After a record breaking 84 degrees, let’s talk about 72!

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Let me get the weather out of the way because I have more important information to give to you.  I hope you’ll take the time to digest it.  That should be easy to do because we have lots of time on our hands and we can’t go out to eat.  We will do all of our digesting at home!  A quick warning, however.  You may get indigestion when you read what comes after the forecast.

Early morning rain will give way to mostly cloudy skies and temperatures in the mid 50’s…much cooler than yesterday’s record breaking 84 degree heat.

Many a student of mathematics, science, medicine, finance, and economics is aware of the “Rule of 72”.  Simply put, it gives us a quick way to estimate how long it would take to double our money if we knew the rate of return.

For example, if you could invest $1000 and you knew that you could lock in a compound interest rate of 6%, then your money would double in approximately 12 years.  Why 12?  72 divided by 6 equals 12.  That’s how simply the rule works.  You would have $2000 in about 12 years.

Suppose you could earn interest on the same $1000 compounded at 9%.  72 divided by 9 equals 8, so in about 8 years your account value would be $2000.

The “Rule of 72” is an easy and quick way to get answers to those types of questions without having to do any complicated mathematics.  It’s because of the complicated mathematics that the rule allows you to make quite accurate projections, but that’s a story for another day.

I bring this to your attention because I feel that there are too many people who misunderstand the danger in not addressing the exponential growth rate regarding COVID-19. I thought using the “Rule of 72” could simply illustrate the high importance in making an effort to understand.

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Here goes!

  • As of yesterday afternoon, when I composed this post, the total number of cases of COVID-19 infection throughout the world, outside of China, that were diagnosed just on this day alone, was 26,072.
  • The number of cases diagnosed just on the day before, Thursday, was 20,550.
  • The day before had 15,735 diagnoses.
  • One week ago, last Friday, there were 10,896 cases diagnosed on that day alone.
  • If you are seeing the dramatic increase in numbers from day to day you may notice that the case load is not increasing at a constant rate of change.  It’s not like putting your car on cruise control.
  • The average rate of change for just the last week alone is 9.7% per day.
  • For the case of our projections let’s round that off to 10% per day.
  • The “Rule of 72” implies that if the rate (10%) remains the same, then the 26,072 cases of the coronavirus is projected to double the daily number of diagnoses in about one week.  Why one week?  72 divided by 10 is 7.2 days.
  • I don’t think hospitals are ready for 52,000 new patients next Friday.!
  • And I’m sure hospitals don’t want another 104,000 new cases on the first Friday in April either.

If, as Dr. Anthony Fauci suggests, the growth rate can be reduced through social distancing, we better the chances that our medical facilities will not become overwhelmed.  That is what he means by flattening the curve.

Epidemiologists estimate that up to 40,000,000 Americans will become infected with COVID-19. We do not want them all to be ill at the same time. We must stretch it out!


Suppose each citizen faithfully practiced social distancing and used common sense and good hygiene.  If we could reduce the growth rate to 6% for starters then the total number of cases would double every 12 weeks instead of every week.

If we could reduce the growth rate to 2% then the number of cases would only double every 36 weeks.  We can handle that, America!  Yay, Math!!!!!


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