How to Use the Rule of 72

Thứ sáu - 26/04/2024 23:11
The Rule of 72 is a handy tool used in finance to estimate the number of years it would take to double a sum of money through interest payments, given a particular interest rate. The rule can also estimate the annual interest rate required...
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The Rule of 72 is a handy tool used in finance to estimate the number of years it would take to double a sum of money through interest payments, given a particular interest rate. The rule can also estimate the annual interest rate required to double a sum of money in a specified number of years. The rule states that the interest rate multiplied by the time period required to double an amount of money is approximately equal to 72.

The Rule of 72 is applicable in cases of exponential growth, (as in compound interest) or in exponential "decay," as in the loss of purchasing power caused by monetary inflation.

Method 1
Method 1 of 4:

Estimating "Doubling" Time

  1. Step 1 Let R x T = 72.
    R is the rate of growth (the annual interest rate), and T is the time (in years) it takes for the amount of money to double.[1]
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Method 2
Method 2 of 4:

Estimating the Growth Rate

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Method 3
Method 3 of 4:

Estimating Exponential "Decay" (Loss)

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Doubling Time Chart

Sample Doubling Time Chart
Method 4
Method 4 of 4:

Derivation

  1. 1
    Understand how the derivation works for periodic compounding.[7]
    • For periodic compounding, FV = PV (1 + r)^T, where FV = future value, PV = present value, r = growth rate, T = time.
    • If money has doubled, FV = 2*PV, so 2PV = PV (1 + r)^T, or 2 = (1 + r)^T, assuming the present value is not zero.
    • Solve for T by taking the natural logs on both sides, and rearranging, to get T = ln(2) / ln(1 + r).
    • The Taylor series for ln(1 + r) around 0 is r - r2/2 + r3/3 - ... For low values of r, the contributions from the higher power terms are small, and the expression approximates r, so that t = ln(2) / r.
    • Note that ln(2) ~ 0.693, so that T ~ 0.693 / r (or T = 69.3 / R, expressing the interest rate as a percentage R from 0-100%), which is the rule of 69.3. Other numbers such as 69, 70, and 72 are used for easier calculations.
  2. 2
    Understand how the derivation works for continuous compounding. For periodic compounding with multiple compounding per year, the future value is given by FV = PV (1 + r/n)^nT, where FV = future value, PV = present value, r = growth rate, T = time, and n = number of compounding periods per year. For continuous compounding, n approaches infinity. Using the definition of e = lim (1 + 1/n)^n as n approaches infinity, the expression becomes FV = PV e^(rT).[8]
    • If money has doubled, FV = 2*PV, so 2PV = PV e^(rT), or 2 = e^(rT), assuming the present value is not zero.
    • Solve for T by taking natural logs on both sides, and rearranging, to get T = ln(2)/r = 69.3/R (where R = 100r to express the growth rate as a percentage). This is the rule of 69.3.
    • For continuous compounding, 69.3 (or approximately 69) gives more accurate results, since ln(2) is approximately 69.3%, and R * T = ln(2), where R = growth (or decay) rate, T = the doubling (or halving) time, and ln(2) is the natural log of 2. 70 may also be used as an approximation for continuous or daily (which is close to continuous) compounding, for ease of calculation. These variations are known as rule of 69.3, rule of 69, or rule of 70.
      • A similar accuracy adjustment for the rule of 69.3 is used for high rates with daily compounding: T = (69.3 + R/3) / R.
    • The Eckart-McHale second order rule, or E-M rule, gives a multiplicative correction to the Rule of 69.3 or 70 (but not 72), for better accuracy for higher interest rate ranges. To compute the E-M approximation, multiply the Rule of 69.3 (or 70) result by 200/(200-R), i.e., T = (69.3/R) * (200/(200-R)). For example, if the interest rate is 18%, the Rule of 69.3 says t = 3.85 years. The E-M Rule multiplies this by 200/(200-18), giving a doubling time of 4.23 years, which better approximates the actual doubling time 4.19 years at this rate.
      • The third-order Padé approximant gives even better approximation, using the correction factor (600 + 4R) / (600 + R), i.e., T = (69.3/R) * ((600 + 4R) / (600 + R)). If the interest rate is 18%, the third-order Padé approximant gives T = 4.19 years.
    • To estimate doubling time for higher rates, adjust 72 by adding 1 for every 3 percentages greater than 8%. That is, T = [72 + (R - 8%)/3] / R. For example, if the interest rate is 32%, the time it takes to double a given amount of money is T = [72 + (32 - 8)/3] / 32 = 2.5 years. Note that 80 is used here instead of 72, which would have given 2.25 years for the doubling time.
    • Here is a table giving the number of years it takes to double any given amount of money at various interest rates, and comparing the approximation with various rules:
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Rate Actual
Years
Rule
of 72
Rule
of 70
Rule of
69.3
E-M
rule
0.25% 277.605 288.000 280.000 277.200 277.547
0.5% 138.976 144.000 140.000 138.600 138.947
1% 69.661 72.000 70.000 69.300 69.648
2% 35.003 36.000 35.000 34.650 35.000
3% 23.450 24.000 23.333 23.100 23.452
4% 17.673 18.000 17.500 17.325 17.679
5% 14.207 14.400 14.000 13.860 14.215
6% 11.896 12.000 11.667 11.550 11.907
7% 10.245 10.286 10.000 9.900 10.259
8% 9.006 9.000 8.750 8.663 9.023
9% 8.043 8.000 7.778 7.700 8.062
10% 7.273 7.200 7.000 6.930 7.295
11% 6.642 6.545 6.364 6.300 6.667
12% 6.116 6.000 5.833 5.775 6.144
15% 4.959 4.800 4.667 4.620 4.995
18% 4.188 4.000 3.889 3.850 4.231
20% 3.802 3.600 3.500 3.465 3.850
25% 3.106 2.880 2.800 2.772 3.168
30% 2.642 2.400 2.333 2.310 2.718
40% 2.060 1.800 1.750 1.733 2.166
50% 1.710 1.440 1.400 1.386 1.848
60% 1.475 1.200 1.167 1.155 1.650
70% 1.306 1.029 1.000 0.990 1.523

Warnings

  • Let the rule of 72 convince you not to take on high-interest debt (as is typical with credit cards). At an average interest rate of 18%, semiretired credit card debt doubles in just four years (72 ÷ 18 = 4), quadruples in eight years, and becomes completely unmanageable after that.
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