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A professor of mine at university taught us the same thing. It's one of the most valuable things I learned and it is uncannily accurate. Even after 30 years in industry I still fall back on this formula with great results.

Sadly, managers want to believe that you put more effort into estimating deadlines so I'll just whip up a gannt chart retroactively based on the 2/20 rule and they're happy.

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> Even after 30 years in industry I still fall back on this formula with great results.

Isn't this Parkinson's law in action, i.e. work expands to fill the time available for its completion? That would mean you overestimate up to 2.4x each time.

Edit: math.

> work expands to fill the time available for its completion

Oh, so if I estimate every task as 1 minute I'll finish really quickly?

The 2.4x is to overcome one's innate overoptimism (even if you see yourself as a pessimist), and to add contingency for unplanned complexity, bugs and general overhead.

It's a rule of thumb, not an immutable law, so if one has completed a similar task many times then less padding is needed.

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