Position Sizing with the Kelly Criterion

The Kelly Criterion is a mathematical formula to determine the optimal dollar amount to bet in a given wager or investment. Say you’re offered a bet where you are a 60% favorite to win and it pays 2:1 in your favor—Kelly suggests betting 40% of your net worth. If you were offered this exact bet a million times, betting 40% of your net worth each attempt (adjusting as you go) would net you the most amount of money in the end. Thus, 40% is the optimal bet size given those odds and payouts.

I’m betting no one (myself included) would actually bet 40% of their net worth in the above scenario—even if they knew the odds and payouts were legit. Most of you probably read that previous paragraph and thought 40% was insanity. I think one of the more interesting takeaways from the Kelly Criterion is how few people live their life in a way that optimizes their net worth over the long-term (again, myself included). And most of us aren’t a little off the mark, we’re not even in the ballpark of what’s optimal. If the above offer was made to all Americans, the average bet size would probably be less than 1% of each person’s net worth (with many doing a nominal amount like $10 per bet).
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