If your savings account earns 6% per year, how long before your money doubles?
According to the Rule, 72 ÷ 6 = 12 years.
If you want your money to double in eight years, what interest rate is needed?
Same calculation, 72 ÷ 8 = 9% per year.
If a wild horse herd is growing at a rate of 12% per year, how long before it doubles?
Six years.
If it doubles every eight years, what is the growth rate?
Nine percent per year.
The formula doesn’t care if you’re counting horses or dollars.
The growth rate for horses is not the same as the birth rate.
The birth rate will be five to six percentage points higher to account for deaths.
For a herd to grow at a rate of 12% per year, a birth rate of seventeen to eighteen percent per year will be needed.
What happens when the birth rate is zero or nearly so, because the advocates poisoned the mares with pesticide-laced darts?
The herd shrinks, as is the case at the Salt River and Virginia Range.
Western Horse Watchers Association 