bank savings | 3.5% | 20.6 yrs |
| MMF average | 9.15% | 7.9 yrs |
| Top MMF | 11.01% | 6.5 yrs |
| 364-day T-Bill | 9.04% | 8.0 yrs |
At 9.15%, your 100,000 will double to 200,000 in roughly 7.9 years. Using the Rule of 72, divide 72 by your rate for quick mental-math doubling estimates.
How the Rule of 72 Works
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%, 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
Today’s rates — the Rule of 72 shows how long each product takes to double your KES.
| Product | Rate today | Years to Double | Tax on Interest |
|---|---|---|---|
| Money market funds (average) | 9.15% | 7.9 yrs | 15% WHT |
| Top money market fund | 11.01% | 6.5 yrs | 15% WHT |
| 364-day Treasury Bill | 9.04% | 8.0 yrs | 15% WHT |
| Treasury Bonds | See today’s yields | 72 ÷ yield | 10–15% WHT (infrastructure bonds 0%) |
| Fixed Deposits | See today’s yields | 72 ÷ rate | 15% WHT |
| Unit Trusts | See today’s yields | 72 ÷ yield | 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% | 20.6 years | 20.1 years | +0.5 yrs |
| 7% | 10.3 years | 10.2 years | +0.1 yrs |
| 9.4% | 7.7 years | 7.6 years | +0.1 yrs |
| 11.5% | 6.3 years | 6.4 years | −0.1 yrs |
| 13% | 5.5 years | 5.7 years | −0.2 yrs |
| 16% | 4.5 years | 4.7 years | −0.2 yrs |
Need the full compound interest calculation? Try the Compound Interest Calculator
Rule of 72 Calculator — FAQ
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