It's really, really hard to define productivity accurately.

The Merriam-Webster Dictionary defines productivity as "the quality or state of yielding results, benefits, or profits".  This is a great definition when someone asks you what the word productivity means, but it's horrible in application.  Which activities are defined as productive?  Which ones are not?

Most people agree on the extremes of the productivity spectrum.  Organizing a food drive is productive, scrolling through Reddit at work is not, studying for school exams is productive, playing Battleship with your cat (above) is not, etc.  However, as you move closer to the midpoint of what is productive or unproductive, the line between the two blurs.  Is watching Netflix when you have an urgent deadline productive?  Probably not.  How about watching Netflix with your friends as a way to relax?  Maybe.  What about watching Netflix with your family as a bonding experience?  The (un)productiveness of the action becomes more uncertain.

After reading some articles on the subject and mulling it over, I've come to the conclusion that the popular perception of productivity is:

Daily Productivity = Production - Consumption

Production is defined as activities where we have net production (working produces a salary, studying produces knowledge) and consumption comprises activities where we have net consumption (watching TV consumes brainpower, throwing a tantrum consumes energy).

It's immediately obvious that the above equation isn't perfect.  If someone reads a book on a subject that's interesting but not useful, are they net producing knowledge or net consuming interest?  How about video games, do they net produce enjoyment or net consume brainpower?  If someone exercises, are they net producing happiness or net consuming energy?  The answer to these questions varies from person to person, so to account for that, we'll revise our equation:

Daily Productivity ≠ Production - Consumption

Daily Productivity = Production + Benefits - Consumption - Costs

In our revised equation, benefits and costs are broad categories that include all benefits/costs not covered in the production and consumption categories.

However, there are still difficult questions that the equation can't answer.  If someone just... doesn't do anything all day, are they still being productive?  Perhaps, if you're resting to recharge your energy, but our earlier system doesn't account for that.  If person A does twice as much as person B, but in four times the time, is person A more or less productive than person B?  Let's add in time to counterbalance that:

Daily Productivity ≠ Production - Consumption

Daily Productivity ≠ Production + Benefits - Consumption - Costs

Daily Productivity = (Production + Benefits - Consumption - Costs) / Time

The equation is much more detailed now, and we're getting closer to an accurate definition of productivity.  There's just one more major item to consider: opportunity costs.

This next section depends on personal values.  If you view productivity as solely a measure of production vs. consumption, then the above definition works fine.  But, if you want to consider how productive you could have been, then there's one last part of the equation to add:

Daily Productivity ≠ Production - Consumption

Daily Productivity ≠ Production + Benefits - Consumption - Costs

Daily Productivity = (Production + Benefits - Consumption - Costs) / Time

Daily Productivity = [ (Production + Benefits - Consumption - Costs) - (Opportunity Costs) ] / Time

You can never perfectly define anything, but for productivity, you won't get much closer than that.